tank capacitor for induction heater? - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: April 07, 2018, 07:06:59 pm
content: So intuitively I think that a tank capacitor just needs to have a ESR and thermal performance enough not to get hot from switching in a tank circuit for an induction heater.Are there any other parameters like mechanical strain that should be paid attention to when specifying a tank capacitor? I am building something between 5-15kW and I want to know if there are any really good non thermal reasons to try to buy 'high performance' capacitors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 08, 2018, 04:48:22 am
content: You get what you pay for. Cheapest polypropylene caps I think are X2 filter caps. They're made with very thin metallization, to the point where their ESR is sensible just on a regular ESR tester (assuming test frequency is high enough to ignore the capacitance still). These kind are good for maybe a few hundred mA of AC current, before they're already getting uncomfortably hot.Pulse rated caps aren't necessarily what you want. CDE 935C type caps are nice and low ESR, plenty of pulse rating, but they get hot at AC. They're only good for a few amperes at 100kHz. On the right track, but not actually the best option.So it takes some shopping around to find a good family. If you don't see a voltage or current vs. frequency rating graph in the datasheet, it's probably no good. If you do, great, mark that down for comparison. (When you see V(F) curves, usually V drops off linearly with F, beyond some cutoff point; this is simply a constant current, because reactance. Take the slope to find current.)Add up as many as you need, in parallel, and there you go.That's what I did for my first one,https://www.seventransistorlabs.com/tmoranwms/Elec_CapBank.htmlThink that was these,https://www.digikey.com/product-detail/en/epcos-tdk/B32652A6104J000/495-1333-ND/592909which seem to be more expensive than I remember, but it has been a while (gosh, nearing a decade I think). I thought those parts had disappeared, or at least fallen into the abyss (the "52 week lead time" one), but, hey, I guess they're still making them after all, neato.There may be better amps/dollar out there today. That was just what I found back then.Tim

author: Wolfram
author_rank: Frequent Contributor
timestamp: April 09, 2018, 07:20:33 am
content: Purpose-made high current polypropylene capacitors are available and not extremely expensive from Mouser and Digi-Key, here is an example:https://www.digikey.com/product-detail/en/illinois-capacitor/245LC3500KL505HM6/1572-1572-ND/6051321This is likely to be the cheapest solution if you need more than a few tens of kVAR worth of capacitors.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2018, 08:02:59 am
content: Quote from: Wolfram on April 09, 2018, 07:20:33 amOh shit, I didn't realize when DK picked up IC, they got induction caps too!Huh... hmmm...I didn't realize IC had that kind of thing either. I wonder if they're relabels from Celem, or HEI?Compare:https://www.digikey.com/product-detail/en/illinois-capacitor/334HC3700K2EM8/1572-1581-ND/6051330https://www.digikey.com/product-detail/en/illinois-capacitor/106HC5700KR/1572-1610-ND/6051359http://highenergycorp.thomasnet-navigator.com/viewitems/metallized-film-capacitors/che-series-metallized-film-capacitorshttp://www.celem.com/Category.aspx?cid=11&hmenu=29That four-hole, two-plate, open type, with yellow tape no less, is a dead ringer for another I've seen but I can't remember who made it. Guessing that's who they licensed/acquired...Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2018, 08:06:58 am
content: Quote from: blueskull on April 09, 2018, 07:54:37 amBTDT, they get hot.Think that was 100x, of something or other. Surprised they didn't end up melting at the ~10kVAR they ran at.1000 pieces would probably be reasonably well within ratings, but would also run an annoyingly low frequency, not to mention the tedium of assembling them.Far more worthwhile to get pulse / snubber rated caps. A typical one of those will handle 2-3A in the same size (10-15mm lead spacing), and should sell for under $0.20/A.Even more worthwhile still, to get one of those monolithic conduction cooled blocks. A hundred bucks for no labor? Beats anything I can do!Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2018, 08:12:12 am
content: Sure, but that was at 120Hz.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2018, 08:24:56 am
content: Quote from: blueskull on April 09, 2018, 08:19:22 amBut they aren't carrying significant ripple voltage in the half MHz range!I built my 10kW prototype with a half-bridge, populated with polyester for the same reason (though still not quite enough, they got rather warm):https://www.seventransistorlabs.com/Images/Induction1201.jpgTim

