Re: 2N3904 B-E junction as fast diode substitute?
Hey, at least you're talking about a scope, Ozan. Some of us pushed the boundaries quite a lot lately with our discussion of part numbering schemes.
Jim Ford
toggle quoted message
Show quoted text
------ Original Message ------ From: "Ozan" <ozan_g@...> To: [email protected]Sent: 2/22/2021 9:28:15 PM Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute? Keithley 616 has the MOSFET protection scheme Ed described. I checked my Keithley 196, it uses JFETs for protection diodes.
I know I am going off topic but Heathkit IO-4235 oscilloscope has a "feature" that blows up B-E junction of power supply BJT Q101(and releasing the magic smoke of couple of zeners with it) if not loaded. Ozan
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Re: 2N3904 B-E junction as fast diode substitute?
Keithley 616 has the MOSFET protection scheme Ed described. I checked my Keithley 196, it uses JFETs for protection diodes.
I know I am going off topic but Heathkit IO-4235 oscilloscope has a "feature" that blows up B-E junction of power supply BJT Q101(and releasing the magic smoke of couple of zeners with it) if not loaded. Ozan
|
Re: 2N3904 B-E junction as fast diode substitute?
The Heath IO-103 Oscope used L842 (or 2N3854) in a C-E config as input protection for vert and hori channels.?
This scope had a variety of problems that I won't go into here, but one problem was related to a few of those transistors not providing consistent performance (capacitance and breakdown voltages).? I think I finally got that fixed.? Other serous problems included hori-vert channel cross coupling, excessive vert? thermal drift, and a few others.? A litany of design errors.? The only Heathkit that truly disappointed me, of the more than 14 kits I built.
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Re: 2N3904 B-E junction as fast diode substitute?
Somebody told me JFETs are pretty good but I dont have a data sheet handy.
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Re: 2N3904 B-E junction as fast diode substitute?
Ed, I swear that I remember seeing exactly the same model radio! I think it was sold in Woolworth's. On Mon, Feb 22, 2021 at 10:02 PM Leon Robinson <leon-robinson@...> wrote: I've seen some radios that had transistors that weren't even connected just to up the count.
Leon Robinson K5JLR
-------- Original message -------- From: "Ed Breya via groups.io" <edbreya@...> Date: 02/22/2021 6:00 PM (GMT-06:00) To: [email protected] Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute?
Dennis, here's a transistor trick that I'm sure was never taught in engineering. More of a marketing lesson. Back in the day, the number of transistors in a radio was a huge selling point - the more the better. Eventually they got cheap, so you could throw extra transistors in for practically anything, upping that banner spec. I once found a cheap line-powered "8 transistor" AM radio, where besides the usual six needed for the function, they counted two more used as rectifiers in the power supply, off the transformer.
Ed
|
Locked
Re: Slightly OT: How can I dissolve Potting Compound? FOLLOWUP
I have welding equipment including Oxy-Acetylene and Helium, CO2 and 25/75 gas for MIG and TIG and the same companies sell all of that also sell liquified gasses so I doubt that I'd have any trouble getting liquid N2. Liquid O2, possibly, but not liquid N2 which in realtively inert and doesn't support combustion.
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Show quoted text
On Mon, Feb 22, 2021 at 9:31 PM Chuck Harris <cfharris@...> wrote: The large dewars are nice, but they have to have a very hard vacuum inside if you want them to hold LN2 for any amount of time... We're talking oil diffusion levels.
Then, there is the problem of getting your local supplier to let you have some. They consider it rather dangerous, and most won't sell it to individuals. So, become a company.
-Chuck Harris
- wrote:
I passed up a good size dewar at a garage sale for $5 about a year and a
half ago! :-/ At the time I just thought that I'd never use it and I'm getting to the age where I need to be getting rid of stuff instead of acquiring more; so, for once, I listened to the logical half of my brain instead of my imagination.
I had a mole removed a few years ago and the Dr used liquid Nitrogen and
he keeps his in one of the old green metal Aladdin thermos bottles like the
one that used to be sold with their metal lunch boxes. He said that that thermos would keep some of the liquid N2 all day long. They only need a tiny drop of it to freeze moles and warts. The thermos are small but they might be large enough for a small assembly.
On Mon, Feb 22, 2021 at 1:46 PM Paul Amaranth <paul@...> wrote:
LN2 is apparently pretty cheap if you have a supplier to sell it to you. I knew I should have picked up a dewar from the university surplus store in the old times. If they ever open it up again ...
Paul
On Mon, Feb 22, 2021 at 10:40:26AM -0800, Dennis Tillman W7pF wrote:
Very clever! I never thought of that. There was a time I could have done
this and I know exactly how it works.
Yes, there is a risk, but as you point out it involves no dangerous chemicals or careful temperature regulation and messy chipping away of very
hot epoxy.
