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In search of the "Safest" bench test setup
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I am revising my bench test configuration and am seeking the opinion
/recvommendations of the safety experts here on the board. My main test gear is a 465B scope with it's case/frame NOW hardwired to a water pipe. I power it with DC. I also use a 6015 Tek probe to test HV in DUT ( other scopes) My questions are: If I run the 465B with AC should I run it through a Variac? If so, is the variac considered an "isolation xformer? Should the device under test also be grounded to the water pipe? Can I power the DUT with the same variac? What is the "perfect" ( safest) bench test configuration Thanks for any and all comments /recommendations. Ron Simmons |
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Stefan Trethan
On Mon, 29 Jan 2007 17:01:16 +0100, HBcubed <rjsimmons@...> wrote:
My main test gear is a 465B scope with it's case/frame NOW hardwiredIt would be better to ground it through the electrical ground (ground prong or screw on an outlet). It is not always a good idea to use the plumbing for ground, since great hazards are created if the plumbing is not properly earthed, or some pipes are plastic (or are later replaced for plastic). It depends on your local regulation if it is still legal to use the plumbing. My questions are:Why would you? A variac is used for VARIable AC. Your scope should be made to run off mains voltage. Many scopes have internal adjustment for "low" "normal" or "high" line voltage, i don't know the 465B. If so, is the variac considered an "isolation xformer?Most variacs are not, they are so called autotransformers with no isolation between input and output. There are also isolated varicas, which have a second fully isolated winding under the outer output winding with the wiper. But often an isolation transformer will be put in front of a variac to provide isolation and adjustable voltage. Should the device under test also be grounded to the water pipe?Depends. Usually your DUT should be grounded, via the electrical system. Sometimes if the DUT is powered via an isolation transformer the ground is intentionally removed. You should not need that if you don't exactly know how or why. Can I power the DUT with the same variac?You can certainly power the DUT with the one variac, you do not need one for a functioning bench scope that is used as test gear and meant to run on your line voltage. What is the "perfect" ( safest) bench test configurationA bench configuration can not make it perfectly safe, although it can make it unsafe. An universal bench configuration (which i use) is the scope grounded and powered by regular mains ("just plugged in"). The DUT is powered through an isolation transformer (ideally with isolation monitor so you are made aware of a fault), possibly combined with a variac to bring things up slowly and check for supply range etc. On my own bench all connections are jumpered with 4mm jumpers, so i can remove ground from the DUT, but ONLY if i know what i am doing. I didn't need that much, if ever. It should be said that a GFI protected outlet can actually be safer than using an isolation transformer, in some cases. If you are not using the isolation transformer you should _always_ use a GFI to power the DUT (i highly recommend using one for everything, test gear and all). Note that GFIs do not function with isolation transformers, it's one or the other. I will rebuild my bench soon and then it will have V/A/W/phase angle meters for the DUT power. I have collected nice industrial panel meters and i will build my very own small "nuclear power station control panel", always wanted that ;-). I've also gotten hold of a nice isolation monitor which will be part of the new bench. ST |
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Hi Stefan, Hi Ron,
you are perfectly right - maximum safety depends on the situation. In general laboratory test setups sometimes cannot be made as safe as one is used to regarding the normal installation. Therefore I everytime rethink and recheck my setup before switching all on. For a detailed discussion maybe you can provide us with your local installation details and wether you have a Protective Earth connector and maybe a one phase outlet referenced to neutral wich in turn is connected to earth ? Having an isolation transformer available sometimes is very useful but it complicates things. If you follow the general rule and ground your 465 (preferably to the ground of the electrical installation) you know that the probes ground connector is also earth which is fine as touching it is safe. If you then use an isolation transformer for the EUT (which I do not recommend) the EUT has no reference at all. In this case when you attach the probes ground somewhere at the EUT you reference this point to ground whatever this point is in the circuit. This could be harmful or not depending on the situation. However in many cases you neglected the bonus of the insulation transformer then. For example (dangerous example! only for virtual