Power supply and oscilloscope problem

michouthefirst · 27th November 2012, 01:20 PM

I have a power supply with an AC output (0-22V, 4A). I want to use this output to simulate the secondary of a power transformer, not having to worry about circuit protection for now.

I wanted to use my oscilloscope (a Tektronix analog scope) to look at the effect of load variation from my regulation circuit.

I have the following problem: when I connect the tip of the oscillloscope probe, then the common, the voltage of the power supply drops significantly (as can be seen on the power supply display) and after 5 seconds the circuit breaker of the unit goes off. I made several test, I even succeeded at blowing the 2 amp fuse of the unit.

After some tests, I also noticed that using the DC output has the same effect. I switched probes, same results. I also verified with a multimeter (battery powered) there is no voltage drop between the oscilloscope and the unit grounds.

When I use my PC oscilloscope, this does not happen.

Does anybody have an idea of what could be wrong here? I never had any problem with the scope before.

DUG · 27th November 2012, 03:09 PM

Here is my guess:

The analog scope ground is connected to the AC safety ground and your power supply is the equivalent of a btl output. You are actually shorting one of the outputs to ground.

Solution(s):

If your scope has two inputs then connect the two probes to the two AC feeds and use the A-B function to view the difference.

OR build a differential to SE opamp cct that divides by 10

Always watch out for smoke.

michouthefirst · 27th November 2012, 04:09 PM

Do you mean that one of the AC outputs is directly connected to the ground? Could reversing the outputs fix the problem?

I could check to see which output has zero voltage with ground.

And thanks for the smoke warning

DUG · 27th November 2012, 04:19 PM

maybe

just a guess...I'm not there

michouthefirst · 27th November 2012, 04:28 PM

Ok, I'll just try that. Thanks for the info, it never came to my mind that one of the output could be gronded.

DF96 · 27th November 2012, 10:43 PM

Unless told the contrary you should always assume that a power supply output, whether AC or DC, has a ground reference. In some cases this may be absent or switchable.

michouthefirst · 28th November 2012, 12:41 PM

Thaks guys for your replies.

27th November 2012, 01:20 PM	#1
michouthefirst diyAudio Member Join Date: Jul 2012 Location: Gatineau	Power supply and oscilloscope problem I have a power supply with an AC output (0-22V, 4A). I want to use this output to simulate the secondary of a power transformer, not having to worry about circuit protection for now. I wanted to use my oscilloscope (a Tektronix analog scope) to look at the effect of load variation from my regulation circuit. I have the following problem: when I connect the tip of the oscillloscope probe, then the common, the voltage of the power supply drops significantly (as can be seen on the power supply display) and after 5 seconds the circuit breaker of the unit goes off. I made several test, I even succeeded at blowing the 2 amp fuse of the unit. After some tests, I also noticed that using the DC output has the same effect. I switched probes, same results. I also verified with a multimeter (battery powered) there is no voltage drop between the oscilloscope and the unit grounds. When I use my PC oscilloscope, this does not happen. Does anybody have an idea of what could be wrong here? I never had any problem with the scope before.

27th November 2012, 04:19 PM	#4
DUG diyAudio Member Join Date: Jan 2003 Location: mississauga ontario canada	maybe just a guess...I'm not there __________________ Doug We are all learning...we can all help

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27th November 2012, 04:09 PM	#3
michouthefirst diyAudio Member Join Date: Jul 2012 Location: Gatineau	Do you mean that one of the AC outputs is directly connected to the ground? Could reversing the outputs fix the problem? I could check to see which output has zero voltage with ground. And thanks for the smoke warning

27th November 2012, 04:28 PM	#5
michouthefirst diyAudio Member Join Date: Jul 2012 Location: Gatineau	Ok, I'll just try that. Thanks for the info, it never came to my mind that one of the output could be gronded.

27th November 2012, 10:43 PM	#6
DF96 diyAudio Member Join Date: May 2007	Unless told the contrary you should always assume that a power supply output, whether AC or DC, has a ground reference. In some cases this may be absent or switchable.

28th November 2012, 12:41 PM	#7
michouthefirst diyAudio Member Join Date: Jul 2012 Location: Gatineau	Thaks guys for your replies.