Thanks for the suggestion even if it is too late in my case. Dennis Tilman W7pF
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of - Sent: Monday, February 22, 2021 3:49 AM To: [email protected] Subject: Re: [TekScopes] Slightly OT: How can I dissolve Potting Compound? FOLLOWUP
Dennis,
A friend of mine is always disassembling things and reverse engineering them so I sent your message to him. He replied and that me that
one of the guys that he used with work with at Litton Laser told him that
he used to removing potting compound by dipping the item into liquid Nitrogen and leaving it there until the nitrogen quit boiling and then took
it out and tapped it with a hammer and that the coating would shatter and
fail off. I have to wonder what that treatment would do to the plastic bodies of transistors and other items but it might be worth a try. IMO you
might need to leave the item in the liquid Nitrogen for a bit less time to
prevent the plastic items from getting too cold and too hard and brittle.
But the good thing about this method is that there are no dangerous chemicals to deal with and even the liquid Nitrogen just boils away so there's nothing to dispose of except the potting compound itself.
FWIW
On Sun, Feb 21, 2021 at 3:37 PM Dennis Tillman W7pF <
dennis@...
wrote:
Thank you one and all for your suggestions on removing the potting. Your suggestions fell into two general categories: 1) Nasty chemicals which I decided had to be avoided at all costs based on your advice. 2) Heat. This seemed like a slightly better approach than the chemical one.
But before I get to what happened when I applied heat I learned many new things along the way thanks to a few of our members and especially, Ed Breya. Ed's comments explained in detail how these HV potted power supplies worked. That led me to an amazing web site that has a staggering amount of information on them about every aspect of lasers of all kinds including their HV Power Supplies. If anyone is interested the site is I have to caution you that I spent hours roaming around in all Sam had to say. He is an incredible resource for anyone who owns a laser. I found many different power supply circuits and details about laser HV modules which is what I apparently blew out that prompted me to ask about removing the potting in the first place.
With suggestions from Ed Breya I tried using the other more powerful potted HV laser supply I had. It was going to try and force too much current through the spectrum tubes so I added additional ballast resistors and connected it to a Variac so I had some control over the High Voltage and the constant current With this arrangement I was able to test every one of the spectrum tubes (I have over 20 different ones each containing a different gas) to determine their breakdown voltage and their steady state current requirements. That told me what I should be looking for to replace the Laser HV supply that I blew out.
In Sam's web site I found the design and schematic for a power supply designed by the legendary Jim Williams of Linear Technology. It was a universal design capable of powering every one of the spectrum tubes I
had.
I ordered the parts to build it.
I still wanted to see what I might learn from removing the potting of my failed HV Laser supply so for $15 I bought a toaster oven at a thrift shop.
I set it up outside, set the temp to 200F (93C) for 30 minutes and poked it with a sharp tool. Nothing happened. At 250F (120C) it got a little softer and a few cracks appeared. At 300F (150C) I was able to chip off the bottom and see the solder side of the PCB. A few of the sides also chipped off.
This was starting to work.
Since the oven was on the ground it was hard to see the temperature knob from the angle I was standing but I raised the temperature once again by the same amount.
30 minutes later when I checked on it, it was smoking. I would rather not say what the reading on the thermocouple was but I will tell you it was way over 260C (500F). When I touched the potting the PC board fell away from the rest of the potting. I knew immediately that was to be expected since solder melts at a lot less than 260C. The parts fell away from the epoxy, by barely touching them but of course there was no way to know what they used to be connected to anymore. The entire thing was a mess.
It did teach me some things. Heat is definitely safer than chemicals to remove potting but realistically even with carefully controlled heat potting is not going to give up its secrets easily or cleanly. I now have an "oven" I can use for soldering surface mount parts with whenever I find something I can't build with a through hole parts.
In the end TekScopes members led me to a solution for an excellent HV Power Supply for my Spectrum Tubes in the form of the Jim William's Linear Technology Application Note 49, August 1992, titled "Illumination Circuitry for Liquid Crystal Displays". His design is shown in Appendix D.
"Figure D1: Laser Power Supply is Essentially a 10KV Compliance Variable Current Source".
Thank you Ed and many others who offered their suggestions
Dennis Tillman W7pF
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Dennis Tillman W7pF Sent: Saturday, January 02, 2021 6:19 PM To: [email protected] Subject: [TekScopes] Slightly OT: How can I dissolve Potting Compound?
I need to dissolve the black HV potting compound of a 12VDC powered Helium-Neon laser inverter power supply I have that stopped working. I would appreciate any suggestions on what works to do this. I¡¯m guessing it may be epoxy. I stuck the tip of a hot soldering iron in it for a few seconds without much effect.
When it was working it turned out to be perfect for powering gas filled Spectrum Tubes. These spectrum tubes (smaller versions of neon signs) filled with a variety of gasses are an excellent source of spectral lines for the 7J20 / J20 Rapid Scan (Optical) Spectrometer to measure.
Spectrum tubes require an initial high voltage (1,000V to 1,500V for example) to break down the gas and start it conducting. Once the gas in the spectrum tube conducts the voltage across the gas drops (250V to 450V for example) and unless you limit the current (to a few mA) it will destroy the tube. Can anyone can point me to a source of information on how to determine the proper voltage and current I need to power these spectrum tubes? Is there a web site or group devoted to Spectrum Tubes?
