Isolation Transformer Process

[lowell]

Well I finally got an iso. I've been working on a few smps and want to become a more versatile repairman. So, I connect the broken gear to the iso correct? After such, how is a scope connected to the primary side of an SMPS? What other issues should I be concerned with?

[Jazz P Bass]

If the iso is hooked up to the DUT, you can now safely 'scope' the unit as you would if you where using a volt meter.

[TomCarlos]

What kind of Isolation Transformer did you get?

Read this thread - Wall Isolation Transformer - and make sure you what the ToddFun video.

Tom

[Jazz P Bass]

What kind of Isolation Transformer did you get?

From another post: https://www.jameco.com/webapp/wcs/st...gDrillDownView

[Randall]

If the iso is hooked up to the DUT, you can now safely 'scope' the unit as you would if you where using a volt meter.

What is DUT?

[TomCarlos]

Device Under Test....

[lowell]

Ok sounds good thanks JPB. Yes Tom I got the one that JPB referenced.

So with the DUT plugged into the isolation transformer, there is now no true EARTH reference. So the rectified mains voltage for the main bridge and filter is +/-195v, and connecting the ground of the scope to -195v or 0v will no longer be likened to shorting those points to earth ground. That's the way I'm understanding it anyway.

Correct?

[TomCarlos]

I believe you have stated it correctly. The neutral on the secondary is no longer (physically) connected to the earth reference. So if you measure the voltage (using a voltmeter) on the secondary, from hot to neutral, you will see 120v. But if you measure the voltage from the hot to the ground, you will get "0." As explained in one of the videos, you might see 70 or something... that is a ghost voltage. If you use a Wiggy (or something that has a solenoid), it won't budge.

Now I hope it is ME who has stated this correctly !!!!

[Jazz P Bass]

Yup.

Have considered 'fusing' the iso?

[lowell]

This iso came fused! It has a 2-prong ac plug attached and an outlet as well for the DUT. Convenient.

So I've thought a bit more about this. If the DUT is in the iso, the scope ground is still EARTH. So, connecting the scope ground to the HOT ground on the primary of a SMPS will not be good. Which makes me think, and correct me if I'm wrong, that the scope should be plugged into the iso transformer.

Also, I downloaded Jestine Yong's e-book switch mode power supply repair. And he seems to be saying that the scope is using the iso.

[Jazz P Bass]

Hooking the scope up to the Iso is pointless.

The idea is to isolate the scope. which has it's chassis at earth ground.
(Please do not even think about lifting the earth pin on the scope!)

Be careful as, even though you have an isolation transformer, there is still the mains voltage across the test points.

Aside: what are you planning on 'scoping' on an SMPS that a simple volt meter cannot 'see'?
Personally, my Tektronix TDS 320 digital scope is too valuable to me to use it on the mains.

[lowell]

I want to scope the PWM output that drives the MOSFET.

[g1]

If the DUT is in the iso, the scope ground is still EARTH. So, connecting the scope ground to the HOT ground on the primary of a SMPS will not be good.

Incorrect. Because the SMPS primary is now floating (isolated by xfrmr) it can be pulled to earth ground by the scope but it will not matter.
If you have seen tube heater supplies that are 6VAC, but referenced to DC instead of ground, it's a similar thing.

[Reader]

Also, I downloaded Jestine Yong's e-book switch mode power supply repair. And he seems to be saying that the scope is using the iso.

Lowell,
I am thinking of ordering that book too. Advert says it has a lot of good and practical information but sounds almost too good to be true. Did you still like it?
TIA,
Bill

[lowell]

Incorrect. Because the SMPS primary is now floating (isolated by xfrmr) it can be pulled to earth ground by the scope but it will not matter.
If you have seen tube heater supplies that are 6VAC, but referenced to DC instead of ground, it's a similar thing.

I'm confused by this. An SMPS primary is already floating relative the EARTH ground. Only the secondary circuitry is referenced to earth ground. So, what am I missing?

Bill, it's helpful but not fullproof. The English isn't great either. But I do feel it has given me more confidence in troubleshooting SMPS supplies. The big points for me are the Blue ESR meter and the Blue Ring meters. I'm still familiarizing myself with it.