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Well I built me an isolation transformer yesterday. Been needing one for quite some time, but commercial units are pretty expensive. I have some used/parts equipment that I got a little while back, so I pulled two transformers (BOTH HP 9100-5000) and the mounting plates from a couple older HP 5370A counters. Cut the mounting brackets in half (wanted half of each to build an attenuator box with). Used an old PC P/S box that I was gonna throw out and mounted the transformers along with one of the plates that had the fuse holder and inlet plug on it. I used some weatherstrip tape to sandwich both of the Xfmrs in between the covers to keep them from moving/vibrating. Then I wired the Xfmrs back to back (secondary to secondary)and wired the primary on the second Xfmr to the 3-prong plug that was the P/S inlet. I wired a 120 VAC neon indicator light and mounted on the inlet fuze panel so I'd know when it is plugged in and getting power. No on/off switch. Then I made a power cord with 3 outlets and a standard 3-prong female connector on the other end. It can run from 120 or 240 (approx.), but will only put out 110-120. Works GREAT! The input voltage from the wall plug is about 119 VAC, and under load (Tek 2337 and an HP 6202B P/S) the ISO puts out 115-116 VAC. Hasn't even got warm! And didn't cost anything but some time, screws, some rivets and solder!
Oh.... and a bit of homework! (The most important part!)
I posted pics in my album "Home Brew Isolation Transformer".
I'm no EE, just learning as I go, so I thought I'd post it up so if some other newbies like myself needed an ISO Xfmr, this could give them a good idea of how to build one.
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Nifty. Before you learn something the hard way, it is advised to never float (isolate) a scope. Instead leave the scope grounded and float the DUT.
Dave Casey
Quoting RonC <thepostman6801@...>:
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Well I built me an isolation transformer yesterday. Been needing one for quite some time, but commercial units are pretty expensive. I have some used/parts equipment that I got a little while back, so I pulled two transformers (BOTH HP 9100-5000) and the mounting plates from a couple older HP 5370A counters. Cut the mounting brackets in half (wanted half of each to build an attenuator box with). Used an old PC P/S box that I was gonna throw out and mounted the transformers along with one of the plates that had the fuse holder and inlet plug on it. I used some weatherstrip tape to sandwich both of the Xfmrs in between the covers to keep them from moving/vibrating. Then I wired the Xfmrs back to back (secondary to secondary)and wired the primary on the second Xfmr to the 3-prong plug that was the P/S inlet. I wired a 120 VAC neon indicator light and mounted on the inlet fuze panel so I'd know when it is plugged in and getting power. No on/off switch. Then I made a power cord with 3 outlets and a standard 3-prong female connector on the other end. It can run from 120 or 240 (approx.), but will only put out 110-120. Works GREAT! The input voltage from the wall plug is about 119 VAC, and under load (Tek 2337 and an HP 6202B P/S) the ISO puts out 115-116 VAC. Hasn't even got warm! And didn't cost anything but some time, screws, some rivets and solder!
Oh.... and a bit of homework! (The most important part!)
I posted pics in my album "Home Brew Isolation Transformer".
I'm no EE, just learning as I go, so I thought I'd post it up so if some other newbies like myself needed an ISO Xfmr, this could give them a good idea of how to build one.
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I have basically the same thing sitting on my desk with two back to back identical transformers. Since the ones I used have dual primaries, I wired the line side primaries in parallel for 120VAC and left the output side primaries separate so that gave me two isolated 120VAC outputs that are also isolated from each other. On Mon, 14 Jan 2013 19:59:21 -0000, "RonC" <thepostman6801@...> wrote: Well I built me an isolation transformer yesterday. Been needing one for quite some time, but commercial units are pretty expensive. I have some used/parts equipment that I got a little while back, so I pulled two transformers (BOTH HP 9100-5000) and the mounting plates from a couple older HP 5370A counters. Cut the mounting brackets in half (wanted half of each to build an attenuator box with). Used an old PC P/S box that I was gonna throw out and mounted the transformers along with one of the plates that had the fuse holder and inlet plug on it. I used some weatherstrip tape to sandwich both of the Xfmrs in between the covers to keep them from moving/vibrating. Then I wired the Xfmrs back to back (secondary to secondary)and wired the primary on the second Xfmr to the 3-prong plug that was the P/S inlet. I wired a 120 VAC neon indicator light and mounted on the inlet fuze panel so I'd know when it is plugged in and getting power. No on/off switch. Then I made a power cord with 3
outlets and a standard 3-prong female connector on the other end. It can run from 120 or 240 (approx.), but will only put out 110-120. Works GREAT! The input voltage from the wall plug is about 119 VAC, and under load (Tek 2337 and an HP 6202B P/S) the ISO puts out 115-116 VAC. Hasn't even got warm! And didn't cost anything but some time, screws, some rivets and solder!
