Test Primary of SMPS with scope

[stepgr]

Hi all,

can someone explain in a simple (if possible ) way , how to measure the primary (hot) side of an power supply without burning the scope or the PSU ?
From reading various posts in here I came to the following procedure :

1. Use an isolation transformer on the PSU and then connect the probes ground
on the hot ground on the PSU and the probe wherever I want in the primary side.

OR

2. Use a dual channel scope in add or subtract mode without the need of an isolation transformer, connecting the two probes ground together and probe with the two probe tips the hot side.

I have an old dual channel 25MHz scope who says max input 300V (DC +AC peak) and I want to do this mostly to learn .

thanks in advance for any tips or recommendations.

[Kiriakos GR]

Re: Test Primary of SMPS with scope

The isolation transformer is a good step but costly at 500W or more.

One special probe at X100 it is what will solve the input voltage limit of your scope, but it is pointless getting one if your oscilloscope have settings just for X1 & X10 probes.

[Uniballer]

Re: Test Primary of SMPS with scope

The safety folks will tell you that it is dangerous to defeat the safety grounding of your oscilloscope and/or the device under test. They say you need fancy differential probes to do it right. And if you have them it works great and is quite safe. So that is what I officially recommend.

I have not personally had great luck using the "two probes in add mode with one inverted" for a differential measurement. I'm not really sure why. Maybe I just didn't work at it hard enough.

My older analog scope (Tek 475A) has a transformer-isolated linear power supply. I can float this scope simply by eliminating the safety ground with one of those 2 to 3 prong adapters. Note that this means if you connect the ground lead of the probe to something 400V above ground then all of the metal parts on the exterior of the scope will be at that potential relative to ground. And the grounds of the two probes are tied together as well. So there is certainly a very real safety concern in floating the scope.

I also have an isolation transformer that will probably work with any scope (linear or SMPS power supply) if you use one of those 2 to 3 prong adapters to lift the ground lead. But the same safety concern applies.

I see no problem using an x100 probe on a scope that doesn't understand it as long as you can remember that the real voltages are 100 times what the scope displays.

[budm]

Re: Test Primary of SMPS with scope

I always put the UUT into the isolation transformer, the transformer is then plugged into GFI outlet.

[Uniballer]

Re: Test Primary of SMPS with scope

I always put the UUT into the isolation transformer, the transformer is then plugged into GFI outlet.

While this does not completely eliminate the safety problems, at least the oscilloscope front panel will not shock you if you touch it and ground at the same time. So this is probably a safer way to use an isolation transformer. My isolation transformer is rated for about 920 VA, and cost about $65 (with shipping within the continental US) on ebay a few years ago. It looks a lot like this one.

[stepgr]

Re: Test Primary of SMPS with scope

The isolation transformer is a good step but costly at 500W or more.

One special probe at X100 it is what will solve the input voltage limit of your scope, but it is pointless getting one if your oscilloscope have settings just for X1 & X10 probes.

Can you give an estimate since we both seem to be in the same country ?
I don't think I'll need more than 500W, my main items to test are PC PSU's
and in general IT PSU's no grater than 400W (from lcd monitors etc).

Also the x100 probe is a must from what I understand in all your answers.

Again thank you all guys for providing useful information (and safety) .

[tom66]

Re: Test Primary of SMPS with scope

I was able to purchase a 500VA continuous, 750VA intermittent isolating transformer for less than £25 here, from a local test equipment shop (Stewart of Reading.) Check local area for any shops selling such.

[Kiriakos GR]

Re: Test Primary of SMPS with scope

Can you give an estimate since we both seem to be in the same country ?
I don't think I'll need more than 500W, my main items to test are PC PSU's
and in general IT PSU's no grater than 400W (from lcd monitors etc).

Also the x100 probe is a must from what I understand in all your answers.

Again thank you all guys for providing useful information (and safety) .

I can not offer any true estimate, copper price rising like crazy.
Your best bet is to order one 220/220 transformer and to make one box for it.
There is a workshop that makes transformers 2Km from my shop.
I would suggest you to find such a source in your area too and to avoid any middle man (electronic parts shop).
If you are not aware of were to search? pay a visit in to any large supply shop for electricians, they will have some one to suggest to you.

Yes the 100:1 probe is a must have, if your scope support it.
I got my own yesterday.

http://www.ittsb.eu/forum/index.php?topic=657.0

[stepgr]

Re: Test Primary of SMPS with scope

I also saw this little item here Pintek DP-25 Differential Probe which seems to be affordable for amateur use (all other diff probes I found go in more than 400 hundred range for some mysterious reason... ). What do you guys think ?

I will look into transformer workshop as you suggested (I already asked in electronic shops and found 50 to 60 euros for 500 VA transformers).

[Uniballer]

Re: Test Primary of SMPS with scope

... Pintek DP-25 Differential Probe ... What do you guys think ?

No personal experience. There is a Pintek DP-25 teardown on EEVblog.

[stepgr]

Re: Test Primary of SMPS with scope

Yes I was reading that also yesterday, seems decent enough for it's price

[Kiriakos GR]

Re: Test Primary of SMPS with scope

@stepgr

Basically you have not mention yet which oscilloscope you do own?
And how high voltage you need to measure.

There is many tools out there for very specific applications.
In some circumstances there is no solution for a free ride, but only the complete ticket at full price.

Therefore be more specific.

[stepgr]

Re: Test Primary of SMPS with scope

I have an old dual channel 25MHz scope who says max input 300V (DC +AC peak) and I want to do this mostly to learn .

You mean what brand it its ? take a look (I said old didn't I?)

[Kiriakos GR]

Re: Test Primary of SMPS with scope

@stepgr

You should turn your calendar 50 years ahead and buy a proper oscilloscope.
Because non of the accessories of 2000-2013 are capable to work with this Acropolis shaped as Oscilloscope.

And this is an advice for all.

[stepgr]

Re: Test Primary of SMPS with scope

@stepgr

You should turn your calendar 50 years ahead and buy a proper oscilloscope.
Because non of the accessories of 2000-2013 are capable to work with this Acropolis shaped as Oscilloscope.

And this is an advice for all.

when it's time for a new, I promise I get one OK?

[Kiriakos GR]

Re: Test Primary of SMPS with scope

The breakthrough of the affordable DSO in the past five years, is that the Oscilloscope become capable to display measurements in numbers with an accuracy close to 1% regarding volts that is good enough.

The much older Oscilloscopes they do not display numbers and they have a far worse accuracy of 5% or more that is bad.

The analog projection on this screen it can be out of adjustment and so you can not be certain of what you see in them, if the Oscilloscope is not tested against a high quality digital function generator.

This Pintek DP-25 Differential Probe is actually an active step down transformer, with an output of few mV which the digital scope according the attenuation adjustment on it, it can multiply and display as to were real voltage.

In conclusion when you will manage to get a proper fresh scope, all the joy will be yours and not my.

From the other hand if these words of truth sounds unpleasantly,
this is not my fault.

[stepgr]

Re: Test Primary of SMPS with scope

You're right , I already told you. It's better to spend some money to get a decent new scope.