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Suitable electrolytics?

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Quantum Singularity

Tue Mar 21 2006, 06:17AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

I have been browsing the net in search of some caps so I can start experimenting on coilguns. Seems about the only cheap-ish caps I have come accross are the sprauge powerlytic 36DX. I have been looking at the 450V models, 2,000uf-4,700uf are commonly available. Depending on circuit parameters I will most likely parallel a few for each stage.

Does anyone have any knowledge on how well the 36DX caps will perform? And are there IGBT's available that will hold up to a bank of a few of these in parallel charged around 400V?

dan

Tue Mar 21 2006, 06:45AM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

I have an old sprague capacitor 36DY, but it's rated for 5,600uF at 450V and it has 1/2'' terminals. Generally you can tell by the size of the terminals if they are pulse capacitors.. the bigger they are the better since it will lower the ESR and ESL. As for what type of IGBT that would depend on the peak current it's going to see.. Remeber IGBT do not handle as much peak current as SCR's. So if you are going to have a kJ per stage I would suggest a nice big IGBT brick rated for at least 300A or more.

Quantum Singularity

Tue Mar 21 2006, 10:25PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

The sprauge 36DX and every other cap I have ever seen 2,000uF or more at 450V all have screw terminals so that doesnt really tell me much. Infact, most photo-flash caps do not have screw terminals and I have seen some decent coilguns with photoflash caps. I think I am just going to get whatever gives me the most J/$. And I have seen the 36DX's refered to as being one of the cheapest large value 450V electrolytics available.

Here is another question that might impact what I buy... If I were planning on paralleling a bunch of caps, say 10 2,000uF caps to give me 20,000uF bank total should I only parallel matching caps? In that case I would want to buy several of the same caps versus buying a couple here and there that are cheap? But then again most electrlytics do vary from cap to cap even in the same models (+/- 20%, etc.).

Here are the Spague's I was talking about:
36dx

Unless someone says these are not good caps I will probably be getting them so speak now...

FastMHz

Tue Mar 21 2006, 10:48PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

Those caps should work...they are similar to what I'm using on my RingRocket launcher, though mine are actually only 350v @ 3600uf. The terminals on the ones in your pic look smaller than mine though, so maybe they aren't designed for as much current.

Even though you can parallel different ones, I'd only parallel identical ones to keep losses at a minimum.

Marko

Tue Mar 21 2006, 10:57PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

That sprague powerlitics are really most seen caps, even I in my country could get some of them.

These are standard inverter grade but they shown to be good in pulse applications as well, if you can get them cheaply its worth, unless you run in some good pulse non-electrolytic cap.

dan

Wed Mar 22 2006, 05:16AM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

You can tell a lot from the terminals. Thoes Capacitors use very small screws indicating that they a not really designed for pulse currents. This shouldn't really be a problem since you will be using multipe in parallel. Real pulse grade electrolitics have huge terminals. (My powerlytic has 5/8'' round contact area with 1/4'' NC bolts.) Also photo flash capacitors are also not really pulse capacitors. Most manufactures use regular inverter grade capacitors. Excpecially the ones used in disposble cameras since they only have to survive a few shots. Also since you say thay are 'cheapest' just solidifys the fact they are inverter Grade Capacitors. Just keep in mind that Since these are not real pulse capacitors the very large forces on the plates durring discharge will cause their capacitance & ESL to shift over time. However I am not saying they don't work because obviously they do. In fact I'm sure they will work great but will not be optimum. I'm also not saying that the advatages of using pulse grade caps justifies their extra cost.

Quantum Singularity

Wed Mar 22 2006, 07:22PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

The caps in my question are 'computer grade' and I was under the assumption that inverter grade is actually better (1 step down from pulse grade). I would rather get pulse grade stuff but in most of my searching I can barely find any that are listed as such, and out of those few I do find they are definately not 'good deals'. And unfortunately alot of caps listed online dont say how big the screw terminals are. Sometimes you can tell in the picture and sometimes you cant.

Maybe I'll hold of until I can find some that will be better. Havent decided yet. Most of the lytics I have found on ebay that I have considered (like the sprague 36DX) are going for around 40-50J per dollar for the cheapest ones.

WaveRider

Thu Mar 23 2006, 12:05PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

I find that computer grade caps work well. The low ESR and high ripple current capacity generally makes for a good CG cap. I have never had one fail (so far)...

dan

Thu Mar 23 2006, 12:44PM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

hmm.. I was under the impression that computer grade and inverter grade ment pretty much the same thing. The only difference being that computer grade capacitors come from computer PSUs.. and I have never seen more then a 560uF @ 400V capacitor inside of a PC PSU..

WaveRider

Fri Mar 24 2006, 08:51AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Computer-grade caps are those fat caps with the large screw terminals.. Generally, they are (were) found in those massive mainframe PSUs. The caps found in PC PSUs are not the same (and are often crap caps...the manufacturers use the cheapest ones that still function...for a few years, at least). I use 3 proper 10000uF 100V computer grade caps per stage in my coilgun and they work very well..

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