Something happened to the inverter and it stopped working. The input is now open. The inverter is a black potted brick 3¡± x 1?¡± x 1¡±. The ballast resistor has continuity so that is not the problem. The original label on the inverter is partially destroyed so I can¡¯t tell what its initial high voltage output was or what it current limits at. All I do know is that it was made by Laser Drive Inc. 5465 Wm. Flynn Hwy. Gibsonia, PA 15044 Model: 1150-6330, S/N: 610574 The input was +12VDC at 0.35A.
I wrote to the company that took over the company that took over Laser Drive Inc. asking if they could tell me the output voltage and limiting current but I didn¡¯t receive a reply.
At this point I am hoping if I can remove the potting compound I can figure out what went wrong with it.
I have a different, bigger Laser Drive Inc. potted inverter which is powered by 115VAC. It puts out 2350VDC at 6.5mA. This causes the Spectrum Tubes to flicker. They do not run continuously. I am guessing that this because 6.5mA is more current than the tube can conduct. The amount of current the tube draws increases in proportion to the inside diameter of the tube but I don¡¯t know much about this matching the power supply to the tube. All I do know is the one that went bad seems to be an ideal match for the spectrum tubes I have.
Battery operated (DC input) inverters are much more desirable than AC input inverters for this application because the AC rectification and poor filtering shows up on the output DC as significant ripple causing the amplitude of each spectral line to be blurred.
Dennis Tillman W7pF
-- Dennis Tillman W7pF TekScopes Moderator
-- Dennis Tillman W7pF TekScopes Moderator
!DSPAM:6033fab334634654433166!
-- Paul Amaranth, GCIH | Manchester MI, USA Aurora Group of Michigan, LLC | Security, Systems & Software paul@... | Unix/Linux - We don't do windows
|
Re: 2N3904 B-E junction as fast diode substitute?
I've seen some radios that had transistors that weren't even connected just to up the count.
Leon Robinson ?K5JLR
toggle quoted message
Show quoted text
-------- Original message -------- From: "Ed Breya via groups.io" <edbreya@...> Date: 02/22/2021 6:00 PM (GMT-06:00) To: [email protected] Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute? Dennis, here's a transistor trick that I'm sure was never taught in engineering. More of a marketing lesson. Back in the day, the number of transistors in a radio was a huge selling point - the more the better. Eventually they got cheap, so you could throw extra transistors in for practically anything, upping that banner spec. I once found a cheap line-powered "8 transistor" AM radio, where besides the usual six needed for the function, they counted two more used as rectifiers in the power supply, off the transformer. Ed
|
Re: 2N3904 B-E junction as fast diode substitute?
Dave, I think that's right - TO-98. The 2N3391 rings a bell too. There were a number of them in that part range, that might be the right ones for good noise. I don't know if any newer, modern versions would work the same, since they'd probably be made with newer, better processes.
Ed
|
Locked
Re: Slightly OT: How can I dissolve Potting Compound? FOLLOWUP
The stainless steel lifetime warranty ones. I have 4, two I pickup at yard sales duds Aladdin replaced. Have another dud one of the original. They will keep Hot coffee drinkable for about 12 hours. gary g
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Re: 2N3904 B-E junction as fast diode substitute?
I thought of that once when I got hold of one of those radios and tested the extra transistors. About half of them were shorted. They were probably rejects or picked off the floor.
?? Bruce Gentry, KA2IVY
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Show quoted text
On 2/22/21 20:45, Ed Breya via groups.io wrote: Maybe the makers of these excess-transistor radios were just being considerate, and provided spares. Or not. Ed
|
Locked
Re: Slightly OT: How can I dissolve Potting Compound? FOLLOWUP
The large dewars are nice, but they have to have a very hard vacuum inside if you want them to hold LN2 for any amount of time... We're talking oil diffusion levels.
Then, there is the problem of getting your local supplier to let you have some. They consider it rather dangerous, and most won't sell it to individuals. So, become a company.
-Chuck Harris
- wrote:
toggle quoted message
Show quoted text
I passed up a good size dewar at a garage sale for $5 about a year and a half ago! :-/ At the time I just thought that I'd never use it and I'm getting to the age where I need to be getting rid of stuff instead of acquiring more; so, for once, I listened to the logical half of my brain instead of my imagination.
I had a mole removed a few years ago and the Dr used liquid Nitrogen and he keeps his in one of the old green metal Aladdin thermos bottles like the one that used to be sold with their metal lunch boxes. He said that that thermos would keep some of the liquid N2 all day long. They only need a tiny drop of it to freeze moles and warts. The thermos are small but they might be large enough for a small assembly.
On Mon, Feb 22, 2021 at 1:46 PM Paul Amaranth <paul@...> wrote:
LN2 is apparently pretty cheap if you have a supplier to sell it to you. I knew I should have picked up a dewar from the university surplus store in the old times. If they ever open it up again ...
Paul
On Mon, Feb 22, 2021 at 10:40:26AM -0800, Dennis Tillman W7pF wrote:
Very clever! I never thought of that. There was a time I could have done this and I know exactly how it works.
