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Proud Mary

Sat Jun 06 2009, 09:46PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

polop wrote ...
without the resistor the spark was meaty but people always reccoment using the resistor why is this? surely the lack of current on the input (due to pathetic transformer) will limit the current output any way. thanks for he help

Many enthusiasts - mostly for financial reasons - operate components very near their maximum ratings - so a large surge current may be sufficient to destroy them.

Also, remember that your spark is not an electric current in one direction only, but an oscillatory train of damped waves which the diodes must hold off when it comes back at them.

Finally, consider the current that must flow when a capacitor is shorted out through a very low resistance - as the resistance approaches zero, the current must also approach infinity!

So the series resistor is there to reduce posssibly destructive stress on the Villard ladder. Did you know that capacitors subject to fast discharge have a finite life-time? Some big pulse capacitors in professional Marx generators have life expectancies of less than 100 shots. So keep stress down if you want your set-up to last.

If you want to discharge capacitors into low resistances - sparkz n arcs - a Marx generator is much suitable to play with.

The Cockcroft and Walton/Villard type of voltage multiplier is the most suitable when you need to supply continuous high voltages at low currents.

Hope this helps,

Harry.

polop

Sat Jun 06 2009, 10:16PM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
Location:
Posts: 51

the rating of the components are well in spec, the caps i am using are 2kv, supply is roughly 300v, diodes are rated for an amp continuosly though the pulses on the output are very short. but i do see that the lifetie will be reduced. i want to also build a marx generator but without sounding like a wuss-- they scare me a bit. i have heard that they can very easily kill you as even on a very small one current can each 10 amps! is his true, have any of you gotten electricuted by a small marx generater and lived to reply to this question? if i kept the caps small (say 220pf 2kv) would the resulting marx generator be realy dangerous (supply being the hv circuit described erlier in the topic)

i ask these questions, though i am a very careful person, but i am afraid that one day my care would be lacking.

Proud Mary

Sat Jun 06 2009, 10:25PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

You do well to remember that high voltages can kill.

If you absolutely MUST make adjustments to a live high voltage circuit, always keep your left hand in your pocket (if you are right handed) as the across-the-body shock through the heart is much the most dangerous.

I would guess that most of our long-term members have had one or more high voltage shocks and none have been killed in the few years that I've been a member.

That doesn't mean to say that they COULDN'T have been killed - they may just have been lucky. But once you've been thrown across the room a time or two, you begin to pay real attention to what you are doing!

polop

Sat Jun 06 2009, 10:36PM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
Location:
Posts: 51

when i say dangerous i mean it in a relitive way. many people on this forum have specific training, degrees and jobs relating to high voltage. so for them a tesla coil with inouts of many kW is "safe", i on the other hand am a student,and electronics is a hobby of mine, high voltage being a part of that. so for me safe means if i were accidently to touch it(assuming my heart is healthy, which i am sure it is) i will be fine, if not a littly surprised and in pain. is it possible to build a marxs generator low power enough to reach those reqirement?

p.s a high value resistor on the output make a marx safe (enough) without ruining its impressivness ?

Proud Mary

Sun Jun 07 2009, 12:03AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Degrees won't save you from getting an electric shock, son, but great care will.

Stand well back from your experiments, figure out how to operate them remotely if you can, and always keep one hand in your pocket as I said .

There's usually no very good reason for adjusting LIVE high voltage equipment, so you just take double or triple care that everything is really switched off and disconnected. .
Take the plug out of the mains socket.

Never forget that capacitors charged with high voltages may retain much of their charge after hours or even days.

But it's not wise to say what is dangerous and what is not. What may harm one person may not harm another.

Your eyes and ears are your main safety tool. Use them. Always stand clear when switching on some new experiment for the first time.

Always, always, take great care that a component like an over-stressed capacitor cannot blow up in your face! Don't let yourself get into a position where such a thing is possible. Make sure all capacitors are fully discharged before touching them. Pressure failure - i.e. blowing up - can happen even for some time after the power has been switched off.

And always take the greatest possible care, just as you would with a hedge trimmer. chain saw, lawn mower, or car.

HV Enthusiast

Mon Jun 08 2009, 07:46PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

You can buy small resonant high voltage supplies that put out up to 15kV that are about half the size of a matchbox. They only cost about $30-40 bucks.

polop

Tue Jun 09 2009, 11:56AM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
Location:
Posts: 51

i've had a look for these small resonant transformers, i cannot find any with your description of them, any links?

HV Enthusiast

Tue Jun 09 2009, 12:09PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Emco High Voltage
http://www.emcohighvoltage.com/

Check out they're Q series HV DC-DC. About the size of sugar cube and 5kV output.
(See attached PDF)

polop

Tue Jun 09 2009, 02:02PM

Registered Member #1537 Joined: Thu Jun 12 2008, 06:44PM
Location:
Posts: 51

that company looks like the type that needs you to order a hundred or so before they sell to you (hidden prices) is there any sources of similar transformers that offered on a one piece basis.

Proud Mary

Tue Jun 09 2009, 02:08PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Venus Scientific Inc and Brandenberg both make miniature kV PCB-mounting modular PSU's but they are expensive. - (afterthought) but no way would you want to risk wrecking them by drawing arcs, so please forget this suggestion.

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