Filtering HV pulses

Thread Starter

JulesP

Joined Dec 7, 2018
374
I have finished the construction of an HV supply (that was in part the subject of another thread) and it is producing hefty sparks (and where a short video of it working can be seen at: https://www.dropbox.com/s/n2z0de5k1pu6fk4/Coil HV.MOV?dl=0 )

My query is regarding the resulting voltages that I measure with a 1000:1 voltage divider probe (see pics). By adjusting the primary voltage (with the MJE3055) I get a range of secondary voltages from +3.8kV to +6kV and -16.4kV to -33.2kV as shown in the images. This negative voltage, with its shorter duration, I presume is some form of BackEMF that I would like to remove from the supply to the plates of my HV plasmolysis cell so there is only one polarity hitting the plates.

Is there an easy way to block these? I imagine most diodes won't take the hit but there might be another way. Alternatively, at some stage, I might need to remove the positive peaks and just keep the higher value negative ones.

Thanks
 

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DickCappels

Joined Aug 21, 2008
7,449
I would just put a diode or stack of diodes with the appropriate reverse voltage rating in series with the pulse. You might need a second diode to ground so as to remove any glitches of the wrong polarity that get through the junction capacitance of the series connected rectifying diode.

1621077262286.png
To let the negative pulses through instead of the positive pulses reverse BOTH of the rectifiers. You might not need the diode to ground depending upon the impedance of your load.

You have measured the voltages now you have to estimate or if possible, measure the current. From that information you can select diodes. You might have to put some diodes in series to get adequate peak inverse voltage capability.
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
You mean like this? If this is ok what rating would I need for the diode to ground?

If I turn this round I presume I can filter out the other polarity of pulses. The diagram below would filter out the negative pulses.

HV Rectifier.jpeg
 

DickCappels

Joined Aug 21, 2008
7,449
First of all, no because the positive pulses will all be shorted out.

While you were drawing your circuit I was drawing one showing the way the diodes should be connected. Sorry I posted text before I made the drawing.

You should not need more than one diode to ground if its peak inverse voltage is not exceeded by the output of the series connected rectifier. You certainly got that part right.
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
The highest rating I could find at a reasonable price was 20kV so should I use two in series to amount to more than my peak voltage, as in my diagram. Yes I should reverse the diode that connects to ground. Would that be ok then?
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
One more thought - can the diode to ground be something less robust like an IN5407 since it’s purpose is simply to short to ground any negative pulse of any voltage, assuming it can handle whatever power is involved?
 

DickCappels

Joined Aug 21, 2008
7,449
Your 1N5407 is only rated at 800V. If you put a pulse of 800V or more through to the output you are likely to clip the voltage and perhaps damage something.
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
Correct me if I’m wrong but the grounded diode is only meant to ground any negative pulses that get through the two main diodes and as such should be unlikely or of ‘low’ voltage. Would you expect some 35kV pulses to get through? I’m referring to the first of the two diagrams here and the converse for the second.
 

DickCappels

Joined Aug 21, 2008
7,449
I am concerned about the voltage coming out of the two diodes in series. If it is 800V or more you might have problems. I do not expect 30 kV on the left-hand circuit, but I think you are aiming to get several KV positive spikes out and those are what should concern you.
 

DickCappels

Joined Aug 21, 2008
7,449
Yes, you need the grounded diode to be able to handle the highest peak (or most negative peak) voltage your circuit generates on the output because that is the reverse bias voltage of that grounded diode.
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
Gotcha, but in an ideal world no negatives should get through the two diodes but as you said earlier only trying it will fully tell.

When I have received the diodes and tried it I will post some traces.
Thank you for your help.
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
True, but the diodes have capacitance. Looking forward to seeing your traces!
I’ve received the HV diodes but when I check them with a meter set to ‘diode’ they read zero. Usually you get the forward voltage drop showing and 0 or open circuit with reverse bias. Does this mean they are faulty?
36FD57C2-EA0B-4E96-8FD3-43BCB50F735E.jpeg
 

DickCappels

Joined Aug 21, 2008
7,449
They are probably fine. In high voltage diodes (I think you were going to order 20 KV diodes) there are many individual diodes in series so the forward voltage will be several volts rather than 600 or 700 millivolts, and that will be out of the range of your meter.
 

Thread Starter

JulesP

Joined Dec 7, 2018
374
They are probably fine. In high voltage diodes (I think you were going to order 20 KV diodes) there are many individual diodes in series so the forward voltage will be several volts rather than 600 or 700 millivolts, and that will be out of the range of your meter.
That makes sense. Yes they are 20kV. Next week I will have a go at testing them.
Thanks
 
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