Oh.... and a bit of homework! (The most important part!)
I posted pics in my album "Home Brew Isolation Transformer".
I'm no EE, just learning as I go, so I thought I'd post it up so if some other newbies like myself needed an ISO Xfmr, this could give them a good idea of how to build one.
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Yep for sure! I'm not isolating my test scope, just the DUT which is a Tek 2337 that I'm checking waveforms on. I've been jolted before from a device that wasn't grounded as I touched a small metal trim rail on my counter top! OUCH!
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--- In TekScopes@..., dcasey@... wrote:
Nifty. Before you learn something the hard way, it is advised to never
float (isolate) a scope. Instead leave the scope grounded and float
the DUT.
Dave Casey
Quoting RonC :
Well I built me an isolation transformer yesterday. Been needing one
for quite some time, but commercial units are pretty expensive. I
have some used/parts equipment that I got a little while back, so I
pulled two transformers (BOTH HP 9100-5000) and the mounting plates
from a couple older HP 5370A counters. Cut the mounting brackets in
half (wanted half of each to build an attenuator box with). Used an
old PC P/S box that I was gonna throw out and mounted the
transformers along with one of the plates that had the fuse holder
and inlet plug on it. I used some weatherstrip tape to sandwich both
of the Xfmrs in between the covers to keep them from
moving/vibrating. Then I wired the Xfmrs back to back (secondary to
secondary)and wired the primary on the second Xfmr to the 3-prong
plug that was the P/S inlet. I wired a 120 VAC neon indicator light
and mounted on the inlet fuze panel so I'd know when it is plugged
in and getting power. No on/off switch. Then I made a power cord
with 3 outlets and a standard 3-prong female connector on the other
end. It can run from 120 or 240 (approx.), but will only put out
110-120. Works GREAT! The input voltage from the wall plug is about
119 VAC, and under load (Tek 2337 and an HP 6202B P/S) the ISO puts
out 115-116 VAC. Hasn't even got warm! And didn't cost anything but
some time, screws, some rivets and solder!
Oh.... and a bit of homework! (The most important part!)
I posted pics in my album "Home Brew Isolation Transformer".
I'm no EE, just learning as I go, so I thought I'd post it up so if
some other newbies like myself needed an ISO Xfmr, this could give
them a good idea of how to build one.
|
On 01/14/2013 02:24 PM, RonC wrote: I've been jolted before from a device that wasn't grounded as I touched a small metal trim rail on my counter top! OUCH! The arm to arm shock that implies can do more than ouch.
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Yes it can! I once had the "enjoyment" of shorting out a pair of pliers once on a hot 460 VAC leg of a power box and I got REALLY lucky! I was leaning over a ditch, touching another box (BIG no no!)Luckily the end of the pliers disintegrated instantly, I got a super shock for a split second. My arms hurt for two days. I was glad I had my GI issue boots on, I think they helped a little. Have learned a lot since then! :~)
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--- In TekScopes@..., John Griessen wrote:
On 01/14/2013 02:24 PM, RonC wrote:
I've been jolted before from a device that wasn't grounded as I touched a small metal trim rail on my counter top! OUCH! The arm to arm shock that implies can do more than ouch.
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--- In TekScopes@..., David wrote:
I have basically the same thing sitting on my desk with two back to
back identical transformers. Since the ones I used have dual
primaries, I wired the line side primaries in parallel for 120VAC and
left the output side primaries separate so that gave me two isolated
120VAC outputs that are also isolated from each other.
Note that dual primarys are bi-filiar wound meaning that two wires are would at the same time, next to each other. Thus the isolation between the two primary windings will not be that great. The capacitive coupling will be very high, and the voltage isolation is limited to twice the working voltage of the magnet wire insulation - probably on the order of 600 V peak. Actuall isolation transformers use windings insulated with a layer of paper. The low capacitance ones actually use two bobins on either side of the iron core. - Steve
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Noted. Thanks, Steve. Fine for breaking ground loops, but don't go floating the DUT at 1kV!
Dave
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-----Original Message----- From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Steve Sent: Monday, January 14, 2013 1:29 PM To: TekScopes@... Subject: [TekScopes] Re: Bench Built Isolation Transformer --- In TekScopes@..., David wrote:
I have basically the same thing sitting on my desk with two back to
back identical transformers. Since the ones I used have dual
primaries, I wired the line side primaries in parallel for 120VAC and
left the output side primaries separate so that gave me two isolated
120VAC outputs that are also isolated from each other.
