Diode bridge measurement

[gulftown17]

Hi, I was measuring some circuits with an oscilloscope (DS1052E) and I saw something that suprised me. Could someone explain whats going on?
So I was measuring Diode bridge (with 1N4007x4 diodes) and as a source I used AC adapter that had an output voltage of 16V AC. On the ouptut of the diode bridge I connected a 820uF 35V capacitor and 10K resistor as a load. What surprised me was that instead of a nice halfwave it was kind of deformed (see screenshots). I tried to play with an oscilloscope and I applied a low pass filter (with cutoff frequency 100Hz, 150Hz, 200Hz, 1kHz and then without the filter). I found out that the halfwave looked least deformed when I set the cutoff frequency to 100Hz, as I was increasing this frequency, it was getting worse. I also found out that this phenomenom only happened when I connected the probe tip to one input of the diode bridge, when I tried the other one, this deformation was not present. I have also found out that when I put a 100nF ceramic capacitor in parallel with any of the diodes it changed the waveform as well. BTW I connected CH1 (yellow) to positive output of the diode bridge, CH2 (blue) to one of the AC inputs of the diode bridge. GNDs of the probes were both connected to the negative output of the diode bridge. Thank you for your help.

Without filter, deformation of the blue waveform with Vpp 4.32V

Again without filter, CH2 connected to the other AC input of the diode bridge, the deformation is negligable

Lowpass filter 100Hz (the AC input where deformation is present)

Lowpass filter 150Hz

Lowpass filter 200Hz

Lowpass filter 1kHz

100nF ceramic cap connected in parallel with one of the diodes

 

[davenn]

not sure of you are not measuring correctly or if you have a wiring error in your diode bridge
but you shouldn't be getting an AC waveform out of the bridge



without the capacitor you should see the purple dashed waveform
with the capacitor you should be seeing the dark black line with the little dips in it

if not then your diode bridge isn't wired correctly or there is a faulty diode or 2


Dave

 

[Colin Mitchell]

You might have your CRO earthed and not floating.

 

[gulftown17]

The yellow waveform on my oscilloscope corresponds to the DC ouput of the diode bridge. It is almost perfectly smooth because Im using high capacitance (820uF capacitor) and low load (10k). The bridge is working and wired correctly. Im worried about the blue waveform which is the input ie probe tip is connected to the AC input of the diode bridge, where it is getting AC input signal. The grounds of both probes are connected to to DC output of the diode bridge.
Colin: Its DSO not CRO hence you can see the screenshots. The oscilloscope is grounded but the circuit is floating because Im using AC powe adapter which only uses 2 pins (live and neutral).

 

[davenn]

OK
would have been handy to have known that for a start ... wouldn't have had to waste my time
with my previous post and diagrams

what you are most likely seeing is the clipping of the AC signal by the diodes
if you disconnect the AC from the bridge input and scope that AC signal, you will most likely see a perfect sinewave


Dave

 

[gulftown17]

Thats what I wanted to know, thank you. I will to look it up .