Author Topic: My terry filter  (Read 164 times)

Offline thedoc298

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My terry filter
« on: October 11, 2019, 01:37:11 AM »
I have a couple of  30kv doorknob caps 500pf. With 15000 across them, that comes out to 3ma or 45watts that is prety hot. The fact that it is rated at 30kv does that mean it is good for the wattage. Never seen any with power rating on the ones on ebay.

Just did some testing and one good thing is its only 7500 across one cap to rf ground and after 5 min its just warm.

Also scored me a pig, only 7200 but it was free, so after get the current one going will switch over to pig power I just need Mrmlstar to roll me a big fat one, inductor.
« Last Edit: October 11, 2019, 02:11:13 AM by thedoc298 »

Offline MRMILSTAR

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Re: My terry filter
« Reply #1 on: October 11, 2019, 05:13:51 AM »
Ha ha. Start looking for those big E and I sections.

I use a low pass RC filter (so-called Terry filter) on my pole pig although it probably isn't necessary. I had the parts so why not, it can't hurt. I use two Murata 500 pF doorknob caps each rated for 30 KV, one for each leg of the HV output. These capacitors work with no problem with my 14.4 KV transformer output. My coil draws 6.5 KVA.

The capacitor spec you are looking for is continuous RMS current handling capability. Unfortunately, this spec is not always supplied. It is related to the capacitor's internal resistance. Doorknob capacitors generally have a very low internal resistance so they can handle a fair amount of continuous current. The biggest problem with them is that as they heat up, the capacitance can change. While important for a tuned circuit, it is non-critical for the single-pole low pass filter which is used for transformer protection.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline thedoc298

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Re: My terry filter
« Reply #2 on: October 11, 2019, 06:54:59 PM »
Ha ha. Start looking for those big E and I sections.

I use a low pass RC filter (so-called Terry filter) on my pole pig although it probably isn't necessary. I had the parts so why not, it can't hurt. I use two Murata 500 pF doorknob caps each rated for 30 KV, one for each leg of the HV output. These capacitors work with no problem with my 14.4 KV transformer output. My coil draws 6.5 KVA.

The capacitor spec you are looking for is continuous RMS current handling capability. Unfortunately, this spec is not always supplied. It is related to the capacitor's internal resistance. Doorknob capacitors generally have a very low internal resistance so they can handle a fair amount of continuous current. The biggest problem with them is that as they heat up, the capacitance can change. While important for a tuned circuit, it is non-critical for the single-pole low pass filter which is used for transformer protection.

I really appreciate all your help, just helps to have a second voice of someone that has done it before. I got that pig in my shop last night and could not believe how heavy they are. I use my welder for a ballast to test it out, still poped my 40 amp fuse after a bit.

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Re: My terry filter
« Reply #2 on: October 11, 2019, 06:54:59 PM »

 


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