What should isolation transformer read like on DMM

[lordvader88]

With the mains line wire connected to the COM probe of a DMM, I checked the output of a 120VAC:120VAC line isolation transformer


what do u think the voltages are when the red probe is connected to the 3 outputs ?


so 120VAC mains line VS each of the 3 secondary wires (its a 3 prong plug)

[helius]

Voltage measurements between isolated nodes can be anything at all. This is not the correct way to perform an insulation test.

[Brumby]

You are going to have some capacitive coupling - so with a high impedance DMM, you are likely to get some worrying numbers - but they aren't going to be very useful.

[ArthurDent]

As others have stated, what you are doing will give meaningless results. If you want to check if the output of an isolation transformer is isolated (it will be) from the input, unplug it and use an ohmmeter. The only continuity between primary and secondary connectors will be on the ground (earth) lead which is carried through for safety reasons.

In your referring to the "three outputs", there is only one output to a 3-prong connector.

[lordvader88]

Well they had the chassis mains earth grounded, and they also attached a ground prong on the output plug to the chassis. It's not connected to the neutral-output tho.

So the voltages I measured were
line to sec. neutral ~80V
" to sec. line ~178V
" to sec. gnd ~120V


Should the output gnd be connected to anything ?

[lordvader88]

here's the data sheet
http://www.farnell.com/datasheets/2238242.pdf

Why would they create a path from the DUT gnd, back to mains neutral ? When attach the scope probe TIP to some spot, couldn't that create an issue? Couldn't some current flow form the TIP back to mains neutral ? Or does it not work that way at all and it doesn't since I can probe the cold side of a SMPS for instance, and not damage anything

I'm forgetting the probe overall is at least 1M ohm back to neutral/gnd, so it is connected

[ArthurDent]

They didn't "create a path from the DUT gnd, back to mains neutral" as you claim. Look at the drawing you linked to. There is absolutely no connection from gnd to either lead of the primary or to either lead of the secondary. The gnd prong on the plug is connected to the transformer metal core and to the gnd pin on the output connector. There is no connection to any winding. If you had performed the simple test I recommended in post #3 with an ohmmeter or continuity tester you would have proved it to yourself.

As every other poster has said, measuring the stray voltages with a high impedance DMM gives meaningless results because of capacitive coupling.

The electrical code for 120 VAC house wiring requires a duplex socket with line, neutral, and a safety ground connection. The gnd (bare copper) wire in the wall socket electrical box is for safety only and cannot carry any current. At your fuse/breaker box out to the power pole and beyond, the ground and the neutral are the same.

[lordvader88]

Yes but gnd on the primary is connected through the panel box to prim. neutral.


So I should leave both gnd's connected ? And go ahead and try it with my scope

I actually measured the ohm's 1st thing between winding/gnd and I actually missed that both gnd's were connected. Then I checked voltages, and posted this.

There's medical ITs, then some that reduce noise somehow between gnd/neutral, and I don't really know differential/common mode stuff yet that I see mentioned.

[lordvader88]

Ah ok I learned some more and took some uAmp current readings between sides and re-remembered some stuff like the 10M resistance of the DMM in voltage mode.

I checked the current with a neon bulb and resistor, then smaller and smaller resistors, like a VOM. And the voltage drops way down the bigger the load resistor.

with 10M ohm it's reads as 180Vrms, with 3.3kOhm it passes 118uArms so should be 0.39Vrms, with 13R ohms it's 165uArms and 2.2mVrms

So it's acting like it should.


Well I have unhooked all mains earth ground from the chassis, and I realized that I can still get a shock from the secondary once the earthed oscilloscope probe clip is attached if I complete the circuit.

[ArthurDent]

"Well I have unhooked all mains earth ground from the chassis......"

You are continuing to defeat safety features that may end up injuring or killing you or others. Your other thread in the forums on a TV transformer where you comment about measuring no D.C. on one of the transformer leads shows a glaring gap in your understanding of the basics of electricity.

You should reconnect all the safety grounds you removed, stick to low power battery circuits, and read and understand some basic electronics theory material until you understand what you are actually doing. What you have described so far is very worrying.

[lordvader88]

Well I'm not planning on touching the transformer and their's folks with lots of experience that recommend doing it that way as well as floating the scope by un-grounding it as-well. And yes that can be dangerous , but thats it.

[Gregg]

You need to step back and look at the whole picture in terms of circuits and what is forming a complete circuit.  Your mains input has the neutral and earth ground connected at some point in the circuit; the wall outlet you plug the transformer into can be considered a source of power.
The secondary wires of the isolation transformer can be considered an isolated source of power; however there is inductive and possibly capacitive coupling inherent in the transformer.  Good isolation transformers have the primary and secondary wound on separate coils to minimize the obvious problem of capacitive coupling. 
What you need to do is consider what potential power may or may not be available at any part of your complete circuit and what the ramifications of probing or putting your bare fingers into the circuit before doing anything rash. When in doubt, draw it out on a sheet of paper; remember to draw the complete circuit, not just the part you are about to commit to a Darwin Award or magic smoke.

[BravoV]

Rather than risking your life, your scope and also your equipment. Why not consider buying a HV differential probes to your scope, its not very expensive these days, and for sure its much safer than using an isolation transformer. Here an example on how convenient to use it to measure mains rated lines/voltage related, the scope was measuring the mains lines 220 VAC at the same socket where it was powered and plugged.

[lordvader88]

I'm no fun at all, I don't even like discharging caps with a screwdriver, since it might damage the cap, or the PCB.

Generally I turn off the DUT if I want to move a probe wire, just in-case I short +2 points. I'm cautious for sure.

I'll be alright for now, but yeah I'll get some diff-probes as well someday. I have a current limiter, auto-transformer, and now isolation transformer, and lot's to learn.

[Gregg]

I'm no fun at all, I don't even like discharging caps with a screwdriver, since it might damage the cap, or the PCB.

Generally I turn off the DUT if I want to move a probe wire, just in-case I short +2 points. I'm cautious for sure.

I'll be alright for now, but yeah I'll get some diff-probes as well someday. I have a current limiter, auto-transformer, and now isolation transformer, and lot's to learn.

I used to be a lot more cavalier when I was younger.  The rule is that your first mistake could be your very last.  Here is a picture of me 12 years ago taking a voltage drop reading in a 3000 amp 480 volt transfer switch with a 2000 amp load on it.  Things were getting hot as noted by the 0.1288 volt drop across a bolted bus connection.  I was wearing 1000 volt gloves, a cotton long sleeve jacket and a face shield.  These days after all of the arc flash safety over-reactions the amount of personal protection required to meet OSHA guidelines would be prohibitively bulky.  The biggest part of staying safe is staying smart and keeping distractions away. 

[lordvader88]

It's like high heights for sure, but u can't see the falls.

I like to use LEDs and neon bulbs to discharge stuff, since they look nice too

[malagas_on_fire]

Hi there.

If using a isolation transformer be aware of the resonant frequency when it is loaded because it can give strange results in frequency dependent circuits such as dimmers, switching power supplies. Use a proper rated voltage ( twice is better) capacitor in parallel to the AC terminals with values between 470nf ~ 2uF to get a more pure sinewave.