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Dual mot SGTC questions

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LAN_403

Toasty

Sun Jan 17 2016, 02:18PM

Registered Member #56785 Joined: Fri Aug 28 2015, 02:54PM
Location:
Posts: 20

Hi guys i am new on this forum and have a few questions about the dual mot SGTC i made and about tesla coils in general.

1. I sometimes see people adding line filters in their circuits to protect other appliences from harmful rf, however i don't see how that can happen. Wouldnt the rf pick the shortest way to earth instead of heading for your desktop, television etc.

2. I have a dual mot SGTC which i made with the TeslaMap program. However my spark gap sputters sort of. It isnt going off continues. I believe this has something to do with my ballast. I use a third mot from which the secondaries are removed as a ballast. Whenever i use a different ballast i get a continues flame at my spark gap.

3. When i use the mot as ballast i sometimes see a racing spark going from my first turn of the primary to the secondary coil.

4. I don't use a rf filter after my mot output. Is this bad? Does it have any effects?

Note: havent fully tuned my tc yet. I just tested it out yesterday.

Thanks in advance, will maybe make some pictures as well.

edit:
If i want to use a rotary spark gap what would be the target rpm i should look at? i have a mmc of about 40 nf right now. I can use a fan motor which is around the 1300 rpm. Is this suitable for rotary spark gap use?

Thanks

Sigurthr

Sun Jan 17 2016, 08:21PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

The hard part is that MOTs vary wildly in terms of output, especially these days where they are skimping on core mateiral and winding conductance. You just need a ballpark impedance for the MOT to work out your primary tank values. Without being to empirically measure the output I can't really advise you any.

I can't comment on RSGs, never used one.

Toasty

Mon Jan 18 2016, 04:21AM

Registered Member #56785 Joined: Fri Aug 28 2015, 02:54PM
Location:
Posts: 20

Yeah the fact that my output is unknown makes it hard to get it to work properly... i can guess that the voltage should be around the 4kv but the current is a different story

Hazmatt_(The Underdog)

Mon Jan 18 2016, 04:43AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

You're going to get flames at your RSG because of the tremendous current output from the MOT.

You will probably need to do two things:

1. From the MOT out, you will need to limit the surge current with a capacitor like the one usually attached to the MOT (.65uF or whatever they used)

2. Increase the tank capacitance. This causes CC charging from the MOT. You have flames because the tank cap is not sufficiently loading your charging system, the capacitor is charged quickly and the MOT can still supply more current, so your arc is not quenching.

Hopefully ballasting the outputs of the MOT's will be sufficient, but if its not you will probably need to go to step 2, which sucks because it shifts the whole design.

Check out other MOT TC's, there are a lot of schematics out there.

Toasty

Mon Jan 18 2016, 07:30AM

Registered Member #56785 Joined: Fri Aug 28 2015, 02:54PM
Location:
Posts: 20

Yeah, but something worth noting is that when i use a third mot as ballast and i fire the whole thing up the gap fires like it should for maybe 2 seconds.... after that it keeps firing irregularly. I think i will try to upgrade the mmc to maybe 80 nf and see how that works out.

Benjamin

Mon Jan 18 2016, 08:39PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

For my dual mot SGTC I don't use a ballast and I use a voltage doubler that consists of two microwave oven capacitors, one on each mot hv output, and a diode string across the other side of the capacitors. The diode string can be 4 microwave oven diodes in series or 30-40 1n5408's in series. The tank circuit is connected directly across the diode string with another mot secondary is series as a choke. I use a rotary spark gap running at about 400-500 BPS and a 50nf mmc capacitor bank. This system works great, it can run off a single 120v 15a circuit and makes sparks up to about 5 feet long. I have found the doubler circuit to be very reliable as long as the diode string has LOTS of voltage headroom. The doubler is very easy to build and should improve your results greatly!

Toasty

Tue Jan 19 2016, 04:45AM

Registered Member #56785 Joined: Fri Aug 28 2015, 02:54PM
Location:
Posts: 20

Benjamin wrote ...

For my dual mot SGTC I don't use a ballast and I use a voltage doubler that consists of two microwave oven capacitors, one on each mot hv output, and a diode string across the other side of the capacitors. The diode string can be 4 microwave oven diodes in series or 30-40 1n5408's in series. The tank circuit is connected directly across the diode string with another mot secondary is series as a choke. I use a rotary spark gap running at about 400-500 BPS and a 50nf mmc capacitor bank. This system works great, it can run off a single 120v 15a circuit and makes sparks up to about 5 feet long. I have found the doubler circuit to be very reliable as long as the diode string has LOTS of voltage headroom. The doubler is very easy to build and should improve your results greatly!

that sounds awesome, maybe the extra voltage will make it a bit easier to trigger the spark gap more reliable.

edit:
So after a little but of searching (a lot), i discovered that dual mot without a doubler is really hard to get it to work. I guess i have to make a doubler now with the mots.... but will a single mot cap work well as a choke? Orbis there a better circuit availible?

Benjamin

Tue Jan 19 2016, 06:37PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

Check out this page

for a good doubler design. A few notes about the circuit though; 1. It looks like the mot primaries are in series but to run it off of 120v they need to be in parallel. 2. I used 4 microwave diodes instead of 2 and didn't use the ceramic capacitors (Cf) and it works fine. 3. The 100W power resistors can be replaced with another mot using only the secondary with the primary unconnected. 4. I don't think that a safety gap or oil submersion are necessary when you are only using two mots. I hope that this helps!

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