Note that dual primarys are bi-filiar wound meaning that two wires are would at the same time, next to each other. Thus the isolation between the two primary windings will not be that great. The capacitive coupling will be very high, and the voltage isolation is limited to twice the working voltage of the magnet wire insulation - probably on the order of 600 V peak. Actuall isolation transformers use windings insulated with a layer of paper. The low capacitance ones actually use two bobins on either side of the iron core. - Steve
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On Mon, 14 Jan 2013 21:29:01 -0000, "Steve" <ditter2@...> wrote:
--- In TekScopes@..., David wrote:
I have basically the same thing sitting on my desk with two back to
back identical transformers. Since the ones I used have dual
primaries, I wired the line side primaries in parallel for 120VAC and
left the output side primaries separate so that gave me two isolated
120VAC outputs that are also isolated from each other.
Note that dual primarys are bi-filiar wound meaning that two wires are would at the same time, next to each other. Thus the isolation between the two primary windings will not be that great. The capacitive coupling will be very high, and the voltage isolation is limited to twice the working voltage of the magnet wire insulation - probably on the order of 600 V peak.
Actuall isolation transformers use windings insulated with a layer of paper. The low capacitance ones actually use two bobins on either side of the iron core.
- Steve
I considered that when I built it. The primaries look like they are wound on alternating layers with paper insulation between them and *not* bifilar. Of course they lack the electrostatic shield between them. I considered using a couple of big plate transformers that I picked up many years ago which are rated to operate with one side of the high voltage output grounded (not always the case) instead of the center tap but they weigh too much unless I build a little cart.
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A couple of years ago, I was working on an old Motorola service monitor. I connected a coax patch cord with grabber clips to the input jack. I clipped the center conductor to a piece of wire running across the shack ceiling, and for support, clipped the shield clip to my fluorescent shop light. Well, at least that was the plan.
When the hook on the clip met the frame of the shop light, there was a loud arc and I pulled back a grabber minus the little phosphor-bronze hook! Since I knew the light was grounded, and the monitor was plugged into a grounded outlet, I was puzzled.
The frame on the service monitor was hot. How could that happen? I replaced the RFI filter box on the 117V line input and the problem went away. Curious, I sawed open the box and saw what had happened. Apparently, the device had taken a lightning hit sometime in its checkered past. The inside of the box was a charred mess. The ground conductor was vaporized, and the hot wire of the line input was welded to the case of the box.
I now have a standard GFCI receptacle in a box with a cover plate laying on the bench, connected to my main power bus with a short cord. Everything under test on my bench now gets plugged into that receptacle.
It could be a lifesaver, folks!
Mike Harmon, WB0LDJ
mharmon at att dot net
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Thank you, Mike!
Will be adding GFCI to my Variacs & Iso-outlets.
Bernd
?
In a message dated 1/14/2013 4:40:41 P.M. Pacific Standard Time,
mharmon@... writes:
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I now
have a standard GFCI receptacle in a box with a cover plate laying on the
bench, connected to my main power bus with a short cord. Everything under test
on my bench now gets plugged into that receptacle.
It could be a
lifesaver, folks!
Mike Harmon, WB0LDJ
mharmon at att dot
net
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You can put GFCIs on normal-voltage outlets, but they won't work on variable ones that go below a certain point. The GFCI circuit needs a minimum line voltage to work properly since it has to be able to fire a solenoid plunger that trips the disconnect.
You can probably modify them to work, or find special industrial type ones that can operate over any range, but regular ones as-are will likely not work below a certain line voltage - perhaps one-half the normal amount. It shouldn't hurt the GFCI to run at any lower voltage, but be aware that it may not trip.
Ed
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--- In TekScopes@..., tubesnthings@... wrote:
Thank you, Mike!
Will be adding GFCI to my Variacs & Iso-outlets.
Bernd
In a message dated 1/14/2013 4:40:41 P.M. Pacific Standard Time,
mharmon@... writes:
I now have a standard GFCI receptacle in a box with a cover plate laying
on the bench, connected to my main power bus with a short cord. Everything
under test on my bench now gets plugged into that receptacle.
It could be a lifesaver, folks!
Mike Harmon, WB0LDJ
mharmon at att dot net
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And obviously it won't work after an isolation transformer, the GFI
needs a grounded grid.
I consider it routine to check anything I fix for ground faults with
an isolation tester (megger) to avoid nasty surprises when put back
into service on regular mains supply.
ST
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On Tue, Jan 15, 2013 at 5:19 AM, Ed Breya <edbreya@...> wrote: You can put GFCIs on normal-voltage outlets, but they won't work on variable ones that go below a certain point. The GFCI circuit needs a minimum line voltage to work properly since it has to be able to fire a solenoid plunger that trips the disconnect.
You can probably modify them to work, or find special industrial type ones that can operate over any range, but regular ones as-are will likely not work below a certain line voltage - perhaps one-half the normal amount. It shouldn't hurt the GFCI to run at any lower voltage, but be aware that it may not trip.
Ed
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Stefan, The GFI works on differential measurement, regardless the presence of ground.
