# Circuit for measuring static charge

I want a cheap tool for investigating the source of and solutions for static electric shocks (from a vehicle).

This link shows a particularly attractive solution; just a FET, LED, resistor and battery.

I have some questions though.

1. Will it work with both positive versus negative charges? If not, how could it be modified?
2. How easily can the FET be damaged? Is there any way to protect it?
3. How could the circuit be modified to give a better idea of the magnitude of the charge? (For the sake of vaguely determining the painfulness of the shock - voltage or amount of charge or both? I don't know). Simply measuring the drain/source current with a multimeter perhaps?

I don't know if it will be more useful to have the device detect charge at a distance, or by contact - perhaps the indication of magnitude would be most useful as it would enable both.

• Well, the FET is good for "sensing" static electricity due to it's extremely high gate impedance. It won't drain much of that energy at all, while still showing that it is there. Both P and N-channel FETs are available, so both + and - fields are detectable (assuming the sensor is grounded.) On the other hand, voltage measurement does nothing to show how much current is behind that voltage. It could be 15kV but 1fA, so "barely" a shock at all. To measure the current, one way is to discharge through a known resistance and measure/calculate it from the voltage difference across the resistor. – rdtsc Apr 23 '15 at 16:17
• Cars are conductive, thus grounded. The tires are loaded with carbon black pigment to promote thermal conduction and UV resistance, so they are conductive. The only reason you get a static shock from a car is that you yourself get charged up as you maneuver out of the seat (it is the friction of you rubbing against the upholstery as you get out). Hope this information helps you. I know it is not exactly what you asked. – mkeith Apr 23 '15 at 19:20

The MPF-102 used in the linked circuit is an N-channel FET see

http://en.wikipedia.org/wiki/Field_effect_transistor

Here is the MPF-102 data sheet where the manufacturer takes no responsibility for it surviving more than +/- 25V on the gate. With a 1 megohm gate resistor it will tolerate +10000V (forward gate current 10 mA) but we cannot predict what negative voltage it might tolerate.

https://www.fairchildsemi.com/datasheets/MP/MPF102.pdf

For investigating static charges, consider making a small adjustable spark gap as an alternative to a transistor cuircuit that cannot be calibrated. A gap between two spheres can provide a voltage measurement without any electronics or voltage dividers, to an accuracy of about 3%. Intensity of the spark is a guide to its shocking effect.

http://en.wikipedia.org/wiki/Spark_gap#Sphere_gap_for_voltage_measurement

1. The transistor is an N-channel JFET- it will switch at voltages slightly below zero (essentially zero), so negative charge makes the LED turn off, positive makes it turn on.

2. Don't leave out the 1M resistor - it limits the gate current. Use a physically large resistor if possible to prevent arcing. JFETs (unlike MOSFETs) are not damaged by gate breakdown, only by destructive currents.

3. There is no way to know how much charge there is from the voltage without discharging it because you don't know the capacitance. If you wanted to detect the voltage, a source follower could be used but it might not be a good indication of how much shock a person would feel.