Hi everyone.

I was told by someone here (Bluelight I think) that I could use a microwave oven transformer to ballast a carbon arc. I want to try, but in another way. The primary and the secondary of the MOT are in series and I hooked up a 0.8 µF 2000V capacitor in series with the ballast and the carbon rods. The capacitor has a 10.4 MΩ resistor in parallel.

My theory is that the capacitor would stabilize the discharge (like a CWA would do) and raise the power factor, thus reducing the amount of heat emitted by the transformer and reducing the power consumption.

I've not tested the circuit yet, and I want to know if it would work and how many volts/amps it will give.

This is what I meant:


I think putting a capacitor on the secondary side will bring the arc power down a lot and I'd say it won't be possible to draw any arc between the carbons.

However the MOT will overheat quickly when run like ballast (few tens of seconds max). It's possible to put two shorted MOTs in series and increase the runtime to a few mins probably. Or three, four...

If you're using 120V mains and one MOT, the current's gonna be around 40 Amps with shorted electrodes and will probably drop a few A when you draw the arc out. With 2 MOTs, the current will be under 20A.

The circuit I want to test is the following.

My intention is to only use the resistance of the transformer to reduce the current. The capacitor would help to do the job.

Well, then I can assure you that won't do anything The MOT secondary has a huge inductance and from 120 volts will draw almost nothing.

Well if I get only 1/4 of an Amp, then the carbon arc will produce almost no light. Do you have a rough idea of how many Amps it'd give?

Well if I get only 1/4 of an Amp, then the carbon arc will produce almost no light. Do you have a rough idea of how many Amps it'd give?

You mean your schematic? I'm not really sure but would say under 0.1A

oh, worse than I thought LOL. How about a circuit with the primary, the capacitor and the arc in series (shorted secondary)

With the shorted secondary you get only the primary resistance (the leakage inductance is not so high), what will yield most likely too high current and for sure excessive power dissipation in the MOT. Using two parallel 1kW incandescents (halogen,...) as resistive ballast (same technique as blended lamps) will do electrically the same job and the dissipation of the power would be much easier (it will be radiated away).

What you might do is to make an air-gap in the core (to lower the MOT's main inductance) and then use the primary as serial reactor ballast, with the secondary open circuit.
Larger airgap yield higher current...

This is what I meant:


I think putting a capacitor on the secondary side will bring the arc power down a lot and I'd say it won't be possible to draw any arc between the carbons.

However the MOT will overheat quickly when run like ballast (few tens of seconds max). It's possible to put two shorted MOTs in series and increase the runtime to a few mins probably. Or three, four...

If you're using 120V mains and one MOT, the current's gonna be around 40 Amps with shorted electrodes and will probably drop a few A when you draw the arc out. With 2 MOTs, the current will be under 20A.

Well, I don't have the possibility to modify the MOT in any way. Maybe if I give you the specifications I want, you might know what to do as I've no idea now:

I'd like to have a 120V AC ~5A ballast. I have two microwave capacitor at my disposal, one is 0.8µF and 2000V with a 10.47 MΩ resistor. The other one is a 0.86µF and 2200V with (I think) a 8.46 MΩ resistor. I'm wondering if I could use one of the capacitors (or both in parallel) to reduce the current and the ballast overheat.

Vince....its alot easier to Ballast from the Primary than the Secondary Side...try doing what i did.....if you can...find a Transformer that has a 240 and 120 Volt Coils...feed power (120 Volts) into the 240 Volt Side and connect the 120 Volt side to the Microwave Oven Transformer's Primary...the step up/step down transformer i used had a 1KW Rating...and it gets hot in about 10 Minutes...my setup lowers the current enough to preven the Breakers from Tripping but this setup will still get fairly warm.

With the shorted secondary you get only the primary resistance (the leakage inductance is not so high), what will yield most likely too high current and for sure excessive power dissipation in the MOT. Using two parallel 1kW incandescents (halogen,...) as resistive ballast (same technique as blended lamps) will do electrically the same job and the dissipation of the power would be much easier (it will be radiated away).

Well, with a shorted 230V MOT, the power is around 5000VA and 1500W, so most of the power still gets "lost" in the shunts...

Well, I don't have the possibility to modify the MOT in any way. Maybe if I give you the specifications I want, you might know what to do as I've no idea now:

I'd like to have a 120V AC ~5A ballast. I have two microwave capacitor at my disposal, one is 0.8µF and 2000V with a 10.47 MΩ resistor. The other one is a 0.86µF and 2200V with (I think) a 8.46 MΩ resistor. I'm wondering if I could use one of the capacitors (or both in parallel) to reduce the current and the ballast overheat.

I don't know the MOT rating, but if the secondary is "1500V" and below, you might do an experiment:
Connect capacitor (one or both) to the secondary and then the use the primary as your reactor ballast. What the setup does: It transform the capacitor's value to the primary to approx. 200..300uF, what would yield about 5..10A. Something would be subtracted due to the MOT's main inductance, but this wouldn't be much...

The MOT was originally used with the 2200V capacitor, so no problems with them...

Could I use the primary with one the capacitor and the load in series? As the capacitor has a resistance, it'd reduce the current and it would raise the power factor if I'm right. The secondary would be open circuit.

These capacitors have too low value, so too high impedance to pass any usable current directly in the 120V circuit (the 0.86uF one about 40mA).

The MOT was originally used with the 2200V capacitor, so no problems with them...

Could I use the primary with one the capacitor and the load in series? As the capacitor has a resistance, it'd reduce the current and it would raise the power factor if I'm right. The secondary would be open circuit.

I also have a 8,7 µF 300V AC dry-type capacitor which came from a shorted out H38 ballast. It would probably work.