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Old 26th September 2002, 08:34 PM   #1
sam9 is offline sam9  United States
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Default Using an Isolation x-former w/ a 'scope

I recently got my hands on a good used oscilloscope at a good price. I'm aware of the safety directive to "use an isolation transformer" when one is taking a measurement from a live unit such as a power amp.

I just realized I have *assumed* that ment to power the unit to be tested from the isolation transformer. The phase quoted above is an actual quote from two different sources. It never says which is to drawn AC from the x-formet the unit being tested or the oscilloscope! It just says "use".

Before I fry myself or some equipment, would some one confirm or correct my assumption.
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Old 26th September 2002, 09:16 PM   #2
haldor is offline haldor  United States
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Hi sam9,

The warnings about using an isolation transformer with a scope come from working with equipment that uses off-the line power supplies where the AC input power is directly rectified without going through a transformer first (common in TV's). Problem is the scope chassis is grounded through the power plug and when you hook your scope probes ground wire to the TV's chassis ground sparks can fly.

Audio amplifiers normally have transformers in their power supplies, so there is no need to transformer isolate the scope. It's possible some high power PA amps with switching power supplies might be using an off-the-line power supply, but I have never seen one (all the switcher based models I've seen do provide transformer galvanic isolation). If you see a large power transformer in the amp or the amp power cord has the 3rd pin ground connection, then you should be safe to connect your scope to the amp. If in doubt, take a voltmeter and measure between the ground of your scope and the signal ground inside the amp. There shouldn't be any significant voltage potential.

P.S. Do be very careful inside your amp. High power amps use voltages that can be high enough to electrocute you. Plus the power supply filter caps can hold dangerous voltage levels for quite some time after you unplug the power. Responsible makers install bleeder resistors to quickly discharge the caps, but doing this costs money (price of the power resistors plus a larger transformer to power both the bleeder resistors and the amp) so don't assume they are there until you have verified it for yourself.

Another thing to watch out for is internal heat sinks that are live. If the heat sinks are not attached to the chassis, then double check to make sure they aren't connected to one of the supply voltages. This is done in some amps to let them get away with using smaller heat sinks (no electrical insulation between the transistors and the heat sink means heat is more effectively transfered from the transistor to the heat sink). These will really get your attention the first time your finger strays into contact with them.

With tube amps I would also be cautious about the max input voltage capability of your scope. Some tube circuits can have potentials of 500V which may require a special high voltage scope probe for safety.

Phil
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Old 26th September 2002, 09:34 PM   #3
sam9 is offline sam9  United States
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Default Thanks

I have built one amp project and managed a couple minor repairs in the past. Safely, although cap I thought was discharged surprised me once. I use the "one hand in the pocket" rule and stand on a rubber pad.

I've never seen a piece of audio gear without a power transformer (at least from the inside) so when reading the admonitions, it never occurred to me there could be such a thing as one without. Infact, I couldn't visualize what was the mechanism that lead to a hazard since I always had a picture in my mind that included the power supply transformer. Still I didn't want to assume I knew better than the safety warnings!

Thanks for the clarification.
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Old 27th September 2002, 03:06 PM   #4
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Default Living Dangerously

I have worked on power amps with a power line direct connected SMPS stage, but the DC power supply of the amp stages of all such amps is isolated from the mains supply by the SMPS transformer.
You can pretty much safely assume that any audio line level/speaker level or connections will be mains isolated because these connections are touchable.
In televisions with a live chassis, you will find that any in/out connections are AC coupled.
On such TV's the aerial socket is capacitor coupled both legs, and AV inputs/outputs are transformer or opto isolator coupled.
For accurate measurements the Device Under Test needs to be grounded to the system and test equipment earth.
For measurements on live chassis equipment, the isolation transformer should supply the DUT, but beware of parasitic couplings causing noise errors.

Eric.

Always wear insulating shoes when working on live equipment, and do not touch any grounds whatsoever (other equipment, bench, chair etc) or allow other grounded persons to touch you.
Also use single handed procedures (ie - one hand in your pocket).
I often contact live chassis whilst observing these precautions and without harm.
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Old 27th September 2002, 05:03 PM   #5
dice45 is offline dice45  Germany
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Eric and haldor,

All,
i am not all too much a fan of the one-hand-in-a-pocket practise as working with one hand, slipping with the tool, jerking around, already got me electrocuted. Cause was i tried to do something with one hand that needs two.

Since then, i wear shoes with rubber soles, i wear a rubber glove on the left hand. Equivalent to one hand in the pocket but both hands available.
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Greets,
Bernhard
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Old 27th September 2002, 06:16 PM   #6
haldor is offline haldor  United States
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Quote:
Originally posted by dice45
i wear a rubber glove on the left hand. Equivalent to one hand in the pocket but both hands available.
Sounds like it might be a reasonable idea. If you can handle the reduction in sensitivity might as well wear a pair of gloves.

Phil
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Old 27th September 2002, 06:24 PM   #7
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Default Dangerous ground

Rubber shoes are not going to do a thing for you. Please no more advice on electrical safety from well meaning but unqualified posters. Too much is at stake. Someone find a good link about electrical safety and post it. I have been bit several times and I have a BSEE and a course in Biomed. This is not a topic to joke about.

H.H.
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Old 27th September 2002, 06:40 PM   #8
haldor is offline haldor  United States
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I agree that rubber shoes or mats are not likely to make any difference. I hadn't considered rubber gloves before. The problem is that to be safe they would need to be pretty thick and I doubt if it would be practical to do anything requiring fine motor skills wearing lineman gloves. It is pretty easy to get a wisker of solder or wire to puncture a rubber menbrane so a pair of latex gloves is probably not much protection.

I've never tried insulating my body while working, I just keep one hand clear (so if I get plugged in it doesn't travel through my chest) and power down before I try to change anything. The only thing you should be doing with power on is measuring with a meter/scope, or adjusting pots. Modifying the circuit in any way with power on is asking for a slagged amp and a trip to the emergency room.

Phil Ouellette
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Old 27th September 2002, 08:36 PM   #9
sam9 is offline sam9  United States
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Default Darwin award nomination

This appearently was not staged but for real

http://electrical-contractor.net/ubb...ML/000137.html

Give the word "dumb" a new meaning.
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Old 27th September 2002, 09:25 PM   #10
sam9 is offline sam9  United States
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Default some links

here are a few links. Probably too general but its a start if anyone wants to compile more.

http://www.repairfaq.org/REPAIR/F_safety.html


http://www.hawaii.navy.mil/Safety/El...l%20Safety.htm


http://www.reprise.com/host/circuits...nce_safety.asp

http://www.reprise.com/host/circuits/voltage_safety.asp
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