thinking about effects!) if you attached the probe ground of your 465 to the HV testpoint of a 547 scope you can recalculate the voltages to ground or chassis. In this case the chassis of the 547 will be at 1850 Volts what is definitely not what you want. You might not survive this experiment and in addition you have these 1850 volts also across some (many) important isolation tracks including the isolation transformers itself. In virtually all cases I know any isolation against ground is designed to withstand mains voltage. OK, it should have been tested against high voltage around 1.5kV for one minute (exact figures depending on appliance) but this applies to special circumstances and to new quality equipment. In most cases (maybe not all cases) it should be the best choice to ground the EUT and the measurement scope. Any other setup needs very special consideration. Looking forward to further discusion, regards, Stefan Junghans |
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bobkrassa
Once you have adopted the standard recommendations - dry, nonconductive
floor and workbench, GFCI outlets, true isolation transformers, you are still not protected from completing a circuit within the DUT! Adopt the "one hand rule" as an inflexible habit - when the power is on to the DUT, keep one hand in your pocket at all times. This rule will also improve your work habits as far as making reliable test connections, but primarily, it prevents your heart from becoming part of the circuit between the HV point on the DUT and the ground on the DUT. Bob Krassa KC0TDS |
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Stefan Trethan
On Mon, 29 Jan 2007 20:58:23 +0100, Stefan Junghans <magnetron@...> wrote:
In most cases (maybe not all cases) it should be the best choice to Yes, i agree. However you take it as a given that using an isolation transformer requires removing earth to the DUT. While some isolation transfomer units are made that way - with a 2 prong outlet, that is not the most sensible thing to do. You do not need to remove the DUT earth at all, and the isolation is still extremely useful. Let me explain: Imagine a circuit in a box. Does not matter what, but the circuit inside is not connected to the metal case. The metal case is firmly earthed, but the supply is isolated. Now your finger is stuck into the case, touching some component by accident. Your hand also touches the case (or not, doesn't matter, but it's likely). There will be no current, the circuit point you touched will simply be pulled to earth. You were protected against first fault without even a jolt or power shutoff (like a GFI would give you). It is most important here to realize that any fault between the circuit and case would not stop operation, and would cause a hazardous DUT without your knoweledge. There would be no protection, no switchoff now, in case of an accident with your finger. This is why i think it so important to install an isolation monitor with any isolation transformer. This device monitors the resistance between earth and the isolated supply, and warns you as soon as the resistance is low. It is used with most isolated supplies in industry and medical applications. An isolation transfomer is _very_ safe for first fault, but if there is a first fault already without you knowing it is _very_ unsafe all of a sudden, less safe than a GFI, without isolation monitor (which is by US standards still "normal", but considered unsafe here in Europe). You can also make scope measurements with the DUT earth intact. The probe ground will now tie your reference in the circuit to ground as well (you may have to disconnect y filter caps if the small current is a problem). Note that as soon as you stick your probe ground somewhere, THIS IS THE FIRST FAULT. Your DUT is now less safe than a GFI protected DUT. Some measurements are only possible that way, without diff probes or isolation amps. It is still a reasonably safe way to operate, the case is still grounded, and you assume the circuit to be hot anyway. It's nothing like floating scopes and cases all over the place, but when using that method you should still take care. It's the same danger level as any device in the US not connected to a GFI, with the case open and someone working on it. Not great, but not too crazy either. Anyway, i hope i could make clear why i feel it is a bad idea to remove earth from a DUT even under isolated power. Keeping the earth connection allows proper first fault indication with an isolation monitor, while still offering all the protection. Also, for me personally, having the chassis tied solidly to earth is a comfort factor. ST |
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First, thank you Stephan T , Stephan J and Bob K for your excellent and
