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Capacitor ESR questions

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StridAst

Sun Jan 23 2011, 07:38PM

Registered Member #3623 Joined: Sun Jan 16 2011, 10:13PM
Location: Utah, USA
Posts: 11

Ok, I've got a usual newbie sort of question here, but I'm having no luck with the search function as ESR is too short to search, and adding words seems to find everything I'm not looking for. So you have my apologies if this is an easily answered via search type of question.

I'm trying to figure out how much ESR is too much as I build a coilgun. I've got a couple panasonic caps at 1800uf 450vdc listed as having 122mOhms. I'm only thinking of using one of these in the coilgun for the initial coil. I'm thinking on a 2 stage coilgun, however I'm still at the conceptualizing point for now. I'm not planning on building till I am confident I know what I am doing.

The questions are: Is 122mOhm too much ESR? Also how do I factor this in with the various sims on Barry's site? do I add it to the resistance it calculates the inductor as having?

I've been getting the feeling that 122mOhms is too much ESR for linking in a series for a railgun. But I'm not certain how to work it into figuring out for coilguns. Eventually I want to try making a railgun, but thats down the road after I get a coilgun working and get a better feel for things.

If it helps, here is a link to the ebay auction for the panadonic caps.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=270686463328&ssPageName=ADME:L:OU:US:1123

Thanks in advance for any advice

Turkey9

Mon Jan 24 2011, 08:18AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Barry's simulator calculates the current and voltage waveform in a series capacitor, resistor, inductor network. The resistance of the wire and the capacitor can be thought of as an external resistor in this network. So in short you would just add it to the calculated resistance of the wire.

I'm not 100% sure on this, but I believe the biggest factor in electrolytics not being the right choice for a coil gun is the fact that they can be damaged by the extreme current that a railgun creates. I don't think esr should be the limiting factor there.

TheMerovingian

Mon Jan 24 2011, 09:53AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

FOr a coilgun, this ESR can dissipate up to 20% of your power. I often prefer to use the MMC route, i.e. several small photoflash capacitors in parallel (each with a resistance of 80 Ohms * uF, for example my (20x120uF) 2400uF bank 330V has approx 35mOhms of ESR). Of course two your panmasonic caps in parallel will have 65 mOhms, that may be ok for a coilgun (not for a railgun)

Dr. Shark

Mon Jan 24 2011, 05:31PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Welcome to the board, StridAst! Don't worry about asking newbie question here, as long as you don't expect others to do your homework for you, people here are very friendly.

For a critically damped circuit (fastest possible discharge with no ringing) ignoring wire resistance, Wiki

says that you are looking at L = 4*C/R^2 = 500mH. This gives you a discharge time constant following something like R/(2L) = 120ms. Way too long as I would assume you are shooting for single digit milliseconds. Making the inductance smaller will make the circuit underdamped, in the limit giving you an RC circuit with tau = R*C = 0.2ms.

You can get a plenty fast discharge, just not with the optimal waveform. There will probably be some suck-back, but I think in practice you will be fine.

Sulaiman

Mon Jan 24 2011, 08:05PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

How much esr is too much?
There are two problems with capacitor esr
1) the esr causes the circuit to perform below expectations
...... anywhere from a slight efficiency loss to failure to operate
2) the esr causes the capacitor to heat up due to I^2.R heating
...... generally not a problem for single-shot,
...... increasing problems with increasing repetition rate.

If you know the esr you can consider it a part of the operating circuit so it is fairly easy to compensate for,
if you don't know the esr then use the figure for a similar unit,
I measure the esr of in-use electrolytics almost daily and from my experience, for physically large electrolytic capacitors (say >=1"dia)
if there are no physical signs of electrolyte leakage/loss
the measured esr is usually lower than the manufacturer's specification even when the capacitor is a decade or more old.
Generally, the smaller the electrolytic capacitor the worse it is.
For hobby projects I REALLY reccommend using electrolytics well below rated voltage and ripple rating, use reliable sources, use 105C, Long Life, low-esr wherever budget allows - it may be the difference between months and decades in terms of reliability.
For coilguns etc. just get whatever you can as very few members here can afford to design the coilgun then get the correct capacitor!

P.S. Tantalum (bead or smd), film and ceramic capacitors are more reliable but so far I've NEVER found an OS-CON capacitor faulty.
I'd use OS-CON (organic polymer) capacitors in my hobby stuff if they weren't so expensive.

A personal record ! a rant and a ramble on the same day!
I'm becoming a grumpy old man ! (as in the uk tv series)

StridAst

Tue Jan 25 2011, 04:09AM

Registered Member #3623 Joined: Sun Jan 16 2011, 10:13PM
Location: Utah, USA
Posts: 11

Thanks for all the helpful replies people! Now back to more researching. The sheer volume of info I still have to assimilate is rather staggering. Electronics has never been my forte. Hoping to fix that over time. Now it's back to more lurking and reading.

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