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Registered Member #1208
Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154
I vust bought a variac on ebay and am trying to drive a MOT with it. using a lightbulb ballast. when i turn up the variac, the light bulb gets brighter. is this normal? also, when i short the mot's output, the lighbulb gets very bright. is it possible that i am pulling more than the 52 rated watt through the lightbulb. also, since i hope to charge a cap bank with this, do the diodes in the full wave rectifier need twice the rateing of the desired output voltage. ex. for 2kv out do my diodes need to be rated for 4kv or just 2kv? also, in a cascade, if 2kv in, do the diodes need to be rated 2kv, 4kv, or 6kv? thanks for forgiving my ignorance.
Registered Member #63
Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425
ramses wrote ...
I vust bought a variac on ebay and am trying to drive a MOT with it. using a lightbulb ballast. when i turn up the variac, the light bulb gets brighter. is this normal?
Yes, somewhat -- as you turn up the variac, the primary is drawing more magnetizing current; since MOTs are built with barely enough iron and not enough copper, the idle primary current is probably pretty high.
wrote ... also, when i short the mot's output, the lighbulb gets very bright. is it possible that i am pulling more than the 52 rated watt through the lightbulb.
That's the right thing to happen, the bulb is limiting the power that will flow, and no, it won't draw more than its rating. Think about it... (Imagine a bulb in its regular application, then realize that in your application you're adding -more- impedance into the circuit).
wrote ... Also, since i hope to charge a cap bank with this, do the diodes in the full wave rectifier need twice the rateing of the desired output voltage.
Normally I'd say "hell why not", but your diodes only need to be rated for about double the input voltage. Considering the MOT secondary could well be putting out at least 2kV, 5kV or even 10kV diodes would be appropriate and conservative.
Registered Member #1208
Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154
thanks, but wouldn't the forward biased diode begin conduct before the reversed biased diode? also, i the light bulb filament was broken from the top post, and still connected to the bottom post. i think the filament was touching the other post about halfway along the filament's length. that would also explain why the light bulb blew the next time i turned it on. i don't plan on driving the MOT fully anyway(thus the variac), and i will only want ~1.8kv out. since i just also bought 100 2kv 1.5A diodes from EBay, do you have any advice on doubling up diodes in series? Thanks, Ramses
ps. sorry about the spelling errors on my first post. I suck at using Graffiti (palm's writing thing).
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Matt Bingham wrote ...
since MOTs are built with barely enough iron and not enough copper, the idle primary current is probably pretty high.
Are you sure they are actually built with copper? Aluminium has slightly higher resistance but is a lot cheaper so why not use it (yes, many MOTs have aluminium primary or even secondary windings)
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n00b mot questions and other things
Aluminium has slightly higher resistance but is a lot cheaper so why not use it 
(yes, many MOTs have aluminium primary or even secondary windings)