Yes, there is a risk, but as you point out it involves no dangerous chemicals or careful temperature regulation and messy chipping away of very hot epoxy.
Thanks for the suggestion even if it is too late in my case. Dennis Tilman W7pF
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of - Sent: Monday, February 22, 2021 3:49 AM To: [email protected] Subject: Re: [TekScopes] Slightly OT: How can I dissolve Potting Compound? FOLLOWUP
Dennis,
A friend of mine is always disassembling things and reverse engineering them so I sent your message to him. He replied and that me that one of the guys that he used with work with at Litton Laser told him that he used to removing potting compound by dipping the item into liquid Nitrogen and leaving it there until the nitrogen quit boiling and then took it out and tapped it with a hammer and that the coating would shatter and fail off. I have to wonder what that treatment would do to the plastic bodies of transistors and other items but it might be worth a try. IMO you might need to leave the item in the liquid Nitrogen for a bit less time to prevent the plastic items from getting too cold and too hard and brittle. But the good thing about this method is that there are no dangerous chemicals to deal with and even the liquid Nitrogen just boils away so there's nothing to dispose of except the potting compound itself.
FWIW
On Sun, Feb 21, 2021 at 3:37 PM Dennis Tillman W7pF <dennis@...
wrote:
Thank you one and all for your suggestions on removing the potting. Your suggestions fell into two general categories: 1) Nasty chemicals which I decided had to be avoided at all costs based on your advice. 2) Heat. This seemed like a slightly better approach than the chemical one.
But before I get to what happened when I applied heat I learned many new things along the way thanks to a few of our members and especially, Ed Breya. Ed's comments explained in detail how these HV potted power supplies worked. That led me to an amazing web site that has a staggering amount of information on them about every aspect of lasers of all kinds including their HV Power Supplies. If anyone is interested the site is I have to caution you that I spent hours roaming around in all Sam had to say. He is an incredible resource for anyone who owns a laser. I found many different power supply circuits and details about laser HV modules which is what I apparently blew out that prompted me to ask about removing the potting in the first place.
With suggestions from Ed Breya I tried using the other more powerful potted HV laser supply I had. It was going to try and force too much current through the spectrum tubes so I added additional ballast resistors and connected it to a Variac so I had some control over the High Voltage and the constant current With this arrangement I was able to test every one of the spectrum tubes (I have over 20 different ones each containing a different gas) to determine their breakdown voltage and their steady state current requirements. That told me what I should be looking for to replace the Laser HV supply that I blew out.
In Sam's web site I found the design and schematic for a power supply designed by the legendary Jim Williams of Linear Technology. It was a universal design capable of powering every one of the spectrum tubes I
had.
I ordered the parts to build it.
I still wanted to see what I might learn from removing the potting of my failed HV Laser supply so for $15 I bought a toaster oven at a thrift shop.
I set it up outside, set the temp to 200F (93C) for 30 minutes and poked it with a sharp tool. Nothing happened. At 250F (120C) it got a little softer and a few cracks appeared. At 300F (150C) I was able to chip off the bottom and see the solder side of the PCB. A few of the sides also chipped off.
This was starting to work.
Since the oven was on the ground it was hard to see the temperature knob from the angle I was standing but I raised the temperature once again by the same amount.
30 minutes later when I checked on it, it was smoking. I would rather not say what the reading on the thermocouple was but I will tell you it was way over 260C (500F). When I touched the potting the PC board fell away from the rest of the potting. I knew immediately that was to be expected since solder melts at a lot less than 260C. The parts fell away from the epoxy, by barely touching them but of course there was no way to know what they used to be connected to anymore. The entire thing was a mess.
It did teach me some things. Heat is definitely safer than chemicals to remove potting but realistically even with carefully controlled heat potting is not going to give up its secrets easily or cleanly. I now have an "oven" I can use for soldering surface mount parts with whenever I find something I can't build with a through hole parts.
In the end TekScopes members led me to a solution for an excellent HV Power Supply for my Spectrum Tubes in the form of the Jim William's Linear Technology Application Note 49, August 1992, titled "Illumination Circuitry for Liquid Crystal Displays". His design is shown in Appendix D.
"Figure D1: Laser Power Supply is Essentially a 10KV Compliance Variable Current Source".
Thank you Ed and many others who offered their suggestions
Dennis Tillman W7pF
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Dennis Tillman W7pF Sent: Saturday, January 02, 2021 6:19 PM To: [email protected] Subject: [TekScopes] Slightly OT: How can I dissolve Potting Compound?
I need to dissolve the black HV potting compound of a 12VDC powered Helium-Neon laser inverter power supply I have that stopped working. I would appreciate any suggestions on what works to do this. I¡¯m guessing it may be epoxy. I stuck the tip of a hot soldering iron in it for a few seconds without much effect.
When it was working it turned out to be perfect for powering gas filled Spectrum Tubes. These spectrum tubes (smaller versions of neon signs) filled with a variety of gasses are an excellent source of spectral lines for the 7J20 / J20 Rapid Scan (Optical) Spectrometer to measure.
