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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
plazmatron wrote ...
I had actually considered using relatively low resistance wirewounds as inductors for a marx gen, but it turned out that water cost a lot less
I have tried choke charging before, but you do need quite large air core inductors for it to work well, and I ended up with a cumbersome Marx gen to say the least.
The only real drawback with water/sulphate resistors, is that resistance changes with temperature, so if your resistor is dissipating quite a few watts, the resistance change as a result needs to be taken into account. (OR you increase the SHC, ie build a physically larger resistor).
For pulsed power applications like in a Marx gen, water resistors can be built fairly small (mine measure 30mm x 6mm), and since they are made from copper and clear tubing, they actually look quite tidy as well.
Les
You could change the electrolyte to change the colour scheme
The problem with inductive charging is that the Marx starts to think of itself as a lumped-constant transmission line, and may act out as a resonant charging system. If the charge on the capacitors, and the current through the inductors isn't exactly nil at the end of the discharge, L and C start talking to each other - backwards and forwards in queasy undulations - the sultans of swing!
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Michael Chen wrote ...
For the moment, Ill use several common resistors in series in order to do a proof of concept. If it works, I'll try to make a water resistor.
By all means do. They work ok for a while with little proof of concept marx's like Mikes "quick and dirty marx", but for the long term, you will need a better solution.
For water resistors I am using 4mm ID plastic tubing ~30mm long with crimped copper tubing electrodes in each end. Up here, ordinary boiled tap water measures approximately 1M per resistor.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
Proud Mary wrote ... The problem with inductive charging is that the Marx starts to think of itself as a lumped-constant transmission line, and may act out as a resonant charging system. If the charge on the capacitors, and the current through the inductors isn't exactly nil at the end of the discharge, L and C start talking to each other - backwards and forwards in queasy undulations - the sultans of swing!
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Proud Mary -
Did I read that correctly - your Marx is powered by an 8kV, 180ma NST? Sounds like that must be a beast of a transformer; is 180ma a common secondary current rating for UK NSTs?
Plazmatron -
You warned against the use of carbon composition and metal film resistors in Marx charging circuits; did you mean carbon film? I would have thought that carbon composition resistors, with their physically large volume of solid resistance element would be the most robust type for this application. If you did mean carbon composition, what is the failure mode, or why are they unsuitable?
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Herr Zapp wrote ...
Plazmatron -
You warned against the use of carbon composition and metal film resistors in Marx charging circuits; did you mean carbon film? I would have thought that carbon composition resistors, with their physically large volume of solid resistance element would be the most robust type for this application. If you did mean carbon composition, what is the failure mode, or why are they unsuitable?
Regards, Herr Zapp
I meant carbon composition. It isn't power dissipation that causes failure, it is the high voltage stresses on the composition. Carbon composition contain all sorts of things in the composition as well as carbon. I seem to remember some paper I read a while back on the failure modes under these kinds of stresses, and it included internal "tracking", growth of "dendrites" within the composition and other weirdness.
In commercial Marx generators, Wirewound, or sulphate resistors are used, as suitable compositions make for very expensive resistors.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
To El Pinko:
Pinky's Brain wrote ...
Would a small series resistor help?
Bound to! Critical damping is the way to go, so long as by 'small resistor' you mean a very generously-rated, fireproof, resistor of low ohmic value.
To Herr Zapp
Herr Zapp wrote ...
Proud Mary - Did I read that correctly - your Marx is powered by an 8kV, 180ma NST? Sounds like that must be a beast of a transformer; is 180ma a common secondary current rating for UK NSTs?
I've no idea how common they are, or were, I should say, in this size. Tunewell was the main UK manufacturer of the pitch-potted NSTs in a tin box. Luckily for us, the centre tap wire rises up out of the tar pit like Excalibur, and connects to a ceramic insulator assembly from which it can be disconnected with a screwdriver.
I suppose the very large neon signs that once decorated city centres must have needed lots of current, but it's not an industry I know anything about.
I also have a Tunewell 5kv-0-5kV 30mA, which seems the most common size. The grey painted steel sheet box is the same for both sizes, but I have not the smallest desire to sweat off all that tar with a steam hose to find out what's underneath - like hauling a squirming and obviously unhappy alien out of its bio-armour shell on Independence Day.
Nowadays Tunewell make only electronic NSTs, but you can still find the old shunted 50Hz types on ebay, where I bought both of mine for about £20 each a few years ago, together with 30 metres of 10kV screened cable, which the seller threw in as an extra.
Registered Member #2887
Joined: Sat May 29 2010, 11:10PM
Location: Panama City, Panama
Posts: 107
After experimenting, I found out a cheap way to make a high voltage relay . (my attempt with acrylic didn't work.) Lego is a very good insulator, and my next material to try to build a relay. If everything goes right, I'll have a working hv relay this weekend.
For the spark gap, I'll be using a PVC pipe segment as a housing, with acorn nuts and some screws as the terminals. Ill post pictures soon.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
wrote ...
4) Any other foreseeable problem?
Absolutely. A Marx Generator puts out probably the nastiest EM pulse out of all high voltage circuits.
This means you don't run in your house wherever you have sensitive wiring / cables running near the room. This includes thermostat wires, network cables, alarm system cables, TV cable, etc... These all act as high impedance sources (antennas) that will collect that EM energy and induce very large voltages on these wires.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes. I've seen one trash a laptop at a range of 20 feet.
(Admittedly it was Mike Harrison's million-volt Marx, the laptop was plugged into the wall with its AC adapter, and the pulse from the Marx probably got into the wiring. But either way, it still turned a nice Sony Vaio into a doorstop.)
Registered Member #2887
Joined: Sat May 29 2010, 11:10PM
Location: Panama City, Panama
Posts: 107
Sorry for the long delay, but I have been busy with school and stuff.
Would some mov's be enough to assure there is not going to be kickback through mains? I'm going to try to make the hv relay, as it cannot be bought in here (Panama), and shipping is expensive here. I know that if I do not disconnect the Marx from the flyback, sooner or later the diodes inside the flyback are going to die, but I must (at the very least) be certain that no significant kickback will go back through mains.
You may want to go a bit higher than 15kv to get consistent gap operation
Thanks for the feedback, I raised the voltage to around 30kv. Ill see if I get another flyback to raise current as well.
PS: If anyone knows how to make a good and cheap HV relay, could hint me? Preferably without solenoids, as I haven't seen any in my local electronic store.
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Marx generator questions
