# Basic Capacitor Question

Discussion in 'General Electronics Discussion' started by j82long, May 10, 2012.

1. ### j82long

2
0
May 10, 2012
Hello,

Is it possible to charge a capacitor using something like a 12 volt car battery that once charged would release a 150 milliamp current through a circuit? If so how do I find this type/size of capacitor. I would like to use the smallest available.

Thank you for your help.

2. ### Laplace

1,252
184
Apr 4, 2010
Yes, such things are possible. But first consider how long the 150mA current must flow, and whether the circuit will still operate correctly as the capacitor voltage decreases over that time period. Capacitance is defined as the charge stored per volt, C=Q/V, where charge (in coulombs) equals current (in amperes) integrated over time (in seconds). C=I*T/(delta V) is the equation which works both for charging and discharging the capacitor, but you are mainly interested in discharging at 150mA rate. Just understand that once discharge starts, the capacitor voltage keeps decreasing (it's not like a battery). So if you are discharging through a resistance circuit, as the voltage decreases the current decreases as well. So choose a capacitance value where the decrease in voltage will be acceptable by the end of the period of interest.

Last edited: May 10, 2012
3. ### j82long

2
0
May 10, 2012
Laplace,

Thanks for your help!

You mentioned 2 things in your reply that made me realize I should have included more information into my original text. I am very new to all of this so please excuse if these sound like dumb questions, I am still trying to understand the basics of how all this works.

1. Does a capacitor have to be in a circuit to discharge? For example, is it possible to charge a capacitor on a 12 volt battery, then remove the capacitor and carry it into another room and discharge it there by touching the two leads of the capacitor?

2. If I were to do this would it dump all the current at once in one instantaneous 150 milliamp charge?

I understand that capacitors can be very dangerous so I am trying hard to avoid doing anything dumb here.

Thanks!

4. ### BobK

7,682
1,686
Jan 5, 2010
1. Yes, the capacitor can be removed from the charging circuit and attached to another circuit in which is it discharged.

2. The discharge rate depends on what it is connect to. If you short circuit the leads, it will discharge very quickly. If you put 1MoHM resistance between the leads it will discharge very slowly. 150ma is not a charge, it is a current. Charge is measured in Coulombs.

3. A capacitor is dangerous only if it is charged to a high voltage, not at 12V.

Bob