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Registered Member #627
Joined: Thu Apr 05 2007, 04:05PM
Location: London
Posts: 32
Hello all
I would like to build a marx generator that gives around 10cm discharges. I have a flyback circuit I would like to use as the power supply. I would reduce the input voltage to try and get around 5kv output. Its this one here: http://www.youtube.com/watch?v=JOTn7A4BFfs
With an input voltage of around 5kv, I'm thinking I will need some resistors capable of withstanding 5kv, and also capacitors.
Does anyone have some info on what 1M resistors would be suitable, and where I might be able to buy them in the UK?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
People tend to use doorknob caps for Marx generators. You can use any cap that can withstand the input voltage. I don't think there is any special type of doorknob you need.
For resistors, you can just string together multiple 1-2W resistors. The total value of the string isn't too critical but a higher value would make the Marx fire at a lower rate. Some people have used inductors for charging the caps because it appears to be a low impedance to the DC but when it fires, the inductors look like a high impedance because of the fast rise time.
You could go for about 80kV with needle discharge points to get about a 10cm discharge (I think that's right).
Registered Member #627
Joined: Thu Apr 05 2007, 04:05PM
Location: London
Posts: 32
Thanks Myke. I am going to use 1000pf 6kV caps. Will try between 15 and 20.
How will the resistors hold up? I'm looking at 2W Carbon Film Voltage Rating:500V dc
May seem a silly question like putting 5kV through a 500v resistor but I cant see any 5kV resistors. Putting 10 in parallel seems a bit crazy and I haven't seen any designs like this. People just seem to use single 1M resistors. Perhaps with the low current it will be OK? I'm going to try using a Cockroft-Walton Voltage Multiplier like in an ioniser.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
If you mean 5kV RMS then your 6kV capacitors will fail toot sweet.
I am not at all sure what has prompted you to choose 1M, since the resulting pulse will have a very long time constant, which entirely defeats the point of using a Marx, but there are always plenty of suitable high voltage resistors on ebay, for example:
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
The 1M resistors don't slow down the pulse because the capacitors discharge in series bypassing the resistors. You want the resistors to have a high enough impedance so they don't interfere with the amount of energy that is outputted.
People tend to ignore the 500V rating on resistors and just use 3-4 1-2W resistors in series. I've put over 2kVAC across a 1M 2W carbon comp resistor before and all it did was heat up (Was a bleeder resistor I forgot to take off the caps of a high voltage power supply).
Proud Mary, Marx generators are charged by a DC source so the supply shouldn't put out anything higher than 5kV. It's always a good idea to have extra headroom for safety reasons so caps with a breakdown voltage of >6kV should be fine.
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