Isolation Transformer Process

[The Dude]

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?......

That's it in a nutshell. Possibly what you are missing is that when the DUT is isolated, there is no longer an earth to the chassis. Therefore you can't inadvertently tie the primary ground to the chassis ground with your scope probe. Hopefully that makes sense?

[lowell]

That makes sense. But not sure if that's the real reason. Won't the scope need to be referenced to HOT ground on the primary side in order to scope the PWM output that drives the MOSFET?

[The Dude]

Yes, and that's why you need to be isolated.

If your chassis is grounded and the scope is grounded, when you attach scope ground to primary/hot ground, you would be connecting hot ground to chassis ground. With the chassis "floating"/isolated, the scope ground does not tie the two together. Am I making sense yet?

[lowell]

So I must be unclear on what EARTH is. I thought earth ground was ZERO volts...PERIOD. Any voltage touching it will be eliminated. So I would think touching EARTH to -165v would mean blown circuit braker. And I'm guessing your response will be that nothing on the primary side is referenced to EARTH ground, therefore it won't be an issue. I'm still unclear on why this is. Sorry. And thanks for humoring me.

[The Dude]

I edited the post above. See if that works.

Edit: This might make things worse, but being ground isolated which is a concern for SMPS is not the same as being completely transformer isolated where you have no direct connection to the mains. Hopefully someone who is better with words, can explain this better than myself so that you can make sense of it. My apologies for confusing the issue.

[The Dude]

I'll add one more thing before shutting up. You can unintentionally ground an amp that you want isolated by inserting a signal from another piece of test equipment that is grounded. Be careful not to do this when scoping around an SMPS.

[lowell]

Helps a little. As long as COLD ground and HOT ground don't mix we're ok.? Thanks for the help.

[lowell]

I guess though...I still don't get it because COLD ground isn't "in play" on the primary side. So referencing scope ground to -165 doesn't have anything to do with COLD ground on the secondary.

[Jazz P Bass]

It may help if you stop misusing the term "Ground".

In ANY circuit, you need a reference point to make measurements.

If you are measuring a +15 Vdc supply, the reference is Zero Volts.
The fact that Zero volts just happens to be tied to Safety Ground is irrelevant.
You could disconnect the Safery ground & it will still read +15Vdc with respect to Zero volts.

When working on the SMPS that you have, the primary circuit reference is -165Vdc.
Hot Ground is a very confusing misnomer.

[lowell]

Yes I get that...surprisingly. I know I sound amateurish so bear with me and thanks for the help. So to be totally clear: If I plug the SMPS DUT into the iso, and the scope into the wall, I can "ground" my scope at the -165Vdc reference point without sparks flying?

[eschertron]

Yes I get that...surprisingly. I know I sound amateurish so bear with me and thanks for the help. So to be totally clear: If I plug the SMPS DUT into the iso, and the scope into the wall, I can "ground" my scope at the -165Vdc reference point without sparks flying?

I'm still trying to get oriented on this topic; if my understanding so far is correct, then as soon as you connect the scope's ground ref to the (transformer isolated) DUT, then that point becomes [earth] ground. Doesn't matter if it's -165vdc relative to the DUT zero volts rail. Everything from there on is measured relative to the scope's 'zero' and not the SMPS's 'zero'. Did I get that right?

[gbono]

another solution is to use isolated scope probes or a portable (battery powered) scope (only works for low voltage under 30V) - both are $$$ compared to an isolation transformer but safer for user and equipment.

[g1]

So I must be unclear on what EARTH is. I thought earth ground was ZERO volts...PERIOD. Any voltage touching it will be eliminated. So I would think touching EARTH to -165v would mean blown circuit braker. And I'm guessing your response will be that nothing on the primary side is referenced to EARTH ground, therefore it won't be an issue. I'm still unclear on why this is.

I'll post the schematic of the primary part of the SMPS you are speaking of, it will be easier to explain (hopefully, sorry if it just confuses things).
All voltages must be in reference to some other point. We often use ground as the reference but it does not have to be. Common is often a better term. But there is always an unspoken reference point. It sounds like the circuit has +/- 165V ? This would be with respect to a common point. You can also consider the -165V to be 330V below the + end (or the +165 is 330V above the - end). It's all relative to the point of reference.
Pin 1 of the far left connector is neutral. With no isolation, it will be connected to earth ground if your house is wired correctly. The line from pin1 is connected to the junction of C109/C111. Your scope ground is also connected to earth. If you connect the scope ground to the bottom of C111 (primary 'common') you are shorting out C111 as both ends are now connected to earth.
Plugging the DUT into the Iso tranny disconnects the DUT's neutral from earth. The smps primary is now fully floating. You can connect your scope ground anywhere and the DUT will not care. It will just float to a new position.

[lowell]

Ok so the junction of the main rectifier caps, aka Neutral, is technically EARTH via my house mains wiring? So Neutral and EARTH are the same?

[Enzo]

Neutral and earth are wired together in the walls of your house and so are basically the same on the outside of the iso. Inside the iso, NOTHING is connected to neutral or hot. Neutral and hot are mains terms. Inside the SMPS, what used to be neutral and hot are now an isolated AC source. it is not connected to any earth now. So now we can connected any one point in the circuit to our earth as a reference for our earthed bench. yes, you can use a battery scope that has no earth connection.

In the SMPS, the control circuits on the primary side the -165 is the common reference for the control circuits. So now I can connect my scope ground to that point, and the 5v or 12v the IC uses is measured on my screen. If the SMPS were connected directly to the mains, then the -165 would be -165 with respect to the earth, and that means the "+12" for the IC is -153 with respect to earth. But on an iso, I can just earth the -165 point and my +12 measures +12.

In the stock SMPS, the input does not float. The neutral side has that grounded relationship with the world. The -165 and +165 are real voltages with respect to earth. On the iso, those two voltages are no longer with respect to earth, they are simply with respect to the neutral pin of the mains input.


Here is why I never put the bench gear on the iso instead of the DUT. In a stock SMPS, there is that -165v rail, we need to use it for a common for the control circuits. If I iso my scope. I can still connect its ground clip to that -165 without issue because the scope no longer has an earth reference. But when I do that, that -165v with respect to earth in the SMPS is also the potential on the ground of my scope, and the frame of the scope. SO if I am touching ground myself. and happen to touch the frame of my scope, it is the same as if I touched the -165v in the SMPS. Dangerous and painful.