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knowledgeable inputs. They should be required reading for all on the board. The more I read, the more I discover that there IS NO ONE WAY. The ONE safety practice I have adopted (note-..I'm still here posting) is that when measuring any HV I: 1. With all power off to everything, discharge the test point I plan to probe to ground. In other words , I treat it like a big capacitor. As we know , even with all protection in place, if you "get across" a big cap in a DUT you can be on the way to the undertaker 2. I then attach an insulated alligator clip to the test point desired and clip the other end to my Tek 6015 probe tip. 3. Then without touching anything, I stand back and apply power and observe the reading then power off, discharge test point etc. I have the good fortune to have an excellent 6015 which is fully charged with Freon 114. The probe has a very large tip which is not optimal for trouble shooting oscilloscopes but perfect , I imagine, for the "tesla coil crowd". This is why I "wire it in" to the circuit first instead of "probing" with a veritable "baseball bat" Thanks again to respondents...one can not know too much about safety. Ron Simmons -----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Stefan Trethan Sent: Monday, January 29, 2007 1:28 PM To: TekScopes@... Subject: Re: [TekScopes] In search of the "Safest" bench test setup On Mon, 29 Jan 2007 17:01:16 +0100, HBcubed <rjsimmons@...> wrote: My main test gear is a 465B scope with it's case/frame NOW hardwiredIt would be better to ground it through the electrical ground (ground prong or screw on an outlet). It is not always a good idea to use the plumbing for ground, since great hazards are created if the plumbing is not properly earthed, or some pipes are plastic (or are later replaced for plastic). It depends on your local regulation if it is still legal to use the plumbing. My questions are:Why would you? A variac is used for VARIable AC. Your scope should be made to run off mains voltage. Many scopes have internal adjustment for "low" "normal" or "high" line voltage, i don't know the 465B. If so, is the variac considered an "isolation xformer?Most variacs are not, they are so called autotransformers with no isolation between input and output. There are also isolated varicas, which have a second fully isolated winding under the outer output winding with the wiper. But often an isolation transformer will be put in front of a variac to provide isolation and adjustable voltage. Should the device under test also be grounded to the water pipe?Depends. Usually your DUT should be grounded, via the electrical system. Sometimes if the DUT is powered via an isolation transformer the ground is intentionally removed. You should not need that if you don't exactly know how or why. Can I power the DUT with the same variac?You can certainly power the DUT with the one variac, you do not need one for a functioning bench scope that is used as test gear and meant to run on your line voltage. What is the "perfect" ( safest) bench test configurationA bench configuration can not make it perfectly safe, although it can make it unsafe. An universal bench configuration (which i use) is the scope grounded and powered by regular mains ("just plugged in"). The DUT is powered through an isolation transformer (ideally with isolation monitor so you are made aware of a fault), possibly combined with a variac to bring things up slowly and check for supply range etc. On my own bench all connections are jumpered with 4mm jumpers, so i can remove ground from the DUT, but ONLY if i know what i am doing. I didn't need that much, if ever. It should be said that a GFI protected outlet can actually be safer than using an isolation transformer, in some cases. If you are not using the isolation transformer you should _always_ use a GFI to power the DUT (i highly recommend using one for everything, test gear and all). Note that GFIs do not function with isolation transformers, it's one or the other. I will rebuild my bench soon and then it will have V/A/W/phase angle meters for the DUT power. I have collected nice industrial panel meters and i will build my very own small "nuclear power station control panel", always wanted that ;-). I've also gotten hold of a nice isolation monitor which will be part of the new bench. ST Yahoo! Groups Links |
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Stefan Trethan
On Mon, 29 Jan 2007 22:30:53 +0100, Ron Simmons <RJSimmons@...> wrote:
First, thank you Stephan T , Stephan J and Bob K for your excellent andI haven't needed my HV probes much, but you are right they are huge beasts. I discovered mine (non tek) have a screw-on tip, so i could probably make up a short lead with a clip, to be connected in the discharged state. I guess the tek probe might have a screw-on tip as well. ST |
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At 21:48 29/01/2007, Stefan Trethan wrote:
I haven't needed my HV probes much, but you are right they are huge beasts.Virtually the only use I ever make of my, also non-Tek, HV probe is to check the CRT accelerating voltage on Tek scopes. As my probe also has a screw-in tip I have made up a Tee piece, using one lead with male connector from a failed HV tripler, one lead with female connector from a failed CRT and a further piece of HV lead connected to a compatible screw to replace the probe's tip, all connections heavily insulated. Now all I have to do is to replace the tip with the Tee, disconnect the anode lead, plug in my Tee and switch back on without having to get anywhere near anything nasty. Ben -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.432 / Virus Database: 268.17.12/655 - Release Date: 28/01/2007 13:12 |
