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Better diff probe to avoid inaccurate scopes

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Faringdon:
Hi,
The attached is the scope of the secondary coil of a isolated 240VAC , 150w  one transistor forward with HV txformer reset.
Its got an oscillation in it of T=10ms. As can be seen
This is not real.
Its due to the leakage current to earth in the diff probe.

Do you agree, we must float the scope to avoid this and get a proper waveform?

We cannot afford a better probe, and an isolation transformer and 100:1 probe gives the same problem.

TA041
https://uk.farnell.com/pico-technology/ta041/probe-active-differential-powered/dp/1667343?gclid=CjwKCAjwq-WgBhBMEiwAzKSH6BTgsoaHaPZhLv3tN57SIsaMBnTLFUtTgh5geS-qpZcRgl3VZM1zuRoCTxgQAvD_BwE&mckv=s_dc|pcrid|605279997276|kword|ta041|match|p|plid||slid||product||pgrid|143566925168|ptaid|kwd-314725982045|&CMP=KNC-GUK-GEN-SKU-MDC-Test-Tools-PS

nctnico:
Never float a scope. And a scope still has at least tens of nf of leakage to mains so it will be worse.

Show us the schematic,test points you want to measure and tell us precisely why you want to measure at those points.

Faringdon:
Hi,
Thanks, its just a 150W, 240VAC one transistor forward, with high voltage transformer reset.
Its isolated.
We wish to  measure across the secondary coil.
Also, we wish to measure the primary, post rectifier  DC Bus.
Both these measurements are totally ruined by the TA041.
The SMPS is fed from an osolated AC source.
But its the same if we feed it from the raw mains.
The Diff probe is lousy. (TA041)
--------------------------------------------
Here is another scoping problem with TA041......The blue trace in the attached two scopes is the post rectifier DC bus capacitor (5u6 Film) of a 240VAC 150W One transistor forward with HV transformer reset. (with 150W load). Its taken with a TA041 Diff probe.

In one, taken up to 5 seconds after switch ON , the blue trace is always running down to a “valley” of some 165V.
In the other, the blue trace is sometimes going down to some 40V "valley".
The reason for the 160V "valley" (or any “valley” voltage above 40V) is the leakage current in the TA041 Diff probe.

Do you agree, we must float the scope to avoid this and get a proper waveform?

We cannot afford a better probe, and an isolation transformer and 100:1 probe gives the same problem.

bdunham7:
If the DUT is isolated what exactly do you gain by floating the scope??

Weston:
By what mechanism do you think "leakage current" is causing this? The differential probe has a 4M input impedance, if you are probing a low impedance node such as the DC bus voltage it seems unlike that any current though the input impedance of the probe would cause issues.

If the waveform is distorted is it possible that you are exceeding the common mode input voltage of the probe? That would distort the waveform produced by the probe.

If the DUT is floating and has no ground reference the common mode voltage is poorly defined and could reach some high DC potential. Floating the scope would not necessary fix this, no would a better differential probe. You need to provide some sort of ground return for the DUT to limit the voltage it could float to.

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