Dose a Capacitor have to be pre charged every time? - diyAudio
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Old 12th December 2008, 06:19 AM   #1
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Default Dose a Capacitor have to be pre charged every time?

I am putting in a system on a boat where the DC is disconnected on the pannel when not in use. The stereo may sit idle for a month at a time. So if I precharge the capicator and put it in the system, won't I have to precharge it before every use if the power is disconcected between uses?

I did not know there were soft start capicators out there when I bought mine, otherwise I would have bought that.

thanks
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Old 12th December 2008, 09:58 AM   #2
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Default Re: Dose a Capacitor have to be pre charged every time?

Quote:
Originally posted by boogeronmyfinge
I am putting in a system on a boat where the DC is disconnected on the pannel when not in use. The stereo may sit idle for a month at a time. So if I precharge the capicator and put it in the system, won't I have to precharge it before every use if the power is disconcected between uses?

I did not know there were soft start capicators out there when I bought mine, otherwise I would have bought that.

thanks

Assuming you talk about the power supply reservoir capacitor, why would you need to precharge it at all?

What's a soft-start capacitor?

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Old 15th December 2008, 09:54 PM   #3
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Default soft start capacitors

A large capacitor draws a large amount of power and will arc or cause other damage with the large inrush of current.

A soft start capacitor has a series of resistors and a relay configured so that the power to the capacitor is not connected by the relay until a certain voltage has been achieved.
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Old 16th December 2008, 04:37 AM   #4
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Use a 12V tail light bulb in series with the capacitor to precharge it before connecting fully to the 12V supply. Old car audio trick.
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Old 17th December 2008, 05:03 AM   #5
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Tail light bulb would work great but do i have to do that every time I use the stereo, cuz I know I will forget and let the inrush blow the cap.
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Old 20th December 2008, 08:37 PM   #6
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It must be a huge capacitor that you're using. Say if you're using a 1Farad cap, and off resistance of 1 million ohms, then your looking at a tau of 11.6 days to drop to 1/3 of the battery voltage.

So if it's sitting disconnected for a month it will be pretty much discharged. You could use a soft start circuit. Although I've never made one, i imagine putting a resistor between the battery and capacitor and having a relay bypass the resistor when the capacitor is charged would work.

Try searching for a slow start circuit.
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Old 24th December 2008, 07:52 AM   #7
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You could keep a 12V battery connected all the time for your boat, and have the battery on a safe , very low mA topping charge to keep the battery full, and the capacitor can stay connected.
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Old 24th December 2008, 08:52 AM   #8
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Here's a circuit I designed for your soft-start:

This will take care of your inrush current. The comparator (LM339) turns on the relay after the capacitor has charged up to the battery voltage, minus a 0.6V diode drop.

The circuit will stay latched (on) until you disconnect the battery and drain the 1F cap.

You can use an Op-Amp in place of the LM339 if you prefer. You can also use a 10 ohm, 10W resistor in place of the light bulb.
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