# Electrical – How to measure the charge of a pulse imposed on a signal

chargedischargepartial discharge simulate this circuit – Schematic created using CircuitLab

Hello All,
So my question is regarding how I can determine the charge of the square voltage source. For example, I need to build a detector that would detect when the pulse of the square wave is at 50 pico Coulombs, how would that be calculated. How can I determine the charge of the square wave from its voltage.
The square wave is imposed on the 60Hz voltage.

Charge, Q, coulombs, is current x time.

$$Q = I \cdot t$$.

For varying current, to measure charge passing through a point you integrate the current with respect to time.

$$Q = \int I dt$$

Your problem will be extracting this from the 60 Hz but you have given no information on the setup.

Basically, if I have a pulsating current, how can I find how many charges are pulsating, causing this AC current? I want to know how many coulombs are going back and forth. – Sam

To measure the charge going back and forth you need to measure the 40 kHz component of the current. Since it's a square-wave then the charge per half-cycle is given by \$Q = I_{PEAK} dt = \frac {1}{2}\frac {i_{PEAK}}{40,000} \$.