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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Hey
Will I be able to drive a marx generator with what I assume is a solid state NST (4kV 20mA 20kHz), it cant really arc without shortcircuting the wires on the secondary, but can draw like 40 mm arc after that.
the ceramic capacitors are rated 4kV 1nF and the resistors are rated 2,5kV 1,5Mohm 0,5W
28. april: video uploaded,
15. april: THE WORKING PRODUCT descriped further down.:
Registered Member #1083
Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256
Seems fine. Just make sure you get some suitable high voltage fast-switching diodes. The caps seem a little maxed out, you'd be running them right on the edge of their rating. Also, when AC is rectified, the voltage is the peak voltage of the input ac. The peak of 4kV is 4000x1.4 which is around 5.5kV. A little too stressful for those caps, I think. It would be best if you used a voltage divider or a variac and run it at about 1/2 to 2/3 input voltage.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
I agree with what Andy said, you need a couple of high voltage diodes (or strings) and you are all set. I'd probably connect two caps in series per stage though, and use the full voltage of the NST, or even double the voltage with a full wave doubler . The reason is that it is a lot easier to get the spark gaps firing reliably i f you have 10kV across them, with only 5kV it really becomes a hit-and-miss affair.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
It's always a good idea to place a large reservoir capacitor on the base feed charging line, and then physically disconnect the power supply (by means of a vacuum relay if you can get one, or a very large one-shot spring-loaded switch) immediately before triggering the first gap.
This will prevent reflected energy being coupled into the power supply, (which may well destroy it), and also stop the Marx using the mains as an earth return, which can have unpredictable results. The Marx should ideally have its own low impedance earth unless it is a very small one.
Don't forget you can wire two of your 1nF 4kV capacitors in series to effectively create one capacitor of 0.5nF 8kV, which will hold off your supply nicely.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Thanks for the replies
I have some 1N4007 I could put in series to rectify, 5 of them should be enough for 4kV.
I will only make half the steps planned then, and put resistors/capacitors in pair series.
Would adding a line filter before the nst be adviseable and should it be connected so load/line are switched around compared to normal installation or does it work the same both ways, ive heard both things said...
This is a rather small project and basicly one of my first with HV next efter shortcircuting a nst.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
That's not going to work, the 1N4 series only works at low frequencies such as 50Hz. You better try to find a proper high voltage diode or a bunch of fast diodes to string up.
Line filter in front of the NST is probably not going to do much, but depending on how well the Marx works you may fry you NST and should put protection after it, as Harry said above.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
MadsKaizer wrote ...
I have some 1N4007 I could put in series to rectify, 5 of them should be enough for 4kV.
Once you have rectified the output of your 4kV NST you will have about 5.64kV, and since the claimed 4kV is likely to be only an approximate figure, you should base your idea on the diodes having to hold off at least 6kV.
As Dr. Shark has said, the IN4XXX series rectifiers can not switch fast enough for an electronic NST, but you can use its fast equivalent UF4007, which you can buy easily on ebay. I bought some myself last week at 10 for £1 (about 1.25 EUR).
Like 1N4007, UF4007 can block up to a maximum of 1kV DC, but it is good design practice always to rate components well within their safe operating area, so I would suggest using at least 8 of them to rectify your 4kV to reduce risk of premature failure and the dissapointment this causes.
You could construct a break-before-make firing device, so that your power supply is always disconnected before the first gap is triggered.
If you do not trigger the Marx. but wait until it builds up and discharges itself, you won't be able to disconnect the power supply, and will risk damaging it.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Im too used to only work with 50Hz so its just another constant in my world, I have alot to read and study before my understanding of HV is anywhere near where I want it to be.
Order for the last components I needed for my 555 circuits and some for the marx was placed some days ago, so ill wait till next time i need something to order more from RS. afterall money is tight as a student.
I got 3 of the said NST, got them real cheap so I can afford to loose one due to risky experiments.
I checked out the UK ebay and there is some nice stuff around a fair prices from f.ex. "HV supplier". If I were to order some rectifier(s) would you advice on a single large one or multiply smaller ones in a string, prices aint that different for same result it seems.
future plans would be to try this scrap one from parts I got to get some experience, burn something and then build a better one when I fully know what to do.
I hope im not too annoying with my lack of knowledge and eager to see some sparks :o
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
MadsKaizer wrote ...
Im too used to only work with 50Hz so its just another constant in my world, I have a lot to read and study before my understanding of HV is anywhere near where I want it to be.
Order for the last components I needed for my 555 circuits and some for the marx was placed some days ago, so ill wait till next time i need something to order more from RS. afterall money is tight as a student.
I got 3 of the said NST, got them real cheap so I can afford to loose one due to risky experiments.
I checked out the UK ebay and there is some nice stuff around a fair prices from f.ex. "HV supplier". If I were to order some rectifier(s) would you advice on a single large one or multiply smaller ones in a string, prices aint that different for same result it seems.
future plans would be to try this scrap one from parts I got to get some experience, burn something and then build a better one when I fully know what to do.
I hope im not too annoying with my lack of knowledge and eager to see some sparks :o
RS Components is a very expensive way to buy components, and I wouldn't be able to follow my hobby if I had to buy all my components at RS. I just enter the component name or part number into a UK ebay search , and it will usually - but not always - find something at a good price.
With parts like 555, you will have no difficulty in finding a supplier that will sell you them at 5 for £1 (1.25 EUR) so there is no reason not to have spares at hand when the inevitable failures and accidents happen.
Also, if you are just starting out, resistor and and capacitor selection packs are a good way - and usually a cheap way - to have a small stock of each of the preferred values in stock so you can build a lot of things without having to order small numbers of components.
As with life, we all had to begin somewhere, with nothing, so you should never feel bad about asking for advice if our hobby is to stay alive. The thing is, to try to learn all you can yourself, and only to ask for help when you are in difficulty about which way to go next.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
If you need to order abroad anyway, you might consider these HV diodes: I have 100 of them and they work great, much better than failure prone diode strings.
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Marx generator with 4kV SSNST