Spectrum tubes require an initial high voltage (1,000V to 1,500V for example) to break down the gas and start it conducting. Once the gas in the spectrum tube conducts the voltage across the gas drops (250V to 450V for example) and unless you limit the current (to a few mA) it will destroy the tube. Can anyone can point me to a source of information on how to determine the proper voltage and current I need to power these spectrum tubes? Is there a web site or group devoted to Spectrum Tubes?
Something happened to the inverter and it stopped working. The input is now open. The inverter is a black potted brick 3¡± x 1?¡± x 1¡±. The ballast resistor has continuity so that is not the problem. The original label on the inverter is partially destroyed so I can¡¯t tell what its initial high voltage output was or what it current limits at. All I do know is that it was made by Laser Drive Inc. 5465 Wm. Flynn Hwy. Gibsonia, PA 15044 Model: 1150-6330, S/N: 610574 The input was +12VDC at 0.35A.
I wrote to the company that took over the company that took over Laser Drive Inc. asking if they could tell me the output voltage and limiting current but I didn¡¯t receive a reply.
At this point I am hoping if I can remove the potting compound I can figure out what went wrong with it.
I have a different, bigger Laser Drive Inc. potted inverter which is powered by 115VAC. It puts out 2350VDC at 6.5mA. This causes the Spectrum Tubes to flicker. They do not run continuously. I am guessing that this because 6.5mA is more current than the tube can conduct. The amount of current the tube draws increases in proportion to the inside diameter of the tube but I don¡¯t know much about this matching the power supply to the tube. All I do know is the one that went bad seems to be an ideal match for the spectrum tubes I have.
Battery operated (DC input) inverters are much more desirable than AC input inverters for this application because the AC rectification and poor filtering shows up on the output DC as significant ripple causing the amplitude of each spectral line to be blurred.
Dennis Tillman W7pF
-- Dennis Tillman W7pF TekScopes Moderator
-- Dennis Tillman W7pF TekScopes Moderator
!DSPAM:6033fab334634654433166!
-- Paul Amaranth, GCIH | Manchester MI, USA Aurora Group of Michigan, LLC | Security, Systems & Software paul@... | Unix/Linux - We don't do windows
|
Re: 2N3904 B-E junction as fast diode substitute?
?That would be TO-98. 2N3391? (By the way, that number is still in production but TO-92.) Dave Wise? ________________________________ From: [email protected] < [email protected]> on behalf of Ed Breya via groups.io <edbreya@...> Sent: Monday, February 22, 2021 3:48 PM To: [email protected]Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute? Morris, if you really need a noisy "Zener," look into an old series of GE NPN Si transistors, that are in an old-style package. I forget the JEDEC designation, but it's the one that looks kind of like a TO-92, but with a disc shaped base. I think they were around 2N32xx or 2N33xx numbers. Certain ones in that family have huge Zener/avalanche breakdown noise, like hundreds of mV, when biased up with high impedance. They are great for audio noise makers, with little power, and not much amplification need - usually just an impedance buffer, since there's plenty of voltage already. I always save any of that style I find during salvage operations, then when there's a bunch accumulated, I check them out on the curve tracer. I save the good noisy ones, and the rest go to e-waste - I wouldn't use them for any new stuff. Not all of those 2N-numbers do this, and not every one of the "right" part type does either, but there's usually enough yield to gather a pretty good collection. Ed
|
Locked
Re: Slightly OT: How can I dissolve Potting Compound? FOLLOWUP
I passed up a good size dewar at a garage sale for $5 about a year and a half ago! :-/ At the time I just thought that I'd never use it and I'm getting to the age where I need to be getting rid of stuff instead of acquiring more; so, for once, I listened to the logical half of my brain instead of my imagination.
I had a mole removed a few years ago and the Dr used liquid Nitrogen and he keeps his in one of the old green metal Aladdin thermos bottles like the one that used to be sold with their metal lunch boxes. He said that that thermos would keep some of the liquid N2 all day long. They only need a tiny drop of it to freeze moles and warts. The thermos are small but they might be large enough for a small assembly.
toggle quoted message
Show quoted text
On Mon, Feb 22, 2021 at 1:46 PM Paul Amaranth <paul@...> wrote: LN2 is apparently pretty cheap if you have a supplier to sell it to you. I knew I should have picked up a dewar from the university surplus store in the old times. If they ever open it up again ...
Paul
On Mon, Feb 22, 2021 at 10:40:26AM -0800, Dennis Tillman W7pF wrote:
Very clever! I never thought of that. There was a time I could have done this and I know exactly how it works.
Yes, there is a risk, but as you point out it involves no dangerous chemicals or careful temperature regulation and messy chipping away of very hot epoxy.
