Charging 1F capacitor?

[Synomenon]

I just received a 1F capacitor. The instructions say to charge it first before connecting it to my car's battery.

It doesn't say how to charge it though.

Can I charge it by connecting a power and ground wire to it, the connecting the power and ground wires to the respective points in this while it's plugged in?

http://www.dealextreme.com/productimages/sku_2705_1.jpg

 

[futuristicmonkey]

1F ? :shocked:

You'll want to charge it with a suitably-rated resistor connected in series with the cap. If you multiply the resistance (in ohms) by the capacitance (1 Farad in this case, which makes things simple) you'll get the time constant of the circuit, in seconds. Multiply this time by about 3 and the cap will be charged to 95%. It should then be safe to connect it.

If you connect it directly to your car battery without charging it like this first, Bad Things (TM) will happen. You really don't want to do this.

 

[Synomenon]

Ok, but how do you charge this thing?

 

[futuristicmonkey]

Originally posted by: IsLNdbOi
Ok, but how do you charge this thing?

That depends. Information about what you intend to do with it and the specs of the cap would help.

 

[EightySix Four]

Originally posted by: futuristicmonkey

Originally posted by: IsLNdbOi
Ok, but how do you charge this thing?

That depends. Information about what you intend to do with it and the specs of the cap would help.

It's likely a car audio capacitor.

 

[Synomenon]

It is a car audio capacitor, that's why I posted this in "The Garage".

This is it:
http://www.raptor-online.com/p..._individual.php?id=212

 

[PottedMeat]

...I don't understand why you would need to precharge a capacitor. It's there to provide large amounts of current for very short periods when the battery can't. Hooking an polarized electrolytic capacitor backwards is fun though.

 

[Synomenon]

I don't know. I've looked at lots of different capacitors over the past couple of days and they all say they need to be charged before connecting to the car's battery. None of them say how to charge them though.

 

[Howard]

Originally posted by: PottedMeat
...I don't understand why you would need to precharge a capacitor. It's there to provide large amounts of current for very short periods when the battery can't. Hooking an polarized electrolytic capacitor backwards is fun though.

Perhaps the high current flow would blow a fuse?

 

[BUTCH1]

They usually include a charging resistor, this will slow charge it so when when you hook it up the sparking will be minimal. If you don't use one the cap will try to fill instantly upon hookup damaging the contacts..

 

[EightySix Four]

When a capacitor is first installed, it has no voltage in it, so if it is hooked up to a battery, it will act as a short circuit and draw a LOT of current (not a good sight). Capacitors need to be "charged up" first. This is simply done by either putting a resistor (1k-ohm or more) or a test light between the positive post of the battery and the positive terminal of the cap (with ground hooked up to metal). After a few minutes, the capacitor will be charged up, and it can be connected. Every time the capacitor is "drained" i.e the lights were left on in the car and battery is dead, the capacitor should be DISCONNECTED, battery charged, and then capacitor has to be recharged and reconnected.

 

[Synomenon]

So can it be charged from a wall outlet using the adapter above instead of connecting it to the car's battery? I'd still use the resistor. The adapter above converts AC to 12V DC like a car's battery.

 

[EightySix Four]

Originally posted by: IsLNdbOi
So can it be charged from a wall outlet using the adapter above instead of connecting it to the car's battery? I'd still use the resistor. The adapter above converts AC to 12V DC like a car's battery.

I wouldn't. Those 12v adaptors do not put out much power.

 

[Comdrpopnfresh]

Charge it by applying a voltage difference across the terminals. Just be sure, if the capacitor has a bias (voltage: +/- can only be applied one way) you get it right- trying to charge a polarized capacitor can lead to explosions.

 

[Howard]

Originally posted by: IsLNdbOi
So can it be charged from a wall outlet using the adapter above instead of connecting it to the car's battery? I'd still use the resistor. The adapter above converts AC to 12V DC like a car's battery.

The capacitor's maximum voltage is probably much lower than 120V.

 

[EightySix Four]

Originally posted by: Howard

Originally posted by: IsLNdbOi
So can it be charged from a wall outlet using the adapter above instead of connecting it to the car's battery? I'd still use the resistor. The adapter above converts AC to 12V DC like a car's battery.

The capacitor's maximum voltage is probably much lower than 120V.

It's 24v max.

Like I said, do it right and use your car battery, there's no reason not to do it properly.

 

[rdp6]

definitely use a resistor to manage the transient current. There is a cool current vs. time plotting tool at http://webphysics.davidson.edu..._semester2/c11_rc.html which illustrates the transient response of a simple RC circuit so you can see how the charging process looks in terms of voltage and current vs. time.

