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Author Topic: How many watts/amps can a polyvaricon take?  (Read 3765 times)
RUSTYSTUFF
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Posts: 17




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« on: July 27, 2011, 09:35:50 PM »

I'm building a L-Match antenna tuner (from the plans in the 2007 Arrl handbook). For the variable cap i bought a few 0-200pF polyvaricons and I'm wondering how much power could they handle? I figured that QRP levels are fine, but i'm not sure what kind of radio i'll end up with yet. Smiley
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AA4HA
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« Reply #1 on: July 28, 2011, 05:47:18 AM »

According to the Mitsumi datasheet there is not a breakdown voltage rating shown for the devices. I imagine that if you did anything more than QRP you would be arcing across the inside of the capacitor.

http://www.datasheetarchive.com/Polyvaricon*-datasheet.html#contextual

Tisha
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
AJ8L
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Posts: 6




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« Reply #2 on: July 28, 2011, 05:52:18 AM »

I have a couple of different configurations of matching networks that I have built using polyvaricons including L networks for random wires. I have used new and salvaged units (salvaged from transistor radios). I have never seen a problem up to a 20 watt carrier. I have not tried more than that. I would bet that 20 watts is just about the limit but that is just a guess. They are great for pocket size tuners. Hope this helps.
Geoff, AJ8L.
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KE3WD
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« Reply #3 on: July 28, 2011, 08:49:04 AM »

Consider them to be "QRP" grade as far as power handling, with about 20W likely being the upper limit. 

I have used them successfully in QRP on magloop antennas, as have others. 



73
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WB6BYU
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« Reply #4 on: July 28, 2011, 10:07:58 AM »

The limit isn't watts, but volts.  (Or perhaps amps in certain circuits.)

I built an "Ultimate Transmatch" using some big variable capacitors from a 400W PEP transmitter, and arced
them at 10 watts when matching extreme loads on 80m.

The L-match is better behaved, but still the power limitation will depend on the impedance you are trying
to match, and the resulting voltage across the capacitor.  If you are matching a long wire, changing the
wire length will change the power rating.  So you will just have to try it and see.

But I don't remember reading any reports of such capacitors failing in L-network tuners at less than 10watts,
so you're probably safe up to that level as long as you use a reasonable circuit and wire length.  I wouldn't
put them in a "T" tuner and try to load up a 18" clip lead on 80m, however.
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RUSTYSTUFF
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« Reply #5 on: July 28, 2011, 07:47:59 PM »

Thanks for all the info.


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TANAKASAN
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« Reply #6 on: July 29, 2011, 10:27:45 AM »

This post got me curious so purely in the interests of research I set up a signal generator and power amplifier at work then built a low-pass filter using two polyvaricons from my junk box at home and a convenient coil which I found on someone's bench. The whole thing fed into a 200W dummy load. With the input cap half meshed and the output cap fully meshed I had a 3.9 MHz low pass filter so the signal generator was adjusted to 3.8 MHz

Up to 22 watts there was no problem.

At 28 watts I could detect a little warmth from the output cap but the input cap was still fine.

At 35 watts both caps were too hot to touch.

At 41 watts the amp tripped at about the same point as the output cap failed. There were no sparks or fireworks, it just failed short circuit.

41 watts into a 50 ohm load is 45.28 volts RMS 0.91 amps. I suspect that the failure was due to RF heating of the dielectric rather than voltage breakdown. Both variable caps were pulls from scrap radios and of unknown manufacture.

Tanakasan
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