Thanks for the suggestion even if it is too late in my case. Dennis Tilman W7pF
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of - Sent: Monday, February 22, 2021 3:49 AM To: [email protected] Subject: Re: [TekScopes] Slightly OT: How can I dissolve Potting Compound? FOLLOWUP
Dennis,
A friend of mine is always disassembling things and reverse engineering them so I sent your message to him. He replied and that me that one of the guys that he used with work with at Litton Laser told him that he used to removing potting compound by dipping the item into liquid Nitrogen and leaving it there until the nitrogen quit boiling and then took it out and tapped it with a hammer and that the coating would shatter and fail off. I have to wonder what that treatment would do to the plastic bodies of transistors and other items but it might be worth a try. IMO you might need to leave the item in the liquid Nitrogen for a bit less time to prevent the plastic items from getting too cold and too hard and brittle. But the good thing about this method is that there are no dangerous chemicals to deal with and even the liquid Nitrogen just boils away so there's nothing to dispose of except the potting compound itself.
FWIW
On Sun, Feb 21, 2021 at 3:37 PM Dennis Tillman W7pF <dennis@...
wrote:
Thank you one and all for your suggestions on removing the potting. Your suggestions fell into two general categories: 1) Nasty chemicals which I decided had to be avoided at all costs based on your advice. 2) Heat. This seemed like a slightly better approach than the chemical one.
But before I get to what happened when I applied heat I learned many new things along the way thanks to a few of our members and especially, Ed Breya. Ed's comments explained in detail how these HV potted power supplies worked. That led me to an amazing web site that has a staggering amount of information on them about every aspect of lasers of all kinds including their HV Power Supplies. If anyone is interested the site is I have to caution you that I spent hours roaming around in all Sam had to say. He is an incredible resource for anyone who owns a laser. I found many different power supply circuits and details about laser HV modules which is what I apparently blew out that prompted me to ask about removing the potting in the first place.
With suggestions from Ed Breya I tried using the other more powerful potted HV laser supply I had. It was going to try and force too much current through the spectrum tubes so I added additional ballast resistors and connected it to a Variac so I had some control over the High Voltage and the constant current With this arrangement I was able to test every one of the spectrum tubes (I have over 20 different ones each containing a different gas) to determine their breakdown voltage and their steady state current requirements. That told me what I should be looking for to replace the Laser HV supply that I blew out.
In Sam's web site I found the design and schematic for a power supply designed by the legendary Jim Williams of Linear Technology. It was a universal design capable of powering every one of the spectrum tubes I
had.
I ordered the parts to build it.
I still wanted to see what I might learn from removing the potting of my failed HV Laser supply so for $15 I bought a toaster oven at a thrift shop.
I set it up outside, set the temp to 200F (93C) for 30 minutes and poked it with a sharp tool. Nothing happened. At 250F (120C) it got a little softer and a few cracks appeared. At 300F (150C) I was able to chip off the bottom and see the solder side of the PCB. A few of the sides also chipped off.
This was starting to work.
Since the oven was on the ground it was hard to see the temperature knob from the angle I was standing but I raised the temperature once again by the same amount.
30 minutes later when I checked on it, it was smoking. I would rather not say what the reading on the thermocouple was but I will tell you it was way over 260C (500F). When I touched the potting the PC board fell away from the rest of the potting. I knew immediately that was to be expected since solder melts at a lot less than 260C. The parts fell away from the epoxy, by barely touching them but of course there was no way to know what they used to be connected to anymore. The entire thing was a mess.
It did teach me some things. Heat is definitely safer than chemicals to remove potting but realistically even with carefully controlled heat potting is not going to give up its secrets easily or cleanly. I now have an "oven" I can use for soldering surface mount parts with whenever I find something I can't build with a through hole parts.
In the end TekScopes members led me to a solution for an excellent HV Power Supply for my Spectrum Tubes in the form of the Jim William's Linear Technology Application Note 49, August 1992, titled "Illumination Circuitry for Liquid Crystal Displays". His design is shown in Appendix D.
"Figure D1: Laser Power Supply is Essentially a 10KV Compliance Variable Current Source".
Thank you Ed and many others who offered their suggestions
Dennis Tillman W7pF
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Dennis Tillman W7pF Sent: Saturday, January 02, 2021 6:19 PM To: [email protected] Subject: [TekScopes] Slightly OT: How can I dissolve Potting Compound?
I need to dissolve the black HV potting compound of a 12VDC powered Helium-Neon laser inverter power supply I have that stopped working. I would appreciate any suggestions on what works to do this. I¡¯m guessing it may be epoxy. I stuck the tip of a hot soldering iron in it for a few seconds without much effect.
When it was working it turned out to be perfect for powering gas filled Spectrum Tubes. These spectrum tubes (smaller versions of neon signs) filled with a variety of gasses are an excellent source of spectral lines for the 7J20 / J20 Rapid Scan (Optical) Spectrometer to measure.
Spectrum tubes require an initial high voltage (1,000V to 1,500V for example) to break down the gas and start it conducting. Once the gas in the spectrum tube conducts the voltage across the gas drops (250V to 450V for example) and unless you limit the current (to a few mA) it will destroy the tube. Can anyone can point me to a source of information on how to determine the proper voltage and current I need to power these spectrum tubes? Is there a web site or group devoted to Spectrum Tubes?
