Using an Isolation x-former w/ a 'scope

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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.
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.


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.
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.


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.
Eric and haldor, :up:

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. :)
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.

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
"Rubber shoes are not going to do a thing for you."
So how come I can charge myself up to a couple of KV by touching a pin on my negative ion generator, and then discharge myself to ground 30 seconds later ?.

Standing on an insulating surface in insulating shoes has worked perfectly fine for me for years so long as you do not do something stupid like touch the grounded aerial cable connector or other grounded equipment or bench metalwork.
When I am working on live chassis equipment I do so whilst standing and I am always careful to make sure that there is no chance of any current paths through my chest area, by not using my left hand at all.
I also have an alligator clip on the negative lead of my DMM, so that I can do single handed measurements and adjustments.
I also announce to anybody in the room with me that I may be live and DO NOT touch me (ie do not try to hand me a beer :) ).
It is not difficult to work safely on live chassis so long as you are aware of correct procedures and possible mistakes.

Bernard, I do not think that the rubber glove idea is good because of chance of perforations - much better to keep the left arm and hand right of the picture.

Eric - still alive and kicking.

BTW - I have been rather badly electrocuted in the past.
I was working on a large Pioneer surround reciever, and so that I could turn it 90* to give access to the rear panel, I placed my hands against each side.
Disastrously I contacted the power input active and neutral Utilux posts, causing current paths across my left hand, and through my chest area to my right hand.
The current between the A & N caused enough heat to burn 2 holes deep into the palm of my left hand, and enough heat to melt the solder on the terminal posts (I had to pick bits of solder out of the wounds) and I still have those scars.
The current path to my right hand caused my arms to clamp across the amplifier chassis and straight ahead and in so doing lifting the amplifier of the bench with both arms fully outstretched, (I could never normally hold this weight of amplifier at full arms length) and caused ALL the muscles throughout my my neck, jaw, throat, chest and back to clamp HARD on.
My hands were clamped flat and hard across the chassis, and I could not pull my arms apart in order to release the amp.
Also I could not yell out to the guys in the next room.
After suffering this for about 30 seconds I managed to stand up, kick my stool backwards away, and spin my body 90* in order to unplug the amp.
This I did and my arms instantly released the amp which continued across the room and fortunately landed flat and no damage incurred.
My back was a bit messed up and I had chiropractic treatment for a few months, and nowadays it's fine.
There is a device called "jordfelsbrytare" in swedish. I am sorry
but I have no idea what the proper english term is. A literal
translation would be something like "ground error breaker".

You typically plug it into the mains outlet and then plug your
DUT, computer or whatever into this device. What it does is to compare the currents
flowing through it and whenever they do not sum up to zero
within a margin of one or a few mA it will immediately break
the mains connection. The reaction time is fast enough that
you will not get injured, although you will probably feel it.

I don't really know, but it seems to me this would supply
sufficient safety. Some install such a device for the electric
network in the whole house. It is also strongly advised to
use one whenever you are working with electrical equipment
outdoors, in bathrooms etc.

Another related issue when we are talking safety. Don't forget
to wear eyaglasses guys. Although I do not belong to those
who have had the nasty experience of blowing up a large
electrolytic, I have had my face targeted with "plasitc missiles"
from opams, leaving a crater all the way down to the chip.
I have also managed to convert a TO92 BJT into one 1-pin
device and one 2-pin device :). I suppose an exploding TO3
could be really nasty. These things happened a long
time ago, when I was young, not having the sufficient knowledge
to learn electronics other than by empirical experimentation.
mrfeedback said:
"Rubber shoes are not going to do a thing for you."
So how come I can charge myself up to a couple of KV by touching a pin on my negative ion generator, and then discharge myself to ground 30 seconds later ?.

If you are standing on the usual wooden or tile floor then you could do this barefoot. The thing you need to be concerned about is touching a live conductor at the same time that some other part of your body is in contact with the grounded enclosure or chassis. That can put a current through your heart which is often fatal.

I have never been electrocuted (tingled a couple of times), but I once did see someone get electrocuted. I was repairing a batch controller at a cattle feed lot in Gila Bend, Arizona in 1981. The operator didn't want to me to power down his system to reconnect a cable inside the control cabinet. Instead he reached in through a broken access panel on the side of the cabinet to make the connection fo me. Unfortunately for him the back of his arm came in contact with the motor contactor inside. The main conduction path was through his worn out, sweat-soaked, steel toed boot that was in contact with the control panel enclosure. The man lost two toes and a hunk of flesh from the back of his arm, but we were able to resuscitate him.

Made a believer out of me.

Would you know if it would give sufficient safety when
poking around inside a connected amp say?

NO, NO, NO!!!!!!!!

Such a breaker will only break when a current goes from live to protective earth.

Here comes a little explanation in Swedish ( I will and can not write it in english because I*m not sure off translation and foreing electrical systems):

Jordfelsbrytare bryter bara om du får en ström från fas(L) till jord(PE (gul/grön)). så om du skulle komma i kontakt med nolla(N) och fas har du fullt ös tills säkringen (10A?) löser, ca 500 ggr dödlig dos.


yes you are right, of course, that it only works in some cases.
It should work in the cases mostly discussed in this thread, ie.
when you get accidentally grounded by something outside
the circuit. It won't protect you, of course, from high voltages
within the circuit, and neither would an isolation transformer.
It won't always protect you, but it will in some cases, and it
should not do any harm, I think, at least as long as you are
only using one wall outlet for all equipment.

BTW, I don't think you swedish explanation is quite correct, or
else I have misunderstood how these things work. It just
sums the currents in live and neutral and breaks if these differ.
It doesn't matter where the difference current goes, that is,
it does not have to go through the safety ground (or whatever
it is called).
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