full wave bridge rectifier-or oscilloscope, what's wrong?

Thread Starter

Fuintur

Joined Nov 17, 2017
4
Hello,
I've just purchased some bridge rectifier: http://pdf.datasheetcatalog.com/datasheet/wte/KBPC3510W.pdf , this one, the KBPC25 25A version, two of them. I have this oscilloscope DSO138 (http://akizukidenshi.com/download/ds/jye/UserManual_138_new_s.pdf) I started using last summer, which I'm not an expert with. It has these probes. I'm feeding its AC leads from a 220V-48V (it can also be configured to 380V-48V) transformer (actually an old AC arc welding machine). When set on 220V-48V I'm getting these readings out of it:

This is the raw AC signal with the probe set on x10:



-Rectified output with the probe set on x10 DC coupling (??):


-Rectified output with the probe set on x1:



It seems like the last waveform is the right one, apart from a smallish kink once every two waves. The wave I get from the probe at x10 seems very wrong (16.51V max would be if on x1 it was 160V peak! and well, it's just very unequal, it reads 50Hz because it's not matching the two half waves). Well, actually the oscilloscope can't handle more than 50V peak and it should be reading 67V peak on the last image, so it probably just shows 1.93Vpeak because it has gone crazy, but still the thing keeps working, and the Vrms (47,53V) seems about correct.

As I said the transformer can also be configured to 380V-48V. If nevertheless I feed it from 230V it becomes a 230V-29V transformer. I've been making the same batch of readings with the transformer on this 230V-29V setting:


-Raw AC wave with the probe on x10:


-Rectified output with the probe set on x10 (??):



- Rectified output with the probe on x1


The results for the probe on x10 are even worse than before, but they seem okay on x1.

I've tried the same tests on different bridge rectifiers I have and the waveforms were basically equal to those already displayed.

What's happening? I've calibrated the internal adjustable capacitors of both the oscilloscope and the probes. Can I never again rely on the probe on x10?
 

AlbertHall

Joined Jun 4, 2014
9,824
You may need to connect a load across the rectifier output, say 10k. The impedance of the X1 probe will be 1MΩ and the X10 will be 10MΩ and the leakage of the rectifier may be affecting the readings.
 
Top