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Old 21st December 2011, 10:25 PM   #201
Simon B is offline Simon B  United Kingdom
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Default Bench Isolating Transformers

Bench Isolating Transformers are 1:1 ratio transformers, with good insulation between primary and secondary, with a usually comparatively big power rating. The idea is this: Your incoming mains supply has one end of supply company's transformer secondary connected to earth as close as possible to the transformer, and usually via a spike stuck into the ground. This is then the NEUTRAL wire - because of the connection to earth, it is usually pretty close to earth potential. The other end of the supply company's transformer secondary, the one which isn't connected to earth is the LIVE - it's voltage is 110 volts wrt (with respect to) ground (US) or 240 volts (UK). If you touch it, current will flow through you, through the earth, and back to the transformer. You have completed the circuit, and possibly died. The Bench Isolating Transformer gives you a supply that is "floating" wrt what you're standing on - theoretically, you can touch either end of its secondary without being shocked. Assuming the transformer is correctly wired and not defective. However, you are probably better protected by using a Residual Current Device, that looks for imbalance between the current flowing in the Live and Neutral wires, and disconnects the power if it sees a difference. BUT, and it's a big but, this all applies only to the incoming AC mains supply. Neither RCDs or Bench Isolating Transformers will protect you against being shocked by the sort of high voltages typically found in valve/tube equipment - and they're usually higher voltage than the incoming mains - especially if you're on a 110v supply. There just isn't an easy answer to this one.....so be careful!
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Old 21st December 2011, 10:50 PM   #202
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I can never concentrate enough around high voltages so dont get involved with them.
If its more than 100 volts then I keep clear. It just not worth ending your life prematurely for.
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Old 22nd December 2011, 07:24 PM   #203
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I would like to put in a plug for isolating transformers.
I use a high-quality 5kVA isolating transformer for my bench supply.

The isolating transformer has a big advantage over the RCD, in that with the transformer the equipment that you're working and your test gear are all isolated from the mains earth or ground.

This means that if you accidentally touch the high-voltage DC there is no complete circuit through the ground that you're standing on. This is because the negative of your DC supply is not physically connected to the incoming mains earth.


The RCD, on the other hand, does not have this advantage. The gear that you're working on plus all of the test gear has a direct connection to the incoming mains earth.

The RCD does not sense earth current, it senses a difference between the incoming mains phase and neutral wires. It will not, therefore, sense the DC flowing down the earth wire from your DC power supply through your body etc.


rgds, M.
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Old 22nd December 2011, 07:36 PM   #204
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Quote:
Originally Posted by marcuswilson View Post
The isolating transformer has a big advantage over the RCD, in that with the transformer the equipment that you're working and your test gear are all isolated from the mains earth or ground.


rgds, M.
You also have the advantage that you can put your scope ground on any part of the circuit. For an irs2092 it has various voltages referenced to different places so a floating unit helps.
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Old 22nd December 2011, 08:57 PM   #205
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When I worked on a high-voltage amplifier with a 2660 V supply delivering 37 mA and having a much greater short-circuit current, see Elektrostatic Loudspeakers , I always wore rubber gloves of the type that are used to keep your hands dry when you wash dishes. I don't know if they would actually have helped against 2660 V, but I figured they wouldn't harm. In fact I tried to keep as much as possible of my skin covered with something that insulates.

More importantly, I was very scared of the thing and therefore very careful.
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Old 22nd December 2011, 09:59 PM   #206
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Exclamation Ouch!

Quote:
Originally Posted by MarcelvdG View Post
When I worked on a high-voltage amplifier with a 2660 V supply delivering 37 mA and having a much greater short-circuit current, see Elektrostatic Loudspeakers , I always wore rubber gloves of the type that are used to keep your hands dry when you wash dishes. I don't know if they would actually have helped against 2660 V, but I figured they wouldn't harm. In fact I tried to keep as much as possible of my skin covered with something that insulates.

More importantly, I was very scared of the thing and therefore very careful.
That sort of voltage would most likely poke a hole straight through those gloves.

I've worked on a lot of electrostatic power supplies. Many are very low current (Quad) but some can supply a lot of juice (Acoustat), so one needs to be very careful with them.

The 50KV EHT on old CRT video projectors was most hazardous!

By the way, I never wear gloves, I find them too restrictive and basically useless. Unless you get the real ones that the power company guys use and they are THICK!

rgds, M.
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Old 23rd December 2011, 01:21 AM   #207
Simon B is offline Simon B  United Kingdom
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"I was very scared of the thing and therefore very careful" Wise Words. When working on powered-up equipment I try never to forget that one wrong move might be my last. Sobering, but sane and safe(r) Without the chassis being earthed, a lot of valve equipment turns into a sensitive, broadband AM radio receiver, which is a big problem. The chassis is usually connected to the HT DC 0v line, and in any case fatal shocks need not involve your feet at all - one hand to the other, through the heart works fine. Please take this on board! Don't see an isolating transformer as a panacea - it isn't! I'm not saying that they're never useful, sometimes they are. The best piece of safety equipment you will ever possess is found between your ears (despite the attempts of certain legislators to outlaw its use!).......
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Old 27th December 2011, 02:15 PM   #208
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Here's a isolating transformer:
Click the image to open in full size.

There are two benefits from using them:
- Isolation from earth
- Current limiting

They are used sometimes on bathrooms in mains sockets, so if the device connected to it's defective or improperly grounded, you won't get shocked
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Old 2nd January 2012, 05:40 PM   #209
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One of the most dangerous things may be a -presumed- dead power supply. Watch them bleeder resistors! One time on a 10KW AM transmitter one of the bleeders on a HV cap opened up. Hot sticks are my only true religion. Saved my butt when I went poking around with one on the cap with the open bleeder on it. WABANG! Piece of 1/8" aluminum hook 1/2" wide vaporized about 9/16" off the end. Got an extra dose of hot stick religion.

Even on TV sets, had a cracked circuit board issolate rectified line cap from bleeder. Got 161V on cap discharged arm to arm. Not fun. Now I always poke a high watt low ohm resistor across any caps BEFORE I start working on things.
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Old 9th January 2012, 02:11 AM   #210
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Had an IC explode at 18volts. A CMOS "power driver" capable
of 12 amps for 50 nanoseconds or so. Long Vdd/GND wires (2 inches)
provided enough inductive ringing (above 30 volts for some nanoseconds)
to trigger a "bipolar snapback latchup".

The lab team was using an unusually hunky HewlettPackard bench supply,
and all the energy in the bench supply capacitors
was dumped
into the rather small
(2mm * 3mm) silicon IC.

The top of the plastic DIP-8 package flew to the ceiling, along with 2 of
the 8 leads.

From then on, the team placed a sheet of paper atop the setup.

So, regarding Audio Power Amplifiers: whether tube or solid state,
the power stored in those 100uf 450Volt caps
or in those 100,000uf 90volt caps
is dangerous.
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