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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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how to protect the electrolytic capacitor bank from back emf

Author Post
rajheman
Fri Dec 30 2016, 05:50PM Print View
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how to protect the 24,000j electrolytic Capacitor bank

made by cevyn from back emf, which will have more than 30KA current .


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klugesmith
Fri Dec 30 2016, 06:29PM
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Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
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Back EMF is generally a good thing. That part of the applied voltage is being magnetically useful, instead of just pushing current through wire resistance.

Maybe you mean voltage reversal on the capacitor.
To avoid voltage reversal, make sure the discharge current does not continue after the voltage reaches zero.
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rajheman
Sat Dec 31 2016, 05:34PM
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Dear klugesmith yes I want your protect the electrolytic capacitors from reverse voltage because they are polarized capacitors, which are not able handel that huge reversal charge.please suggest me how to bypass these reverse charge getting back to capacitor bank
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Enceladus
Wed Feb 15 2017, 08:09PM
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You can either place a large high current diode in series with the cap bank, or place a diode inverse parallel with each cap, or you can just use a nice beefy SCR as the main switch. I personally like the SCR approach the best because they are simple, robust, easy to trigger, and are available up to very high voltage and current ratings.
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DerAlbi
Thu Feb 16 2017, 01:05PM
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Enceladus, the diode in series to the cap bank does not work. Think again.
Antiparallel diodes are the only way to go.
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Sulaiman
Thu Feb 16 2017, 01:28PM
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Just to add one more opinion : I would use one suitably over-rated diode per row of electrolytics,

I would also have a resistor across each capacitor for slow discharge over many seconds, e.g. R.C >= 10 sec.
you could put one resistor per row, but one resistor per capacitor is more reliable.
As I'm sure you've discovered, even partially charged capacitor banks can give surprises :)

P.S. I like the charging supply, but hate to see so many crocodile clips around high energy cap banks.
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DerAlbi
Thu Feb 16 2017, 01:51PM
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R.C >= 10 sec.

This dissipates roughly 24kJ in 30sec... resulting 800W average power ^_^. I'd rather burn my hands on DC voltage than an 800W heating element
Not that the idea is bad, however if you have so much energy stored there is a point where you simply have to deal with the dangers involved.
In that manner i disagree that cocodile clips are a problem. They evaporate easily.
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Sulaiman
Thu Feb 16 2017, 02:35PM
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This is a compromise between safety and efficiency;

e.g. If R.C = 10 seconds, for C=2400uF, R=4k166.
Worst case is when the cap bank is maintained fully charged ... V = 4500, P= 4.86 kW, (50W per resistor)
which is just a little too high for the dual MOT supply,
so maybe let R.C = 100 seconds, R=41k666 ... use 47k, Power = 430 W total (use 100x 47k 5W)
or use 100x 100k 2.5W resistors, 215 W total dissipation, but I'd wait 1/4 hour before feeling comfortable

It is wasteful but;
you will be able to approach the capacitor bank within five minutes of being left fully charged
you will not accidentally forget to discharge the bank when not required
the resistances ballance the voltage per row, compensating for leakage current imbalances

Just remember, your cap bank has more energy than a 0.50 BMG ammunition round
and can release it a lot faster !

DerAlbi ... I'd rather burn my hand than be a discharge path for a 4.5 kV 24 kJ cap bank
... even 10% residual charge could easily be leathal (240J @ 450V)
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DerAlbi
Thu Feb 16 2017, 02:44PM
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I get your point no worries I just do not think its practical. hmmh.
I mean everything that can withstand a dump of that energy has to be big and bulky. I still would not connect it continously because this, in my opinion, would not be a good design.
I just thought about.. hmmh.. i mean. uuhh
I may sound stupid, but what about an SCR + a bucket of water with 2 electrodes inside ? You simply can discharge the cap bank when you need it. Will the water explode ?
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Sulaiman
Thu Feb 16 2017, 02:53PM
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Location: United Kingdom
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>= 4500 V thyristors don't seem too common around here,

I often use higher value resistors just for voltage balancing with a separate discharge path,
often I've used a change-over relay contact, common to cap bank,
normally closed contact to disharge resistor, normally open contact to charging supply.
Applying power to the relay coil allows charging, dropping power to the relay discharges the bank.
More efficient but it creates one more set of high voltage wires to avoid :)
and are you SURE that your relay contacts have not welded together ? ...etc.

Based on the above, maybe it would be worthwhile adding
a 1 MOhm 0.5 W resistor (e.g. ) in series with a neon lamp, ( e.g. or )
across each row, just as an extra 'reminder'
You rarely make the same mistake twice with such capacitor banks.

It should be obvious but I will say it anyway to relieve any future regrets,
... be extremely careful... arc-flash burns, electrocution, deafness, shrapnell ... loads of fun !
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Conundrum
Fri Feb 17 2017, 04:50AM
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Yeah, +1 on the whole "shrapnel" thing.
I once had a near miss with a PC power supply where due to a fault the two capacitors were fully charged. Fortunately was wearing yellow sunglasses so avoided any damage. #include "SAFETYGOGGLES.h"
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Enceladus
Fri Feb 17 2017, 08:02AM
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DerAlbi wrote ...

Enceladus, the diode in series to the cap bank does not work. Think again.
Antiparallel diodes are the only way to go.


Yeah, I actually had my doubts about this idea but it seemed to make sense. I Have a small camera flash module based CG whose cap I've been protecting by simply placing the working coil in series with the flash tube. It works well and doesn't seem to back-charge the cap but I know the flash represents lost energy. I was planning on using a series diode with a relay after removing the tube but I decided to take a moment to simulate it after reading your comment and it appears you are quite right. I guess I'll be using an antiparallel diode.
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