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  • Electroscope for Electrostatic Charge Measurement

  • Created: Sep 08, 2014

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Summary

This do it yourself (DIY) electroscope circuit can accurately measure electrostatic charge. The electrostatic charge to be determined is stored on C1, a high quality MKT capacitor that has a value of 1-2μF. The voltage (u) across the capacitor (C1) is related to its charge (Q). The equation is U = Q/C1. The operational amplifier IC2 buffers this very high impedance source. An input lead is linked to one side of capacitor C1 and it is terminated with a test probe. The other side is linked to an earth lead and to a convenient earth point. The IC3 increases the low voltage level at the output of IC2 and it drives the moving coil meter M1 (±100 μA to ±1 mA center zero).


The circuit uses a MCP617 dual operational amplifier (op amp) that has a gain bandwidth product of 190kHz with a low typical operating current of 19µA and an offset voltage that is less than 150µV. The MCP617 uses Microchip's advanced CMOS technology, which provides low bias current, high-speed operation, high open-loop gain and rail-to-rail output swing. The MCP617 operates with a single supply voltage that can be as low as 2.3V, while drawing less than 25µA of quiescent current per amplifier. The MCP617 is available in standard 8-lead PDIP, SOIC and MSOP packages. This amplifier is ideal for battery and loop-powered applications as well as industrial process control, low-power battery-operated devices, portable equipment, data acquisition equipment, test equipment and low-end audio applications.


The operational amplifier IC1 produces a symmetrical supply with a center rail (earth) from the 9V battery. The supply current for the electroscope is in the order of 5mA, most of which is used by the Zener diode D1. Alternatively, a type that can operate at a higher supply voltage can replace the operational amplifiers. For example IC2 and IC3 can be replaced by a single (dual op-amp) TLC272 (see the DIL outline for this device to assign the new pins). IC1 can be replaced by a TLC271 (pin 8 should in this case be connected to earth and pins 1 & 5 left unconnected). The maximum supply voltage for these ICs is 16V so Zener diode D1 can be omitted which will bring the supply current down to 3mA.

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