capacitor
I need to estimate the total charge that flowed from the capacitor for a short duration of 2ms. I am using an ATmega128 for this. Can someone advise me if analog sample/hold in front of an ADC is applicable for this? Or some other analog solution perhaps? I am not very familiar with this measurement procedure. Please see this picture:
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Capacitance in Farads F = Current I x time t / voltage dV
If the load current is limited by resistors and not a constant current load, the current will drop as voltage reduces, so the above calculation needs to be changed to a I x dT / dV calculus form where dT tends to zero. This becomes a bit more messy. So, let's keep things simple:
For the output case with a 3 Volt drop over 10 seconds, assuming constant current load, capacitance works out to:
F = 0.03 x 10 / 3 = 0.1 F = 100,000 microFarads.
Such "super capacitors" or "ultra capacitors" are available online fairly inexpensively these days, if the purpose is for a DIY project. For instance, on eBay, 3 pieces would cost $4.50 from this seller (plus shipping), or a single piece for $1 from a Canadian seller, if that saves you some shipping cost:
That is the capacitor needed.
The charging period and supply voltage have no effect on the capacitance calculation, assuming your supply can provide the required current (300mA), so ignore those.
If capacitor is connected to a closed circuit it will either charge or discharge, depending on the voltage "forced" on it (the equilibrium or steady state voltage, that is reached when there is no current flow through the capacitor). If this voltage is higher than the capacitors \$Q/C\$, such as with case when it is connected to a power supply, it will charge to reach this voltage. If there is no voltage source, or it's voltage lower than \$Q/C\$, the capacitor will discharge to reach this voltage as well. In case of passive elements only this voltage would be \$0\$. So the capacitor will act as a power source, and the current will "go" into capacitor itself (in order to equalize the charges on the capacitor plates) through the passive components while the energy is dissipated as heat and/or light and/or other types of energy, depending on components.
Best Answer
Q = CV so the loss of charge is also proportional to the loss of voltage (providing there isn't a recharge path that might be bolstering up the voltage).
If you know the capacitance then the the calculation is very straightforward.
Using a sample and hold (and ADC) that measures initial voltage (converts to digital and stores it) then measures final voltage (2 milli seconds later) and stores it is pretty much a normal sort of setup for analogue to digital converters. Virtually any off-the-shelf successive-approximation ADCs from TI, ADI, Maxim, NXP and Linear tech will fit the bill providing it has enough accuracy but that is delving into another area not covered in your question.