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aobp11
Hello Stefan, your example is illustrative but not very realistic
for my DUTs: usually TEK 'scopes or plug-ins that need repair. Nearly no internal circuits are floating w.r.t. chassis. Another point is that you and other members praise our European GFI system. I hope everyone recognizes the limited value of GFI as mentioned e.g. by Bob Krassa. GFI doesn't protect against internal current loops through your body. It only protects in case of accidentally touching at the same time one of the primary wires to the mains power transfomer (not using an isolation transformer) and other parts of the circuitry or chassis. Why do those power transformer have so many secondary windings? Otherwise you could install a GFI in the secondary circuit! Albert --- In TekScopes@..., "Stefan Trethan" <stefan_trethan@...> wrote: ----inside is not connected to the metal case. The metal case is firmlyearthed, but the supply is isolated. Now your finger is stuck into the case,touching some component by accident. Your hand also touches the case (ornot, doesn't matter, but it's likely). There will be no current, thecircuit point you touched will simply be pulled to earth. You wereprotected against first fault without even a jolt or power shutoff (like aGFI would give you). |
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Stefan Trethan
On Tue, 30 Jan 2007 18:06:07 +0100, aobp11 <ao_te_z@...> wrote:
Hello Stefan, your example is illustrative but not very realistic Yes, nothing protects against high secondary voltages, but i expect direct mains power is the main cause for accidents. The isolation transformer is still useful for working on the SMPS primary, in the case of scopes. I don't see many practical applications to float the chassis of the DUT, i have never needed it. It would be safe with something like the Tek A6901 Ground Isolation Monitor and when i can find one at a good price i'll probably get it, but i already have isolation amplifiers (for the scope probes) and diff amps, which seem more convenient and safer than floating a DUT chassis around. Also, because of capacitive coupling and stuff, the point to which you tie your scope ground would need to be fairly solid or you'll influence it or introduce noise. Probably only practical with supply rails, which are often DC and easily subtracted from the display. So if you find my example unrealistic, maybe you can supply more realistic ones where floating the scope chassis is actually practical. ST |
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Dennis Tillman
VARIACS ARE NOT ISOLATION TRANSFORMERS!!!
VARIACs are wonderfup things and every electronics professional should own one. But they are not transformers in the sense that you are most familiar with. AC power transformers have two separate windings. This seperation means no currents flow from one winding to the other. This is where the isolation comes from. The primary creates a magnetic field that induces a voltage in the secondary. But there is only a magnetic field coupling the energy across the windings. There is no electrical path. Variacs only have one winding which is connected directly across the 120V line. The output comes from the sliding tap that goes from one end of this winding to the other (and beyond). As the tap is slid along the primary winding the voltage at the output is being tapped directly off the 120V line. To prove this to yourself connect an ohmmeter from the tap to either lead of the primary and measure the resistance (with the Variac unplugged!!!). It is quite low (less than a few hundred ohms). Now do the same thing with a conventional AC power transformer. The resistance measured from the primary to the secondary should be infinite. Usually the Variac's tapped off voltage can range from 0 to 145VAC if 120VAC is being applied to it because the creators of this clever device (General Radio) included additional windings beyond where the 120VAC high side connects to the Variac. An inexpensive isolation transformer can usually be found at a hamfest or equivalent. It is not something that you should have hooked up permanently since it can cause more problems that it solves. But there is occasionally a situation where you need it. An isolation transformer will not protect you from high voltage shocks. That is not what they are designed to do. The isolation transformer will only be as good as the insulation between the primary and secondary. Since both windings are wound together (overlapping), as a rule they will not withstand a large voltage difference and must not be counted on for HV isolation. If I recall correctly,from my youth, HV transformers like you find in a TV have primary and secondary windings that are seperated by physical distance on