Something happened to the inverter and it stopped working. The input is now open. The inverter is a black potted brick 3¡± x 1?¡± x 1¡±. The ballast resistor has continuity so that is not the problem. The original label on the inverter is partially destroyed so I can¡¯t tell what its initial high voltage output was or what it current limits at. All I do know is that it was made by Laser Drive Inc. 5465 Wm. Flynn Hwy. Gibsonia, PA 15044 Model: 1150-6330, S/N: 610574 The input was +12VDC at 0.35A.
I wrote to the company that took over the company that took over Laser Drive Inc. asking if they could tell me the output voltage and limiting current but I didn¡¯t receive a reply.
At this point I am hoping if I can remove the potting compound I can figure out what went wrong with it.
I have a different, bigger Laser Drive Inc. potted inverter which is powered by 115VAC. It puts out 2350VDC at 6.5mA. This causes the Spectrum Tubes to flicker. They do not run continuously. I am guessing that this because 6.5mA is more current than the tube can conduct. The amount of current the tube draws increases in proportion to the inside diameter of the tube but I don¡¯t know much about this matching the power supply to the tube. All I do know is the one that went bad seems to be an ideal match for the spectrum tubes I have.
Battery operated (DC input) inverters are much more desirable than AC input inverters for this application because the AC rectification and poor filtering shows up on the output DC as significant ripple causing the amplitude of each spectral line to be blurred.
Dennis Tillman W7pF
-- Dennis Tillman W7pF TekScopes Moderator
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Re: 2N3904 B-E junction as fast diode substitute?
Maybe the makers of these excess-transistor radios were just being considerate, and provided spares. Or not. Ed
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Re: 2N3904 B-E junction as fast diode substitute?
Ed, your posts are always full of great nuggets like these. Thanks so much!
-- Cheers, Tom
-- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070
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On 2/22/2021 17:34, Ed Breya via groups.io wrote: Tom, if you study some of the old Keithley electrometers, you may find the two E-B junction clamp. I believe it was sort of a trade secret, developed when they started using MOSFETs (the type was probably a trade secret too), instead of electrometer tubes. In nearly every one (a lot) that I studied, the clamp circuit is shown as a black box. Later, the MOSFETs and clamps were combined on a small board, that could probably be figured out by inspection, by going through the trouble and risk to remove it. The sub-parts were never identified - the whole module was to be replaced if bad. I finally stumbled upon it about a year ago in a schematic of something they made, actually showing the details. I'm pretty sure I remember it right, because I had always assumed it was just some low-leakage diodes or JFETs - it only needs to protect the summing node MOSFET gate which operates right close to zero, and keep it from breakdown, which is fairly high - and I was surprised that it was a little different.
It actually makes sense though, and is pretty slick when you think about it. If you use a single diode junction for each direction, it's forward biased with any excursion of the summing node away from zero (usually up to a mV or so in operation), so even though the diode may have a very small saturation current (Is), it's still forward biased, and can have a big effect in a fA circuit. If you use the diodes in reverse bias, against some DC clamp voltages, then you have to deal with diode reverse leakage. By using two transistors, putting the forward clamp in series with the reverse breakdown junction, you get that extra blocking (the breakdown junction will see only very small reverse voltage due to the diode's forward current, so will have tiny leakage), and there's virtually no leakage until the excursion is well past the operating level, where it doesn't matter anyway. The end result is a clamp with extremely tiny leakage near zero voltage in normal operation, which starts to leak a little more as it leaves the normal range (don't care), then clamps solidly as the Zener level (say 7 V total) is approached, where you do care, to keep it well below the MOSFET gate breakdown, typically 20-30 V.
If you have a circuit where you need to hard-clamp at one junction, which is very common, you wouldn't use this kind of circuit. But, for these MOSFET inputs, it's awesome. Also, the whole thing is a passive, two-terminal device, so easy to apply. No wonder they kept it a secret. I'd guess that the surface body leakages on the transistors and MOSFETs is a much bigger problem than the clamp leakage itself - that's how good I think it can be.
I have seen the same or close used in some DMM circuits, so others have used it too. If I can re-find any of the definitive info, I'll post about it. If it turns that out I've imagined all this, I'll post about that too.
Ed
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Re: 2N3904 B-E junction as fast diode substitute?
Tom, if you study some of the old Keithley electrometers, you may find the two E-B junction clamp. I believe it was sort of a trade secret, developed when they started using MOSFETs (the type was probably a trade secret too), instead of electrometer tubes. In nearly every one (a lot) that I studied, the clamp circuit is shown as a black box. Later, the MOSFETs and clamps were combined on a small board, that could probably be figured out by inspection, by going through the trouble and risk to remove it. The sub-parts were never identified - the whole module was to be replaced if bad. I finally stumbled upon it about a year ago in a schematic of something they made, actually showing the details. I'm pretty sure I remember it right, because I had always assumed it was just some low-leakage diodes or JFETs - it only needs to protect the summing node MOSFET gate which operates right close to zero, and keep it from breakdown, which is fairly high - and I was surprised that it was a little different.
