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Calculating peak discharge current, and clamping

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Quantum Singularity

Thu Jun 08 2006, 06:48PM

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

I have been running some numbers on the simulator on the Magnetic Gun Clubs site calc and I had a few questions regarding this calculation. For one, I am unsure what the ESR is on my caps. I was wondering if anyone here has a ballpark number? I have shot off an email to CDE asking them, not sure if I will here back. Also, is there ESL in the caps enough to need to include that in the circuits inductance? And also is there any significant resistance in the SCR? The only data on the SCR datasheet that said resistance was 'slope resistance' and I am not familiar with what exactly that means.

On a different note, the circuit representation their shows the clamping diode across the cap. I was under the assumption the best spot for it is directly accross the coil, is that not correct? Or will it work both ways just the same?

FastMHz

Fri Jun 09 2006, 01:17AM

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

If the diode were across the cap bank, before the SCRs, I don't think it would protect the SCRs from the back emf...mine is directly across the coil, which to me, makes the most sense. Sorry I can't help with your other Qs...hopefully the company will write back.

Electroholic

Fri Jun 09 2006, 01:34AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

if there is no diode at all, the will be an emf kick.
and the cap will be charged BACKWARD!

if a diode is placed at the caps(the right way), the collapsing mag. field will freewheel thru the diode AND the scr.

if a diode is placed right at the coil(the right way), most if not all of the current will freewheel thru the diode.

I use both in my coil guns, just to play safe,

BTW, my coil guns are reluctant type.

If the diode is placed the wrong way, then something interesting will happen, SCR VS DIODE, see who blow first. I only did it once, 50RIA120 vs 4X1N5408 on ~200J
5408s won.

EDY19

Fri Jun 09 2006, 01:44AM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Haha electroholic- robot wars.....junkyard wars.....semiconductor wars

my diode is located very close to the coil, I think this is the best way.

Quantum Singularity

Fri Jun 09 2006, 04:21AM

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

Well, that all reinforces what I thought... just wanted to check. I think I will hook up directly accross the coil then. Like FastMHz says, makes the most sense to me also.

About the ESR/ESL... A rep at CDE did email me back but said he will look into it and hopefully get back to me with some detailed info on monday.

Wilson

Fri Jun 09 2006, 04:56AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

The ESR of most large electrolutic caps are quite low, espeically in the inverter grade ones. Its usually between 40-100mohm from memory, so if you're parralling a few caps, the ESR should no longer be the limiting factor on the peak discharge current.

Quantum Singularity

Fri Jun 09 2006, 05:26AM

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

Well, just for comparison I tried 100mohm and 40mohm on the calculator I linked to in the first post. The later increased the current by about 4,000 amps... so you see it makes a kinda big difference

For an induction launcher setup (what I'll be working on first) I'll probably make banks of three caps in parallel, and have 4 of these banks stacked in series so the ESR will be approximately 25% higher than the individual rating of just one cap alone. This is not set in stone and I do plan on testing different arrangments. I would just like to have some kind of calculation that suggests my diodes and scr's will survive.

Sulaiman

Fri Jun 09 2006, 08:37AM

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

Don't know if it helps;
for photoflash electrolytic capacitors,
esr = (80 Ohms)/(uF capacitance)

e.g. C= 1000 uF, R= 0.08 Ohm (80 mOhm)

thom

Fri Jul 07 2006, 08:06AM

Registered Member #239 Joined: Thu Feb 23 2006, 03:15AM
Location: canada
Posts: 23

I'm not sure how accurate this is, or if its correct at all, but if you have access to a LCR meter you could try the resistance mode on it.

I tested a couple caps: a compact size 4700uF that read 75mOhm, and a large industrial 20,000uF that read 17mOhm.

But ESR meters test at around 100kHz+, while this LCR meter is only 1kHz, so impedance would be an issue.

IamSmooth

Thu Sept 28 2006, 01:04PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Quantum, what SCR are you using to handle that current surge and where did you buy it? Can you post a picture?

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