Can I calculate mA from coulombs?

Discussion in 'General Electronics Chat' started by kenw232, Mar 5, 2015.

  1. kenw232

    Thread Starter Active Member

    May 18, 2009
    I have a charge in a cap bank of 1uF at 1000V. Q=VC. This gives me the number of coulombs in the cap of .001C I think. How can I get the mA value of this? I can't because mA's are the number of coulombs per second and .001C is only happening in a single pulse of period of say 15mS. But...

    So .001C = .001Amps/second or 1mA. 1000mS / 15mS = 66.6. 1mA / 66.6 = .015mA per pulse. I get .015mA per pulse.

    Can anyone confirm this? Sounds right anyway.
  2. wayneh


    Sep 9, 2010
    One amp is a coulomb per second. And a farad is one coulomb per volt. So your 1µF capacitor stores 0.001 coulombs at 1000V.

    If all of that is added or removed in 15ms, that's 0.001 coulomb per 0.015 sec, current = 0.001/0.015 = 67mA. That's the average current over the 15ms pulse. Depending how the charge is added or removed, the current more likely follows a curve. It could be "infinite" at the start (limited by circuit resistances [and inductance]) and approaching zero near the end.
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
    dont forget to include the inductance of the interconnetions, and the stray inductance of the cap, that will slow down the discharge.
    wayneh likes this.
  4. kenw232

    Thread Starter Active Member

    May 18, 2009
    One amp is a number of coulombs (in the trillions?) per second, not "a" coulomb. If its .001C discharged every 15ms (or single pulse) then its .001C * the frequency. Frequency would be 1S / 15mS = 66Hz. .001C * 66Hz = .066C per second which is directly equal to Amps ("One amp is a [number] of coulombs per second"). .066A * 1000 = 66mA. So your right. And per pulse is 1mA then (66Hz / 66mA), not .015mA
  5. kenw232

    Thread Starter Active Member

    May 18, 2009
    Your right, an amp is a coulomb. I'm way off.
    GopherT likes this.
  6. WBahn


    Mar 31, 2012
    An amp is NOT a coulomb!

    An amp is a flow RATE of charge of one coulomb per second.

    This is like saying that a meter is a meter per second.

    Also, frequency in Hz is cycles per second. That is very different from 1/second.

    If you have pulses at 1 pulse per 15 millisecond (abbreviation is ms, not mS -- mS would be milliseimens, which is a measure of conductivity) then this would be 66 pulses/second. If you want to generalize that a bit and call a pulse a cycle, then you can call it 66 Hz.

    If you want the AVERAGE current during each pulse, then (as wayneh said) you would have

    <br />
\text{I_{avg} \; = \; \frac{Q/cycle}{T} \; = \; \frac{CV/cycle}{\frac{1}{F}} \; = \; \frac{CVF}{cycle} }<br />
\;<br />
\text{I_{avg} \; = \; \frac{\(1 \mu F\)\(1000V\)}{ \(\frac{15ms}{cycle}\)\cdot cycle} \cdot \frac{\(\frac{1C}{1V}\)}{\(1F\)}}<br />
\;<br />
\text{I_{avg} \; = \; 66.7\frac{\(1 \mu C\)}{\(1ms\)} \cdot \frac{1A}{\(\frac{1C}{1s}\)} \; = \; 66.7mA}<br />