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HV Question

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Monkeyman87

New Member
I have a HV power supply that can start an arc at approx. 1". This would be about 25Kv if my calculations are correct. After the spark fires across the gap is there any way to induce a high current across the ionized path that the spark made? BTW the supply is a MOT w/ 15 turns on the primary connected to a PC power supply at 12v and switched by a PIC MCU at 500Hz. Thanks

MonkeyMan
 

Styx

Active Member
errr,

Think abt the power!!!!

You say you have 25kv, now ok you can back that voltage off 1c an arc is produced since big volts are needed to produce an arc. But you will still need a fair few volts to maintain, thus

"Think of the power!!!"
 

stevez

Active Member
I don't think that the open or short circuit voltage of a simple AC welder is all that high however, once and arc is established the gap is rather significant. I guess that is most likely because of the established ionized path. Implications are that you might establish the ionized path with high voltage/low current and maintain it with a much lower voltage at much higher current. This almost sounds like HID lighting. Take a look at info on old carbon arc lamps or a carbon arc welder. Again, don't think the voltage is all that high but gap is significant from what I recall.
 

ukeee

New Member
I agree with Stevex, I think that once there was an ionised path the voltage required to mainttain it would drop, allowing you to pass higher currents through. It is possible that the effective resistance of the air would continue to decrease the longer the ionised path was maintained for, this is certainly the case in fluorescent lamps but that is a slightly differenct case as the gas is contained within a tube. I'm not sure what would happen in a gas that was not contained in something.

The gap would be significant, the greater the gap the more voltage would be required to maintain the ionised path.

Hope this was some help, what are you trying to use it for, sounds interesting.
 

stevez

Active Member
Proper welding requires a small gap however just out of curiosity I've let the electrode melt away and the arc will continue until the gap seems quite large-at least 1/2 inch. Part of the reason may be the vaporized metal. One way to find out it try it with a tungsten electrode.
 

Monkeyman87

New Member
I was thinking about making a carbon arc light. How much time after the spark ionizes the air do I have before the low resistance is gone. I thought maybe I could program the PIC to output a high current across the spark gap imediatly after the MOT breaks down. I am looking into Carbon arc lamps on the net and have found some informative sites. I'm going to check out HID, as I don't know to much about it. thanks for the input
 

spuffock

Member
I am reminded of the flash triggering circuit for a ruby laser. The high voltage appears across the secondary of a ferrite cored step up transformer in series with the flash tube and its capacitor bank. The trigger pulse ionises the flashtube, and current starts to flow through the tube. The ferrite core saturates and the inductance of the secondary effectively disappears, allowing the flash current to rise rapidly to its peak value. Hope this helps.
 

Monkeyman87

New Member
The ferrite core saturates and the inductance of the secondary effectively disappears,
How do I saturate the Ferrite? Does saturation depend on the frequency of the current pulse on the primary?
 

gerty

Member
The carbon arc lights (old drive in movie theatre projector) that I've dealt with used 26 volts ac at 160 amps.. We used a welder at one point when the original transformer burned up. the carbons were 3 feet long, 5/8" dia. uncoated and in motorized electrode holders. As the electrode burned away the motor would try to maintain the proper gap. I don't remember what the proper gap was, we could tell by the sizziling sound it made when all was well..
 

spuffock

Member
Saturation depends on the strength of the magnetic field inside the ferrite, which depends on the current in the winding and the number of turns. The core should have a high al value, that is it should give a high inductance for a small number of turns. It must not have an air gap. On applying a voltage to he inductor the current will at first rise slowly, then as the ferrite can support no more magnetic flux, the current will rise much more rapidly.
 
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