Here in USA these GFI's are legal to use in an ungrounded electrical system in residential buildings, as long as a label is attached to the face plate stating "ungrounded".?
Dave
-=-=-=-
On 14 January 2013, at 10:41 PM, Stefan Trethan wrote: And obviously it won't work after an isolation transformer, the GFI
needs a grounded grid.
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Your supply transformer output winding must be grounded, otherwise there is no differential current, draw it up.
It won't work after an isolation transformer.
I am not talking about the ground conductor to the DUT.
ST
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On Tue, Jan 15, 2013 at 7:52 AM, Dave C <davec2468@...> wrote:
Stefan, The GFI works on differential measurement, regardless the presence of ground.
Here in USA these GFI's are legal to use in an ungrounded electrical system in residential buildings, as long as a label is attached to the face plate stating "ungrounded".?
Dave
-=-=-=- On 14 January 2013, at 10:41 PM, Stefan Trethan wrote: And obviously it won't work after an isolation transformer, the GFI
needs a grounded grid.
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You can probably modify them to work, or find special industrial type ones that can operate over any range, but regular ones as-are will likely not work below a certain line voltage - perhaps one-half the normal amount. It shouldn't hurt the GFCI to run at any lower voltage, but be aware that it may not trip.
Ed
-=-=-=-
A few more notes on the use and frequent failure of GFCI devices:
<>
Dave
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You're both right.
The GFCI operation is by virtue of a differential measurement of current with essentially a balun transformer, to make sure that current leaving the hot side returns through the neutral side - wthin the trip level. If you complete the circuit from hot to neutral, you're toast - the GFCI doesn't know if you're a fault or a light bulb.
In normal conditions, one leg is grounded back at the source, so the faults that normally would cause shocks or electrocution involve the more common situation where this imbalance occurs - from the hot circuit to earth ground, so the GFCI will save you.
An isolated source just removes this ground path, but doesn't eliminate the risk of having dangerous voltages present.
Ed
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--- In TekScopes@..., Stefan Trethan wrote:
Your supply transformer output winding must be grounded, otherwise there is
no differential current, draw it up.
It won't work after an isolation transformer.
I am not talking about the ground conductor to the DUT.
ST
On Tue, Jan 15, 2013 at 7:52 AM, Dave C wrote:
Stefan,
The GFI works on differential measurement, regardless the presence of
ground.
Here in USA these GFI's are legal to use in an ungrounded electrical
system in residential buildings, as long as a label is attached to the face
plate stating "ungrounded".
Dave
-=-=-=-
On 14 January 2013, at 10:41 PM, Stefan Trethan wrote:
And obviously it won't work after an isolation transformer, the GFI
needs a grounded grid.
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In normal conditions, one leg is grounded back at the source, so the faults that normally would cause shocks or electrocution involve the more common situation where this imbalance occurs - from the hot circuit to earth ground, so the GFCI will save you.
An isolated source just removes this ground path, but doesn't eliminate the risk of having dangerous voltages present.
Ed
-=-=-=-
So using a GFCI on the "other side" of an isolation transformer, if I touch either hot or neutral conductor (not both) and the other hand is grounded (or there is some other path to ground), there will be no activation of the GFCI interrupter??
Dave
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Nope. Not unless you have enough leakage somewhere to reach the trip threshold of about 3-5 mA. There is no hot or neutral on a floating winding - that is a convention that depends on how it's connected to the external world. If it is truly isolated, you can ground either end, and the opposite one becomes the hot side.
Ed
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--- In TekScopes@..., Dave C wrote:
In normal conditions, one leg is grounded back at the source, so the faults that normally would cause shocks or electrocution involve the more common situation where this imbalance occurs - from the hot circuit to earth ground, so the GFCI will save you.
An isolated source just removes this ground path, but doesn't eliminate the risk of having dangerous voltages present.
Ed -=-=-=-
So using a GFCI on the "other side" of an isolation transformer, if I touch either hot or neutral conductor (not both) and the other hand is grounded (or there is some other path to ground), there will be no activation of the GFCI interrupter?
Dave
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Yes, nothing will happen.
The GFI can not work, you should use an isolation monitor instead.
Typically the DUT will often be grounded (like with the ground clip of a scope probe) and then the isolation transformer powered DUT is more dangerous than a regular GFI protected circuit, there is no protection left at all.
People think "I have an isolation transformer, it's safe to work on this" but often they do not realize they have constructed a hazard.
ST
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On Tue, Jan 15, 2013 at 8:40 AM, Dave C <davec2468@...> wrote:
So using a GFCI on the "other side" of an isolation transformer, if I touch either hot or neutral conductor (not both) and the other hand is grounded (or there is some other path to ground), there will be no activation of the GFCI interrupter??
Dave
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David Hess