separate parts of a non-conducting ferrite core. They also work at 15KHz (which is OK for ferrites) rather than at 60Hz (which usually requires a classic iron core). A good oscilloscope is properly designed to work with the third prong grounded through the wall socket. An inexpensive $7 devvice can be bought at any hardware store to check all your outlets to insure they are properly grounded. This tester will also detect other faults as well. Tektronix would tell you otherwise if there were a better way to connect a scope than through the third prong of the power cord. Attaching your scope chassis directly to a water pipe is almost certainly causing more problems that it solves. If the third prong of the power cord is also connected in the wall socket then the water pipe ground will create a ground loop that will be the cause of some very subtle problems (and incorrect readings) you may never notice. Variacs are not cheap. They are one of the very, very few things that is worth more today than when they first came out in the 1920s or 1930s. A typical 600 watt one in good shape will cost $75-$100. Dennis |
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David C. Hallam
There are variacs that are also isolation transformers. They may be know by
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another name than variac but they are available at a higher price than the standard garden variety variac. David KC2JD/4 -----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]On Behalf Of Dennis Tillman Sent: Tuesday, January 30, 2007 2:39 PM To: 'tekscopes' Subject: RE: [TekScopes] In search of the "Safest" bench test setup VARIACS ARE NOT ISOLATION TRANSFORMERS!!! VARIACs are wonderfup things and every electronics professional should own one. But they are not transformers in the sense that you are most familiar with. AC power transformers have two separate windings. This seperation means no currents flow from one winding to the other. This is where the isolation comes from. The primary creates a magnetic field that induces a voltage in the secondary. But there is only a magnetic field coupling the energy across the windings. There is no electrical path. Variacs only have one winding which is connected directly across the 120V line. The output comes from the sliding tap that goes from one end of this winding to the other (and beyond). As the tap is slid along the primary winding the voltage at the output is being tapped directly off the 120V line. To prove this to yourself connect an ohmmeter from the tap to either lead of the primary and measure the resistance (with the Variac unplugged!!!). It is quite low (less than a few hundred ohms). Now do the same thing with a conventional AC power transformer. The resistance measured from the primary to the secondary should be infinite. Usually the Variac's tapped off voltage can range from 0 to 145VAC if 120VAC is being applied to it because the creators of this clever device (General Radio) included additional windings beyond where the 120VAC high side connects to the Variac. An inexpensive isolation transformer can usually be found at a hamfest or equivalent. It is not something that you should have hooked up permanently since it can cause more problems that it solves. But there is occasionally a situation where you need it. An isolation transformer will not protect you from high voltage shocks. That is not what they are designed to do. The isolation transformer will only be as good as the insulation between the primary and secondary. Since both windings are wound together (overlapping), as a rule they will not withstand a large voltage difference and must not be counted on for HV isolation. If I recall correctly,from my youth, HV transformers like you find in a TV have primary and secondary windings that are seperated by physical distance on separate parts of a non-conducting ferrite core. They also work at 15KHz (which is OK for ferrites) rather than at 60Hz (which usually requires a classic iron core). A good oscilloscope is properly designed to work with the third prong grounded through the wall socket. An inexpensive $7 devvice can be bought at any hardware store to check all your outlets to insure they are properly grounded. This tester will also detect other faults as well. Tektronix would tell you otherwise if there were a better way to connect a scope than through the third prong of the power cord. Attaching your scope chassis directly to a water pipe is almost certainly causing more problems that it solves. If the third prong of the power cord is also connected in the wall socket then the water pipe ground will create a ground loop that will be the cause of some very subtle problems (and incorrect readings) you may never notice. Variacs are not cheap. They are one of the very, very few things that is worth more today than when they first came out in the 1920s or 1930s. A typical 600 watt one in good shape will cost $75-$100. Dennis |
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DON CRAMER
I've used for years a Tenma "Isolation Transformer", like this one in an ebay listing.