Also make certain that your resistor is capable of handling the power for your circuit design, e.g. 12V battery across 1K resistor (assuming totally discharged capacitor): V^2/R is 12^2/1000 = 0.144W (1/4 W rating is commonly available)

Next, you can use 5x the RC time constant to get to ~99% of the charge: 5*R*C => 5*1000*1 = 5000 seconds to 99% charge. 3*RC will get you to 95%, which will probably be plenty safe for your application. An hour may seem like a long time to charge a cap, but you have a large capacitor. You could reduce the resistor value for a proportional decrease in charging time if you like, just make certain the resistor can handle the corresponding increase in power.

 

[Synomenon]

Look guys. I'm not an electrician. I'm not going to design and build a circuit for this. I guess the most simple way is to charge it through the car's battery.

 

[PottedMeat]

Originally posted by: Howard

Originally posted by: PottedMeat
...I don't understand why you would need to precharge a capacitor. It's there to provide large amounts of current for very short periods when the battery can't. Hooking an polarized electrolytic capacitor backwards is fun though.

Perhaps the high current flow would blow a fuse?

Yeah... forgot about that.

Hmm, so the capacitor is always in parallel with the battery? If you ever disconnect the battery and reconnect it you would blow a fuse?

----

This isn't anything special. It's just a resistor in series with the battery and capacitor. What don't you understand about crazySOB297 and rdp6's posts?

 

[EightySix Four]

Originally posted by: IsLNdbOi
Look guys. I'm not an electrician. I'm not going to design and build a circuit for this. I guess the most simple way is to charge it through the car's battery.

battery -> 1 kohm resistor from radio shack -> + terminal on capacitor

- terminal on capacitor -> metal on car


Do NOT plug it directly to the battery, it will be bad.

 

[rdp6]

Assuming your wire resistance and the internal resistance or your battery is a few milliohms (a guess), 12V/.005 = 2400A. Maybe someone can give some better figures for the resistances, but in any case you are looking at a pretty dangerous situation.

It isn't the voltage that kills, it is the current. Don't hurt yourself. No matter how you slice it, if you are going to charge a capacitor, you are doing some electrical engineering. Even if you do it without regards to your safety.

Your circuit goes like this: Battery red terminal to resistor to capacitor to battery black terminal. A very simple series circuit.

http://www.electronics-tutorials.ws/rc/rc_1.html

Maybe more background than you want, but it isn't difficult at all. BTW, the switch in the diagram on the linked page is you completing the circuit , i.e. finishing the wiring.

All this stuff applies to discharging your capacitor, too. Don't short the terminals on a 1F cap at 12V without a resistive load.

I would go with the 1K resistor for a few cents at an electronics store, wire the circuit, and go have some lunch. It won't hurt at all to leave the circuit hooked up indefinitely.

 

[Synomenon]

So I take a wire then:

- connect one end to the car battery's positive terminal
--- connect the other end of the wire to a 1kOhm resistor
--- then take another wire and connect one end to the other end of the resistor and the other end of this second wire to the capacitor's positive terminal

- Then I take a third wire and connect the battery's ground / negative terminal directly to the capacitor's ground / negative terminal

Did I get that right?

 

[Knavish]

Did you try this yet? Hopefully you're charging it mounted, not just sitting on top of the battery or something like that. I sure wouldn't want to be carrying a charged 1F capacitor around (or trying to hook a precharged cap into a system!).

Like this guy says in Yahoo Answers:

You should not hook a capacitor up without charging it. To charge it use a test light, hook the negative of the cap to the negative battery terminal or chassis ground, then connect one end of a test light to the positive of the cap and the other to the positive of the battery. The light will be bright at first, then will eventually dim to nothing, at that point it is fully charged. if you don't have a test light, you can use a resistor, but you will have to check the voltage at the cap to see if it is charged, it should read the same as the battery, around 12.6 volts. If you just hook it up directly without a load, you will get a big spark, like you touched positive directly to ground.

 

[Synomenon]

The cap. has a digital voltage readout built in so I don't think I'll need to use the light bulb method. I didn't think it would be this complicated to add a cap. to a system. I thought all you had to do was charge it somehow then unplug the power from the amp., plug it into the cap., then run a new power wire from the cap. to the amp. and ground the cap. where the amp. is grounded. Also, install the cap. as close to the amp. as possible.

 

[rdp6]

So I take a wire then:

- connect one end to the car battery's positive terminal
--- connect the other end of the wire to a 1kOhm resistor
--- then take another wire and connect one end to the other end of the resistor and the other end of this second wire to the capacitor's positive terminal

- Then I take a third wire and connect the battery's ground / negative terminal directly to the capacitor's ground / negative terminal

That's correct. I'd also follow Knavish's advice wrt mounting the capacitor first. You can use a voltmeter across the capacitor terminals to check the charging progress, which will be initially (relatively) fast, slowing as it reaches the final voltage. With a 1K resistor you are looking at ~50 minutes to 95% of final voltage, depending on actual resistance and capacitance.