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Registered Member #1564
Joined: Wed Jun 25 2008, 05:09PM
Location:
Posts: 11
Hello guys, I’m new to this forum and high voltage experiments.
I would like to make a high voltage DC power supply, perhaps up to 150kv. I intend to make a voltage multiplier and have purchased a plasma driver unit from Information Unlimited as the driver. The driver produces up to 20kv pk @ 20-50 khz variable.
Information on the driver can be seen here:
As the multiplier stage I would like to use doorknob type capacitors partly because they look good and also I think they are probably more durable then the disc type caps
For the diodes I intend to solder a string of 1-2kv diodes in series to achieve the desired value.
My question is about the value of the capacitors and diodes used
Capacitors: how will the capacitance value effect the function and what would be a good match for my driver unit?
Diodes: I have read that fast recovery avalanche diodes 1kv + will be required. I aim to use diodes with a minimum of 1amp value but may consider higher depending what I learn from other members of this forum who have made similar units. What would be the minimum recovery time rating I should consider. My aim is to use fairly cheap diodes for this purpose as I will obviously require many…..any ideas?
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
ok, so it seems you are planning a cockcroft-walton multiplier, so as the values of the caps increase, the overall power would, but at a cost: a higher value means that you must have a lower frequency in order to charge the caps and discharge, meaning a longer period of time having the output at that of the plasma driver..., also the current would likely be higher than typical for a CW multiplier... what kind of power are you looking for?
Registered Member #1564
Joined: Wed Jun 25 2008, 05:09PM
Location:
Posts: 11
Hi DaJJHman, Yes I am looking to use the plasma driver to power a cock croft-walton stage. Higher Capacitance = lower driving frequency? I thought it would require a higher driving frequency, is this due to resonance (higher capacitance = lower resonant frequency?
I'm looking for a voltage perhaps up to 150kv haven't got a specific amperage requirement but as I have already purchased the plasma driver I guess a CW stage which makes the most efficient use of the driver I have.
Edit:
I would want the output to be as stable as possible ie not pulsed DC. Large value capacitors would produce pulsed DC at the stage output?
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
EHT wrote ...
Hi DaJJHman, Yes I am looking to use the plasma driver to power a cock croft-walton stage. Higher Capacitance = lower driving frequency? I thought it would require a higher driving frequency, is this due to resonance (higher capacitance = lower resonant frequency?
I'm looking for a voltage perhaps up to 150kv haven't got a specific amperage requirement but as I have already purchased the plasma driver I guess a CW stage which makes the most efficient use of the driver I have.
Edit:
I would want the output to be as stable as possible ie not pulsed DC. Large value capacitors would produce pulsed DC at the stage output?
I am afraid you misread, I meant the OUTPUT of the CW multiplier has a lower frequency if higher values are used... the driver is perfectly fine... just make sure that if you do go to the max it can put out (15Kv if I did not misread) that each cap is rated for that, and the diodes used, I forget if it is just the rating of the driver or the whole assembly
also, the larger values would not make it pulsed DC, BUT what would happen is that (and this happens with all CW multipliers) while the output overall is continuous, you have some time in the wave output where you have 150Kv, then at the bottom of the "wave" you have the driver alone flowing through the diodes, making it DC [but pulsed more or less, it varies from unit to unit] (the "bottom" as I referred to it as is the time when the caps are charging and the "top" is when they fire)... a smoothing cap might help if that is an issue, but then again it is a large difference for that cap to smooth?
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Actually no matter what your capacitor size and frequency, the capacitors will charge to the supply voltage. If your capacitor values are large, or your frequency is low, it will simply take more cycles to do so.
The DC voltage shifting on the output that DaJJHman is referring to is called "ripple", and for most amateur purposes, such as arc drawing and capacitor charging, it does not matter. It will be minimized, however, by a higher driving frequency.
Also, as you increase the drive frequency you will be able to push more power through a smaller value of capacitor. I don't know the math, but maybe someone else will chime in (or you could try google/wiki) with the formula to calculate what capacitance (minimum) will allow you to take full advantage of the power your transformer has to offer (@50kHz).
Oh and one more thing, even at 20kHz you will still need fast diodes, and 50kHz would be best. You will need to do some searching here...
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
(Edit: With regard to the diodes, I mis-read your original post. It will be a lot less hassle, much cheaper, and more reliable if you simply buy some fast 20 kv diodes on Ebay).
If you don't have unlimited money, your design will likely be constrained by what parts you can get at a reasonable price, more than anything else.
Lastly, you should get a circuit simulator (I recommend CircuitMaker 2000) and look at how the multiplier's output varies with frequency, load, # of stages, and capacitance values. Look to see what capacitors are available on Ebay at a reasonable price and use these values as a starting point for your simulation.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
Oh and I just remembered, for a cheap source of tested-reliable HV capacitors, try here: The author himself has successfully used them in a CW multiplier.
You'll need to make 10 for a 5 stage CW multiplier, but hey, just about all it costs you is your time.
Registered Member #1564
Joined: Wed Jun 25 2008, 05:09PM
Location:
Posts: 11
This would be my second CW Stage I have constructed. The first one used 20kv diodes which I purchased on ebay and disc capacitors 25kv @10nf. I potted the stage in an acrylic tube with paraffin wax, it worked great, first time it I switched it on the output was very strong, hissing loudly and filling the air with ozone......then SNAP, a flash and the hissing stopped, the diodes were fried.
I read CW construction notes on a website (cant remember where), they had exactly the same problem and said the diodes were not robust enough for the application. The recommendation was to solder diodes in series and pot them, that way they can withstand a higher amperage.
Since I fried the last CW stage I want to construct a better one this time, select the components with more thought and hopefully make something that is not so sensitive.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
You were using 20kV diodes with a driver that's rated for 20kV peak? What happens if you mains voltage happens to be a little high that day? You need to de-rate your diodes, and probably your capacitors, a lot more than that. And potting your diodes won't make them withstand higher current, it will only make them less likely to flash over at too high a voltage.
But you shouldn't even really need to worry about the diodes' current ratings. If you don't have a limiting resistor on the CW's output, then the capacitors will instantly discharge through the diodes, which is a huge current pulse that could be in the hundreds of amps. Understand, most multipliers of this type are not used for high-power. They are used for air ionizers, charging capacitors, etc. For arc drawing you will need a few Kohms minimum on the output.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Not sure if this was covered or not, but in a C-W machine, the capacitors and diodes see 2X Vpk, so you need 40kV components (AT LEAST) with a 20kV peak input voltage.
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High voltage DC power supply construction