It actually makes sense though, and is pretty slick when you think about it. If you use a single diode junction for each direction, it's forward biased with any excursion of the summing node away from zero (usually up to a mV or so in operation), so even though the diode may have a very small saturation current (Is), it's still forward biased, and can have a big effect in a fA circuit. If you use the diodes in reverse bias, against some DC clamp voltages, then you have to deal with diode reverse leakage. By using two transistors, putting the forward clamp in series with the reverse breakdown junction, you get that extra blocking (the breakdown junction will see only very small reverse voltage due to the diode's forward current, so will have tiny leakage), and there's virtually no leakage until the excursion is well past the operating level, where it doesn't matter anyway. The end result is a clamp with extremely tiny leakage near zero voltage in normal operation, which starts to leak a little more as it leaves the normal range (don't care), then clamps solidly as the Zener level (say 7 V total) is approached, where you do care, to keep it well below the MOSFET gate breakdown, typically 20-30 V.
If you have a circuit where you need to hard-clamp at one junction, which is very common, you wouldn't use this kind of circuit. But, for these MOSFET inputs, it's awesome. Also, the whole thing is a passive, two-terminal device, so easy to apply. No wonder they kept it a secret. I'd guess that the surface body leakages on the transistors and MOSFETs is a much bigger problem than the clamp leakage itself - that's how good I think it can be.
I have seen the same or close used in some DMM circuits, so others have used it too. If I can re-find any of the definitive info, I'll post about it. If it turns that out I've imagined all this, I'll post about that too.
Ed
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Re: 2N3904 B-E junction as fast diode substitute?
The idea is by no means new.? There was a radio manufacturer named Midwest that used lavish numbers of lower power rated tubes (6K6) falling out of favor and cheap in parallel instead of one high rated popular and expensive (6L6) tube.? The lower plate impedance allowed the use of cheaper output transformers in large sylish cans, small transformer core and lots of pitch. Likewise, using four rectifiers in full wave bridge increased the tube count and allowed the use of cheaper power transformers.?? One solid state application of running up the count was actually functional. Instead of using a thermistor or diode as a temperature sensor, they used one or two transistors mounted on the output transistor heat sink to regulate the bias. Germanium transistors needed careful biasing, and by the time these radios became popular, silicon transistors were taking over.? Germaniums were abundant and cheap, so they were put to practical use as well as deceiving the customer.
????? Bruce Gentry, KA2IVY
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On 2/22/21 19:43, Michael A. Terrell wrote: Ed Breya via groups.io wrote:
Dennis, here's a transistor trick that I'm sure was never taught in engineering. More of a marketing lesson. Back in the day, the number of transistors in a radio was a huge selling point - the more the better. Eventually they got cheap, so you could throw extra transistors in for practically anything, upping that banner spec. I once found a cheap line-powered "8 transistor" AM radio, where besides the usual six needed for the function, they counted two more used as rectifiers in the power supply, off the transformer. There was a 15 transistor radio on the market around 1970.Only six were active. The rest had all three leads soldered together.
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Re: 2N3904 B-E junction as fast diode substitute?
Speaking of noise... could you please start new threads for these interesting but unrelated topics?
I looked at the 2N3904 and 2N5769 data sheets. They have the same max Ic (200 ma). The 3904 data sheet shows "output capacitance 4 pf, input capacitance 8 pf" and the 5769 collector-base capacitance 4 pf. Sound about the same...
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Re: 2N3904 B-E junction as fast diode substitute?
Ed Breya via groups.io wrote: Dennis, here's a transistor trick that I'm sure was never taught in engineering. More of a marketing lesson. Back in the day, the number of transistors in a radio was a huge selling point - the more the better. Eventually they got cheap, so you could throw extra transistors in for practically anything, upping that banner spec. I once found a cheap line-powered "8 transistor" AM radio, where besides the usual six needed for the function, they counted two more used as rectifiers in the power supply, off the transformer. There was a 15 transistor radio on the market around 1970.Only six were active. The rest had all three leads soldered together.
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Re: 2N3904 B-E junction as fast diode substitute?
Got one here somewhere- kept for years just to show people. All three device legs soldered together to the same pad and no other connection. Of course you don¡¯t know till you look at the copper side....... this one's from the early Jap AM pocket radio market. DaveB, NZ
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-----Original Message----- From: [email protected] [mailto: [email protected]] On Behalf Of Tom Lee Sent: Tuesday, February 23, 2021 13:03 To: [email protected]Subject: Re: [TekScopes] 2N3904 B-E junction as fast diode substitute? Jim Williams found an even more shameless example, where there were two dead transistors soldered to ... nothing. But they were counted in the AM radio advertisement. Tom Sent from my iThing, so please forgive typos and brevity. On Feb 22, 2021, at 4:00 PM, Ed Breya via groups.io <edbreya@...> wrote:
?Dennis, here's a transistor trick that I'm sure was never taught in engineering. More of a marketing lesson. Back in the day, the number of transistors in a radio was a huge selling point - the more the better. Eventually they got cheap, so you could throw extra transistors in for practically anything, upping that banner spec. I once found a cheap line-powered "8 transistor" AM radio, where besides the usual six needed for the function, they counted two more used as rectifiers in the power supply, off the transformer.
Ed
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