Grimmis1027

Well, Lets put it this way, over the years, I've built a number of power supplies for electronic circuits, all of them using capacitors as filters to cleanup and remove the AC component leftover from the transformer and other parts of the circuit.

A capacitor in that type of circuit charges up to the peak voltage level and tries to keep it there, filling in any negative going spikes, ripples or short voltage drops, if it's large enough, it will also act like a reservoir.

In a car, it acts mainly as a reservoir, trying it's best to keep the voltage to the amp up during heavy current draw, it will also try to act as a filter, filling in any spikes (negative) and other electrical noise.

But when completly discharged, they act like a short untill they begin to charge, usally only a second or so, that's why they want you to charge it either through a resistor or a 12V trouble light.

Sassmaster

iTrader: (2)

in reality, the cap begins to charge as soon as voltage is applied to it, the massive inrush of current that it takes when it is hooked directly up to the power source is the capacitor charging. The time it takes a capacitor to charge is actually quite easy to measure. Since 1 farad is equal to 1 amp discharged over 1 second, from a battery that will produce 1000 amperes, the charge time, when directly connected is about 1/1000 of a second... which is why you get the big spark and the black mark.

They are like a short in that they have a massive amount of current very quickly, but the current never gets back to the battery... that is why they are an open circuit.

also...when you power up your amplifier power supplies do you charge each capacitor before you apply the rail voltage? I doubt it (unless you like playing with charged capacitors) you apply the voltage and they charge themselves.

on another note, the easiest and safest way to discharge a cap is to remove the fuse under your hood, and then turn on your system... your amps will drain your cap dead in a matter of seconds.

BassHamster

Dude, Chill...... If you hook up a fully discharged cap, and then put your fuse back in the fuse holder, it will pop the fuse. This is why a charge bulb/resistor is used. The manufacturers dont include them in the packaging because they look nice. They slow down the current going to the cap, plain and simple. This is something I have to deal with the noobs doing in my shop. So maybe you should be a little easy on the guys who are just trying to help then, eh champ?

BassHamster

You're absolutely right. The catch is that thoes caps are much smaller (in most cases) and dont cause such a peak current draw when charged.

Tom.F.1

iTrader: (1)

I have a 1 and a 2 farad cap. One came with a resistor and one with a light to charge or discharge them. The instructions say the cap could be damaged if you don't charge or discharge slowly. I've lost the resistor and broke the light that came with it.
So, i used a standard 12 volt incandescent light bulb, for my brake light, i always keep spares in the car, to charge my caps. The bulb lit brightly at first, and dimmed to nothing in less than 30 seconds. I left it there for a couple minutes anyway.
Then i put in the fuse. No spark.

Justin LaRouche

if u use a 25 ohm balast style resistor u can charge it realy safe. just hook up the negative first and use the resitor inline with the power wire till the cap is up above 11volts and u should be fine. u can excellerate the end of the charging process by running the vehicle in order to bring the battery voltage up to 14volts and just shut the car off when u finally conect the cap.

as far as big sparks and bombs! ya u can get a preaty good spark out of a cap and probably 99 times out of a hundred u will not have the cap die. however on the inside of the cap were the electrolitic conects to the terminal of the cap u can ark this point if the resistace at that point is greater than the external conection that u are making. in turn this may produce a usless component. seen it twice and did it once myself

any who hope that helps!

Grimmis1027

If you remember the value of the resistor, or find out from a friend with one, you can pick them up at electronic supply places for a few bucks each.

Pick one rated at 10W or higher, and ask for wirewound type, the value is usally printed on the top or side.

Sassmaster

iTrader: (2)

never blown a fuse yet doing it that way, but I guess if you're using cheap fuses it's possible.


again, a massive inrush for a tiny fraction of a second... a lightbulb will allow the same thing to happen actually. because it takes longer for the filament in the lamp to heat up enough to glow than it does to charge the cap.

again, there's lots of voo-doo out there about caps, and 99.9% of it is total BS and making things safe for people who are completely clueless.

I'm tired of the idiocy though, so believe what you like.

BassHamster

Im not trying to start a fight. I'd just like to respectfully dissagree. IF the cap charges before the "bulb" has a chance to ligh up..... then what causes the bulb to light up, and then slowly dim, and eventually go out. My opinion is that is infact the cap, and the internal caps in the amps charging.

I agree there is alot of BS out there about caps, but but thats why there are industry members and vendors on this forum, to try and help others. And like i said in another thread, there are people out there trying to pass their opinions as fact, and it IS annoying. I went on quite a rant. But simple insults dont help anything, Especially from a moderator.

abnormal72

You can use any value resistor that has a power rating suitable for 14 volts
for example you can use a 10000 ohm 1/4 watt resistor which are about 10 cents a piece or a more expensive power resistor with a lower resistance and higher power rating
The difference being is a larger resisitance takes longer for the cap to charge