uncontrolled half wave rectifier with R=3.77kohm

Thread Starter

w1586569

Joined Nov 30, 2018
12
we have been asked to setup a half wave rectifier with a R=3.77kohm resistor using Powerframes - Power Electronics 70 series unit70-220
http://www.feedback-instruments.com/pdf/brochures/70-002_70-005_datasheet_Powerframes_PElectronics_11_2013.pdf
and we observed a negative clipping part with the half rectified signal.(According to theory we should be getting only the positive half cycles of the input voltage waveform). We would be grateful if anyone could help us with finding the reason behind this.View attachment 164741 ?
 

LesJones

Joined Jan 8, 2017
2,696
I assume your problem is the slope on the bottom of the wave form that should be horizontal. (This is the negative half cycle part of the waveform.) I suspect that you have the scope set to AC coupling rather than DC coupling.

Les.
 

Thread Starter

w1586569

Joined Nov 30, 2018
12
I assume your problem is the slope on the bottom of the wave form that should be horizontal. (This is the negative half cycle part of the waveform.) I suspect that you have the scope set to AC coupling rather than DC coupling.

Les.
Thank you! I think that must be the reason! So this AC coupling affect the input waveform as well I guess ? Cuz when we calculated the diode forward voltage;difference between vin and vout, it was 23v! And theoretically it should be ~o.6v
 

Thread Starter

w1586569

Joined Nov 30, 2018
12
I don't know if this information will help but there's a scale down factor of 5 with this oscilloscope and the vout we observed is +15.4vpeak and -9.2 vmin.(oscilloscope is set to 5v per division)So Actual vout is 15.4x5=77v
 

ebp

Joined Feb 8, 2018
2,332
Multiple things:
I don't see a ~ adjacent to the vertical scale display on the scope screen, as would be expected for AC coupling.
The waveform is consistent with AC coupling, but I suspect not in the oscilloscope.
The "ramp" on what should be a flat bottom on the waveform further suggests AC coupling with a short time constant.
The scope display says 5 V per division. If the input supply was 100 V, the displayed amplitude is clearly wrong whether the scope is actually 5 V per division or 50 V per division at the probe tip.


The frequency the scope thinks it is getting is off by almost 0.2%, which is a big error if the input is supposed to be 50 Hz at AC mains frequency, but perhaps the really ugly stepped display isn't due to the fact it is a digital scope but that the input source is a crudely synthesized waveform.
 

ebp

Joined Feb 8, 2018
2,332
I don't know if this information will help but there's a scale down factor of 5 with this oscilloscope and the vout we observed is +15.4vpeak and -9.2 vmin.(oscilloscope is set to 5v per division)So Actual vout is 15.4x5=77v
It still makes no sense. Peak to peak would be about 123 V which is inconsistent no matter if the 100 V is specified as peak or RMS.
 
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