Measure waveform at Drain Pin of a Power switch IC blow the fuse?

Thread Starter

simpsonss

Joined Jul 8, 2008
173
hi,

In the simple SMPS which output a 5v dc, there is a power switch IC (M02659). I was planning to have a look on the waveform of the drain pin of this IC to get a some sort like switching status(i'm not sure whether is it correct).

After i connect the negative(black) wire of my probe(from oscilloscope) to the GND pin of this IC, and then i switch on the main AC. strip happen and the fuse inside the SMPS blow. ***But i haven put the probe on the drain pin.

i was wonder why strip happen.

hope someone can help me on this.thank you.
 

n9352527

Joined Oct 14, 2005
1,198
The ground of the oscilloscope is connected to the mains' earth. The GND pin of the switching IC is connected to the negative voltage of rectified mains. When you connected the oscilloscope to the switcher you shorted this negative voltage (which is referenced to the main's live and neutral) to the earth! For a full wave rectifier, that means shorting the live to the earth for half the cycle and shorting the earth to the neutral for another half of the cycle. Or either the mains live or neutral directly for a half wave rectifier, depending on the polarity of the mains plug.

Use an isolation transformer, as lannan pointed above, to isolate the switcher (not the oscilloscope!) from the mains. You need the oscilloscope ground/earth for safety.

The switching IC is on the primary live side, so it's ground is the negative voltage of rectified mains and NOT the DC output GND, which is on the on the secondary side. Those two sides are isolated from each other for safety reason.

Last, as lannan said above, never play with mains supply if you do not exactly know what you are doing!
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
The ground of the oscilloscope is connected to the mains' earth. The GND pin of the switching IC is connected to the negative voltage of rectified mains. When you connected the oscilloscope to the switcher you shorted this negative voltage (which is referenced to the main's live and neutral) to the earth! For a full wave rectifier, that means shorting the live to the earth for half the cycle and shorting the earth to the neutral for another half of the cycle. Or either the mains live or neutral directly for a half wave rectifier, depending on the polarity of the mains plug.
ok.got it.thanks for the explanation and advice.

Could u share some info on how to isolate the switcher by using isolation transformer? i want to learn it.

thank you.
 

retched

Joined Dec 5, 2009
5,208
An isolation transformer has the same number of windings(typically) on the primary and secondary. It physically separates the two sides effectively isolating them. That way you get the same voltage on both sides without a direct to earth "ground" on the secondary side. You would connect the primary from the iso-transformer to the primary of the transformer you want to measure. Then MEASURE from the secondary of the isolation transformer.

This decouples them and gives you no step up or down in voltage.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
You would connect the primary from the iso-transformer to the primary of the transformer you want to measure. Then MEASURE from the secondary of the isolation transformer.
Hi retched,

thanks for the reply.
So do the iso-transformer in the market have different type of input AC voltage? I mean do i need to know the AC voltage at the point that i want to measure for my case is the DRAIN pin of the switching IC?

thank you.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
Hi,

ABCD0008.jpg

From the figure i attached, i should get a ripple waveform as what i draw on the point A right? From theoretical i know that. But how could i measure this point A by using oscilloscope? To which ground should i refer for my black color wire of my probe to be connected to? Is it i need a iso-transformer again as what u all mention above?

thank you.
 

n9352527

Joined Oct 14, 2005
1,198
Another way of measuring it without using isolation tranformer is to use a differential probe (observe the voltage rating, and use dividers appropriately), if you have one. It is even more expensive than an isolation transformer.

There is another hackish way, but it is not safe and I won't be part of it :)
 
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Thread Starter

simpsonss

Joined Jul 8, 2008
173
so if i would have to buy an isolation transformer.So the first thing i would need to know is the voltage at the point i want to measure?

thank you.
 
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