Looking for good capacitor prices

[ResonantEMF]

I'm looking for 942C20P15K-F capacitors. I need about 30 of them for my 56.2 nF 16kV MMC. The best price I could find for them was $5.52 each so far. That would cost me $165.60 and I'm sure there has to be a slightly better price somewhere. Would anyone know of such a place? Ideally, I'd like to keep the price below $150.

Furthermore, This is for a dual MOT coil, so the rating is 4x the input voltage. Is this margin overkill?

[304er]

Hi,

Unfortunately I do not have any recommendations for finding at better prices from the usual suppliers.
Even a few years back prices were not exactly cheap either. And they have gone up since then, like so many other things too.

Back in 2016 the best price found was about $3.90 each. Purchased 90 of them and are currently used on my old/new SGTC with a 15 kv 60 ma NST.

Recently, back in November 2021, I purchased 210 of them at about $3.50 each from an eBay seller who is a coiler. He had an unused stash of them bought several years ago. I bought these for a future project in planning of mine.

So while this was a bargain compared to current prices, this was still quite the "investment". Sometimes we have to "Pay to Play" with our hobby here.

On your MMC margin...

A rule of thumb I follow is: RMS volts x 1.414 x 2.5 for the minimum rating.
To get my LTR required my MMC, I ended up at higher than the 53,025 V minimum at 60 kv, a plus, which is better in my opinion.

So your 4 x is very close to the 3.535 x , which is a plus, again better in my opinion.

While you can set up for a much lower voltage value, you are taking your chances if it will "hold up".
If they "blow", money saved here is now lost. And then now...have to spend money to do what is correct/recommended to begin with.

So these are just recommendations, we all make our choices.
Just also remember, even if you go with a possibly a lower voltage level recommended, even if it possibly works with no problems...life of the individual capacitors is reduced in the long run. They only have so much "shot life" to begin with and since at individually higher voltage each, this reduces life, which eventually leads to complete failure.

Life/reliability of capacitors is greatly increased, if set up for higher margin as each capacitor now see less voltage/ "stress". So "shot life" now increases if done this way.

Chris

[davekni]

Furthermore, This is for a dual MOT coil, so the rating is 4x the input voltage. Is this margin overkill?

As Chris said, there is no simple correct answer to how much margin is wise.  My dual MOT SRSGTC has a 9kV rated MMC, which is the peak MOT output voltage I measure with an HV probe.  I'm using resonant charging to hit 9kV.  This coil gets only rare use.  No failures so far.

[304er]

Hi,

I just did a search at Octopart and the current prices for less than 100 are quite jaw dropping I will admit.

Here is a screenshot I did:

https://photos.app.goo.gl/ocvuWvKLUyGBESK2A

So definitely if less than 100 needed you will have to explore elsewhere with some just luck involved too.

Chris

[MRMILSTAR]

A cheaper alternative is to watch for suitable used Maxwell energy discharge capacitors on E-Bay. These show up occasionally.

[304er]

Hi Steve,

I have to agree some here on Maxwell/General Atomics EDC's.

Do you happen to know what all/some are the good series numbers and/or good individual numbers are maybe?
The General Atomics website does not have the best information on their caps in my opinion. A bit confusing I will admit. And many older Maxwell #'s I have come across are now obsolete now...not found anymore under GA's site.
It is hard to find good reliable information on them unfortunately when I have looked at them at times and because of this, I have skipped on several in the past on eBay. Did not want to chance it, even though some prices were good, without good info. I do know of some who have had the "Maxwell Juice Experience" unfortunately.

I know your Maxwell caps on your SRSGTC is a good number to use:

https://highvoltageforum.net/index.php?topic=752.0

If I remember correctly...you actually managed to find a true spec sheet of your Maxwell caps and it had the important spec of maximum rated pulses per second (PPS).

I do not recall your cap number showing up very often, but may have missed them.

Chris

[MRMILSTAR]

As you alluded, not all Maxwell capacitors are suitable for SGTC duty. Many are only rated for 1 PPS. I know that any in the 37xxx series are good. They will all withstand at least 500 PPS and have lifetimes over 100,000,000 shots at rated voltage. Some in the 31xxx series are good. I have a few data sheets for Maxwell capacitors. If anyone sees a candidate capacitor just post the part number and I will see if I have the data sheet.

Another possibility is severe derating. If you take a 1 PPS Maxwell capacitor, or combination, rated for example for 70 KV then it will withstand a much higher pulse rate and have a much longer shot life if you only use it 10 KV. I don't have any papers describing the life extension prediction though.

[Mads Barnkob]

CDE 942C capacitors was never cheap. The only reason that there at points was a huge quantity circulating in the HV community was due to group buys. People on 4HV and pupman list got together and bought in thousands at a time from mouser or digikey.

So if you want cheap caps, arrange a group buy