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safely discharge large capacitors?

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dmg

Tue Feb 23 2010, 05:00AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627

I charged a 200,000 uF cap bank ( a pair of 100,000uf) with 12VDC, I put an LED with an 800 ohm resistor across the terminals, with thier respective polarity, anyway, I had a nice nightlight, damn thing was lit an hour or two.

here is the question, at this capacitance, shorting it will be a bad idea as It can melt/damage the terminals. and quite possibly make the capacitor go boom,

does anyone know any methods to safely and discharge this kind of capacitors in under 1-2 minutes, without damaging the capacitor? (or damaging my pride, by having me covered in warm electrolytic juice...)

as for bleed resistors, I tried 5W, 800 ohm, 20W, 500 ohm and some other resistors in between, (even parrelling some of them) and it had no effect on the drain rate, and the LED remained just as bright as before.

all suggestions are welcome, feel free to write them down.

note: I will use these capacitors in a DC power supply, as smoothening capacitors, and I dont want my indicator lights still lit for an hour after I turned the power off.

Myke

Tue Feb 23 2010, 05:19AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

You could use a 10W 12 ohm resistor that discharges the caps once the power is turned off. It would discharge the caps in about 10 sec or so. There would be about 1A flowing right when you connect the resistor across the caps. You don't want to have the resistor across the caps during the entire time it's on because it would dissipate tons of power.
Another solution is to stick the indicator light on the primary side of the power supply so it will immediately go off when you turn off the supply.
Remember that LEDs drop a certain voltage across them so your resistor would see less voltage across it so therefor less current would flow. If you just had the resistor, the caps would discharge in a shorter amount of time.

This is only if you think you need the bleeder. Personally, I don't think you need it since what ever you are powering with this supply should take up quite a bit of power which would discharge the caps fairly quickly. Also, the voltage is so low that you don't need to be worried about being shocked. Just don't short the rails when the caps are charged.

dmg

Tue Feb 23 2010, 05:32AM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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yes myke, the load isn't too big (about 50-100W) but with the indicator lights, load, and bleeder resitors, it should take up enough power to drain the capacitor fairly fast.

for the LEDs, there are 3 of them in parralel, each have a 1k ohm resistor to limit the current

radiotech

Tue Feb 23 2010, 06:05AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

You could draw it down with a CEMF, That is apply a regenerative current flow that releases when terminal voltage is zero. Then you have control of the curve.

Tue Feb 23 2010, 01:54PM

Registered Member #2372 Joined:
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Posts: 62

Just put a switch with a few 10s of ohms resistor between the terminals to drain it. The drain time goes like R*C so for 0.2F and 30 Ohms you get a 1/e time of 6 seconds. You can make a high wattage resistor by paralleling a bunch of low wattage resistors and the final resistance will just be R/n where R is the resistance of each resistor and n is the number of resistors. A 500 Ohm resistor would take 100 seconds to drain it to 37% of its initial charge, that is too long.

dmg

Tue Feb 23 2010, 02:54PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

something rather strange happened, I tried to put a 5W, 300 ohm resistor with the leads, and I also put a 12V zener diode for voltage regulation across the terminals, when the cap was charged, it was working fine and when I unpluged the power, the capacitor was drained in 5 seconds, I felt the resistor and diode, they were cold, tested them, still working, and the capacitor wasn't warm.

then I charged the cap, and touched the resistor and diode setup to thier terminals, there was no gaint spark, but rather a small, barely visable flicker and the cap was drained very fast. (I had the original LED setup as well, it was glowing untill I touched the terminals, then it dimmed into nothingness.)

I suspect that the zener is responsible for this, but does anyone care to explain how?

radiotech

Tue Feb 23 2010, 06:00PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

An electrolytic capacitor undergoes a chemical reaction whenever a voltage is applied. Dielectric absorption causes voltages to build after discharge to zero.

Electrolytics have a short shelf life;they are the most unstable part we have. I have some that increased their rated capacitance by being connected for years to a lower voltage than they were rated for.

Since experimenters may not always get new parts, a lot of care is needed to get some value from the parts. Reforming is one trick.
All aluminum electrolytics go bad. I've got some rated in the thousands of ufds that got dessicated down to nanofarads by being in a chemical plant. Excellent for learners easily fooled by not paying attention to what units their capacity meters were showing them. The guys who repaired photoflash camera supplies that the press cameras used could tell if a capacitor was good by watching an incand lamp in series as it charged.

Sulaiman

Tue Feb 23 2010, 06:54PM

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

probably you charged the capacitor(s) to a voltage higher than the zener (actually avalanche) voltage.
The current that would have initially flowed in the zener would have been;
(Vcap - Vzener) / Rzener (zener diodes have a small effective series resistance)
This current was too much for the poor little zener and it went short-circuit
quickly discharging the capacitor(s).
Often the discharge current is so large that the short-circuit zener blows open.

If you're only using low voltages then a filament lamp (e.g. automobile lamps) is nice to use for discharge as you can see when the voltage has dropped to near zero.

teravolt

Tue Feb 23 2010, 08:16PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

if you want a good discharge device use a ordinary 12v automotive lamp and you will know when it is discharged. generaly when you get to 50 volts or grater discharge on the big electrolytics is when it gets dangerous. if you want a ruff estiment of discharge time R x C=T

Luca

Tue Feb 23 2010, 08:42PM

Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134

Just a curiosity: why do you need such huge capacity for a DC power supply?

Regards,

Luca

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