author: Wolfram
author_rank: Frequent Contributor
timestamp: April 09, 2018, 09:52:20 am
content: Quote from: T3sl4co1l on April 09, 2018, 08:02:59 amAs far as I know, these are Alcon relabels, from looking at the available products and their specifications. I bought a few and the construction looks pretty solid, but I have not tested them at full current yet. I have some HEC CHF and CHE series capacitors as well, and the construction style is identical.It's too bad that none of the common component suppliers stock Celem, because their CSP150/400 and CSP305/600 series have a very good performance/cost ratio.

author: Circlotron
author_rank: Super Contributor
timestamp: April 09, 2018, 10:16:44 am
content: About 15 years ago I was on the design team for a 3kW resonant mode SMPS. Back then at least, the hot ticket was WIMA FKP1 caps.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 09, 2018, 04:52:05 pm
content: Quote from: Wolfram on April 09, 2018, 09:52:20 amAh, that does indeed look like the line!QuoteRichardson is the only usual-ish one, IIRC. Which... they split the catalog to here I guess,https://www.relltubes.com/products/Capacitors/Synthetic-Capacitor.htmlrell.com doesn't seem to have much of anything, just a corporate info page. Even then, I'm not seeing anything actually for sale, just quote...Tim

author: David Hess
author_rank: Super Contributor
timestamp: April 10, 2018, 08:30:30 pm
content: Quote from: Circlotron on April 09, 2018, 10:16:44 amI went through most of the WIMA film capacitor catalog a couple years ago looking for the ones which the highest high frequency ripple current rating for an application like this and did what T3sl4co1l suggested in ignoring any which did not have an actual specification no matter what the list of features said. It was time consuming and I do not remember the results but some of them are suitable with some design care.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 11, 2018, 03:29:01 pm
content: No useful specifics, but I built a small induction heater a while back and discovered it was the most brutal capacitor test imaginable. Low dissipation factor is the name of the game. I had good results with large polypropylene, though polystyrene did even better. Hard to find those these days, and they have to be foil, not metallized.

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2018, 08:33:30 pm
content: Quote from: Conrad Hoffman on April 11, 2018, 03:29:01 pmPolystyrene are low loss but have a very limited maximum temperature.Foil capacitors are definitely the way to go if expense is not a problem but I found that it was more economical to use a greater number of the best metalized capacitors unless the current sharing physical layout becomes too difficult so fewer parts are required.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2018, 08:40:54 pm
content: Polypropylene has a pretty low MP too, though it does surprisingly well up until then.When they melt, the polymer expands significantly, cracking the encapsulation. The capacitor roll (metallized melty-goo) then extrudes out, and usually thins or tears enough to arc over and start a fire.That was fun, dumping 10A+ into EPCOS DC Link caps.Tim

author: Etesla
author_rank: Regular Contributor
timestamp: April 14, 2018, 05:08:38 pm
content: Along with picking the right caps, I think it is also important to hook them up to your coil in the way I show in the attached picture. I found that when I didn't do this, the caps closest to my coil got significantly warmer than the ones in the back. Just a tip...

author: CopperCone
author_rank: Super Contributor
timestamp: April 14, 2018, 05:11:07 pm
content: Yes, I noticed that with my LCR meter. If you probe it in different areas it is a significant distributed capacitance (I made a giant distributed capacitor out of a bunch of junk I had laying around, hopefully she will hold) depending on where I probed.Based on what I saw, you would want to put the conductor in the middle of both plates. I am kinda thinking now that I should have cut circular plates and arranged my capacitors in a circle rather then a square..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2018, 07:25:57 pm
content: Best is the last two on this page,http://www.celem.com/Page.aspx?id=21&hmenu=37Tim

author: David Hess
author_rank: Super Contributor
timestamp: April 15, 2018, 06:11:44 pm
content: Quote from: Etesla on April 14, 2018, 05:08:38 pmThe same trick is used with batteries and capacitors in high current applications and sometimes with transistors. Audio power amplifiers require similar care in layout to get the lowest distortion and this is hopefully shown in the schematic.