# 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
54
1
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 Expert

Sep 9, 2010
12,382
3,239
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
2,449
428
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
54
1
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
54
1
Your right, an amp is a coulomb. I'm way off.

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6. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
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

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