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Could you use microwave oven caps in some tesla coils?

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LAN_403

803

Tue Aug 10 2010, 06:30PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

I'm not sure about DRSSTCs or SGTCs, but would it work in fbt tcs and VTTCs? It does seem like a good idea, I remember Steve saying these were good to around 8-10 kvolts, and they should have a high current rating.

What do you think?

PAul

klugesmith

Tue Aug 10 2010, 07:52PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

803 wrote ...
...and they should have a high current rating.
What do you think?
PAul

Construction is probably similar to AC motor caps, except higher V and lower C.
The normal current in a MWO capacitor is less than 1 ampere at 50 or 60 Hz.
What is the RF current in your intended TC primary?

Now I will stand aside, and await responses from real tesla coil experimenters.

Myke

Tue Aug 10 2010, 07:56PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

Even though I haven't experimented with TCs, I know that microwave oven caps aren't meant for the HF they would see in operation. The dielectric would heat up causing the oil to expand/boil and then you have a grenade on your hands.

They are not rated to 8-10kV but have been tested to that voltage with no failures. Also they are ok-ish pulse caps for low duty things like can crushing and disk launching. Not HF TC use.

quicksilver

Tue Aug 10 2010, 11:44PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

I concur with the above responses - as I once asked the SAME question!
To the best of my knowledge, Microwave oven caps are not high frequency cap. For a Tesla Coil they are not appropriate. CERTAINLY not for a SGTC. I seriously doubt that they would be appropriate for other types for the same reasons. Now if you had MANY solid state units, you could set up a MCC with Mylar caps but thats second place to good quality MMC standard material.

If you would like I could post specific types & brands.

For other applications, the MOC's can be used. - Like a "can crusher", etc

radiotech

Wed Aug 11 2010, 02:40AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

"Microwave oven caps are not high frequency cap."

Here is a "high frequency " capacitor.
1.0ufd 1000 volts peak 50 kHz.

Here also is a MOC

Edit If we applied the RMS voltages, 700 @ 50 Khz to the HF cap and 875 @ 60 Hz to the MOC , what would happen?

Matt Edwards

Thu Aug 12 2010, 12:42AM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

Although they are not intended for high frequency use, i have seen several diagrams using them for SGTC's along with a few MOTs. If it is all that you have then give them a try. But be cautious of exploding caps!

Mads Barnkob

Thu Aug 12 2010, 03:59AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Oidium45 wrote ...

Although they are not intended for high frequency use, i have seen several diagrams using them for SGTC's along with a few MOTs. If it is all that you have then give them a try. But be cautious of exploding caps!

Are you sure they were used for a primary capacitor and not just in the voltage doubler/trippler etc in the supply part?

HVgeek

Fri Aug 13 2010, 10:05AM

Registered Member #2998 Joined: Tue Jul 13 2010, 08:34PM
Location: Swedish forests.
Posts: 26

When it comes to SGTC:s I definately say NO. My coil fried six of them in series after only a few seconds, and one of them took the shape of an orange whilst hissing and giving off smoke. Don't worry, pcb free. The driver voltage was only about 8 kv, so it was not a simple overoltage failure (rated voltage on caps in paralell would be 12 Kv) .

I am trying to use MOC:s and a protective inductance as a way to smoothe the input of a VTTC though. Haven't gotten it to work yet, but I'll make another thread when I do and share my experiences.

radiotech

Fri Aug 13 2010, 05:25PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

A MOC , during the first quarter cycle, 5 mSec @ 50 Hz charges to the peak DC voltage , to ground, it can ( MOT secondary V * 1.4) through the diode. The magnetron is reverse biased at this time, (no conduction) At 10 mSec the transformer reverses. The high end it - . This rising - voltage is in series with the capacitor's charge, aiding. The magnetron cathode is negative by the sum of the capacitor voltage and the negative transformer voltage. This is a voltagfe doubler circuit. The capacitor never sees AC voltage.
Why doesnt it see AC? The silicon diode does no work. The diode formed by the grounded anode and floating magnetron cathode does work and has a dynamic resistance. The trick here is the time constant of the magnetron dynamic resistance and the capacitor.
The magnetron working voltage approachces the PIV that the rectifier is subject to. Should the heater burn out that is what it would be.

If either the diode or magnetron shorts, the capacitor then sees the full AC, the capacitor tries to resonate the secondary, voltage rises, and the transformer flux rises until the shunt limits it and the fuse blows.

Capacitors explode because of current primarily--voltage is secondary. Current depends on frequency for a given voltage.

For TC builders, A thermocouple ammeter wont lie- seek them out.

Current ratings for capacitors are shown in the table here

quicksilver

Fri Aug 13 2010, 06:46PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Thank you for posting that.
I wish I could get a copy of that book. It's entitled "The SCR Manual? or is that simply a chapter? Who wrote it?

Thanks again.

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