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Registered Member #1180
Joined: Mon Dec 17 2007, 12:26AM
Location: Nope
Posts: 28
Hello denizens of 4HV, I have a proposition to make to you all. I' am designing my first Marx generator, what I am thinking of is a probably a bipolar setup with 12 stages in either tower, I'm tentative about my capacitors (18nF, 12 kV) my question is though, is the diode setup in the power supply (which is an NST) a good plan? what should the specs be for the resistors (simply enormous, or are the equations I should follow along to?) but more to the point, will this lay out even work (sorry about the wretched quality, I only have MS paint and ten minutes to work with) Any and all help is much appreciated, this is my first leap into what I hope is going to lead me to my electrolaser
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
The schematic shows a fullbridge rectifier, but no dc smoothing cap, you would get voltage going from 0v-120v-0v at 120Hz, so wouldn't it still give AC to those transformers?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
This circuit will not work at all, I'm afraid,.
If we simplify the circuit by removing from it the bridge rectifier, and one of the NST's, so that you are left with one voltage multiplier, your problems are not over.
The high voltage output of an NST is, of course, delivered at 50Hz (or 60Hz in America) - the same frequency you put into it from the mains.
Now, all capacitors have a property called reactance - which you can think of simply as their resistance to the passage of an alternating current. In electronics notation reactance is designated by the symbol X, and if it is the reactance of a capacitor we are talking about, then the symbol is Xc.
But unlike the passage of a direct current through a resistor, the reactance of a capacitor depends on the frequency of the alternating current.
This relationship between frequency and the size of the capacitor is expressed in this simple formula:
Xc = 1 / (2Pi*f*C)
From this you will see that at 50Hz just one of your 18nF capacitors will have a reactance of 176.839k ohms - a high resistance, so not much current will be able to flow through it!
Your capacitors would have to be very much bigger - and very costly - to get more than a feeble trickle of current from your voltage multiplier.
In practice, Cockcroft and Walton generators are often driven at frequencies as high as 100kHz, precisely because this allows much smaller and less expensive capacitors to be used. The cost of the 100kHz driver supply circuit is small compared with the cost of the very large capacitors that would be needed were it to operate at 50Hz or 60Hz from a transformer such as an NST at mains frequency.
Registered Member #1180
Joined: Mon Dec 17 2007, 12:26AM
Location: Nope
Posts: 28
If I'm reading this right I just need to find a way to convince the NST's that they should run at 100KHz, short of cobbling a circuit together (which I abhor and can't hardly manage to save my life,) how am I to produce a pulsed DC wave short of a whole lot of jiggery pockery with inductors?
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
err guys, lol what's wrong with everyone today? :D
>voltage going from 0v-120v-0v at 120Hz, so wouldn't it still give AC to those transformers?
Therefore you'll get AC waveform with the amplitude of 60v (undervoltage) and 60v additional constant displacement (eg DC component). This means the xformers will blow anyway ;)
>Your capacitors would have to be very much bigger - and very costly - to get more than a feeble trickle of current from your voltage multiplier.
This is NOT a multiplier, it's a Marx ;) It operates on DC input - the trick is all the caps charge up through the charger resistors and, once the gaps are fired, are connected in serial through all the gaps, while the resistors don't add much effect (there are inductive Marx schematics as well, where the chokes do the same). So, this schematic would definitely work in case the NSTs would be replaced with DC-DC convertors. In the present case however, you just need two rectifying bridges after the NSTs without any on the input. 20kV diodes are not hard to make, you can see those in most CW multiplier projects - so just use that design, you need only 8 diodes after all.
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Why don't you save yourself alot of time and possibly some nst's by just making a zvs flyback driver running off 24vdc and powering the marx with that?
Registered Member #561
Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230
Did you say you where going to use this to power a laser? because Marx generators create pulses. For any gaseous lasers you need continuous power. Also you can't run an NST at 100khz, There designed for 50/60hz.
I'd start by moving the bridge rectifier to the other side of the NST, and get rid of the second NST and Marx generator. If you still really want to make a twin Marx generator do that after a single Marx
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Marx generator power supply