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<> It has a 1:1 isolation transformer in front of an autotransformer, the whole thing rated for 3A. An analog meter shows line voltage in, or voltage or current out. It has a 3 wire line cord; and for the DUT the ground of the 3 hole outlet on the front is "grounded" (not floating). I've only had a couple issues with it. One is you can't see voltage and current simultaneously, though I've made a mod to allow this using an outboard DMM. The other is higher loads can produce a bit of sag in the output voltage (just be aware of it). I've used previously a similar unit from BK Precision, their model 1653A. Though it's rated to only 2A. An example here: <> Their model 1655A is larger (and 3A rated) and can measure leakage, though I've never used one. BK also sells a couple straight isolation transformers for those interested (<>), though once again I have no experience using them. Don ----- Original Message -----
From: David C. Hallam<mailto:dhallam@...> To: 'tekscopes'<mailto:tekscopes@...> Sent: Tuesday, January 30, 2007 11:51 AM Subject: RE: [TekScopes] In search of the "Safest" bench test setup There are variacs that are also isolation transformers. They may be know by another name than variac but they are available at a higher price than the standard garden variety variac. -----Original Message----- From: TekScopes@...<mailto:TekScopes@...> [mailto:TekScopes@...]On Behalf Of Dennis Tillman Sent: Tuesday, January 30, 2007 2:39 PM To: 'tekscopes' Subject: RE: [TekScopes] In search of the "Safest" bench test setup ...VARIACS ARE NOT ISOLATION TRANSFORMERS!!!... |
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Greg_A
I connect 1:1 isolation 115V transformer with variac to the out of it and I
getting isolation and variance in voltage from 0V up to 150 VAC, again completely isolated from line input.... Should be safest way to work with AC voltage line. Greg At 02:51 PM 1/30/07 -0500, David C. Hallam wrote:
Emacs! |
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aobp11
Hello Stefan,
You are right in mentioning the SMPS. For the moment this is the only dangerous circuitry I can think of that is isolated from the chassis. Sorry for the confusion, with "not very realistics" I only meant to say that I am nearly always checking circuits that are referenced to the chassis (had SMPS problems only at the secondary side). Of course also then there is a risk to touch parts at the mains side. I don't promote a floating chassis. Albert --- In TekScopes@..., "Stefan Trethan" <stefan_trethan@...> wrote: primary, in the case of scopes.realistic ones where floating the scope chassis is actually practical. |
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Kuba Ober
1. With all power off to everything, discharge the test point I plan toYou're right, but it doesn't have to be big, or even a designated capacitor. Mind the parasitics! The CRT with its parastic anode capacitance is enough to zap you big time. Cheers, Kuba |
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Michael Bender
Kuba Ober wrote:
Also while the "shock" might not kill you, falling backwards1. With all power off to everything, discharge the test point I plan toYou're right, but it doesn't have to be big, or even a designated capacitor. Mind the parasitics! The CRT with its parastic anode capacitance is enough to zap you big time. or bumping into something from your reflexive action might cause you to break your neck or get impaled on something, or have something big fall on top of you. mike |
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But the charge on even a pretty large CRT, at least in the olden
times, before flatscreens, had pretty impressive HV like 25 kV, and you can get a nasty surprise, but not worse. After all, it's DC,and the capacitance isn't all that large. Now, I know this is going to provoke a lecture and thread ad infinitum (you know who you are), but really, we did take at least *some* care, like making sure a five tube ac/dc set really had the chassis at neutral and that neutral and hot weren't interchanged. Of course you're going to get shocked if you reach stupidly into a power supply that has a 300-0-300 VAC transformer secondary, but if you're lucky, it'll be the dc which *will* fling you across the room, not lock your muscles and kill you. People learn fast from such experiences, at least those who didn't grab the transformer secondary with both hands. I know that I was immensely pleased to have the transistor come along just at the right time to spare me shocks but mostly wiring the accursed filaments. Regards, Jonathan --- In TekScopes@..., Kuba Ober <ober.14@...> wrote: plan to1. With all power off to everything, discharge the test point I capacitor.probe to ground. In other words , I treat it like a big capacitor. AsYou're right, but it doesn't have to be big, or even a designated Mind the parasitics! The CRT with its parastic anode capacitance isenough to zap you big time. |
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