Precise Measurement of Electrostatic Charge
This circuit has the ability to accurately measure electrostatic charge. The charge under consideration is stored on capacitor C1, which is a high-quality MKT capacitor with a value ranging from 1 to 2 µF. The voltage (U) across capacitor C1 is directly proportional to its charge (Q) and is determined by the equation U = Q/C1. To handle this very high impedance source, operational amplifier IC2 functions as a buffer.
An input lead is attached to one side of capacitor C1 and terminated with a test probe, while the other side is connected both to an earth lead and a suitable earth point. IC3 amplifies the low voltage level present at the output of IC2 and subsequently drives the moving coil meter M1, which has a range from ±100 µA to ±1 mA with a center zero. The circuit is equipped with switch S2, which enables the selection of two measurement ranges. When S2 is closed, the amplification factor is 5, whereas, in the open position, the amplification factor becomes 10.
Internal Impedance of P1 and Alternative Measurement Methods
The internal impedance of P1 is set at 2.2 kΩ. Alternatively, instead of using P1, a digital multimeter can be employed, eliminating the need for resistor R7 (2-20 kΩ). The electroscope’s status is indicated by low current LED D2, confirming its operational state. The operational amplifiers utilized in this setup are MAX4322 from Maxim. These devices allow for common mode input voltage that extends to the supply rails, and the outputs can drive from rail to rail. Considering the maximum supply voltage of 6.2 V, zener diode D1 is incorporated to limit the supply voltage, ensuring optimal performance. Detailed specifications can be found in the datasheet available at www.maxim-ic.com.
IC1 generates a symmetrical supply with a center rail (earth) sourced from the 9-V battery. The electroscope’s supply current is approximately 5 mA, primarily consumed by zener diode D1. Alternatively, a higher supply voltage-compatible operational amplifier can be used. For instance, a dual op-amp like TLC272 can replace IC2 and IC3 (refer to the DIL outline for pin assignment). IC1 can be substituted with a TLC271 (with pin 8 connected to earth and pins 1 & 5 left unconnected). These ICs can handle a maximum supply voltage of 16 V, eliminating the need for zener diode D1 and reducing the supply current to 3 mA.
Operation of the precision electroscope is simple:
- Switch on S1, LED D2 lights.
- The test probe is touched to the earth lead to discharge capacitor C1 before a measurement is made. Alternatively, a small push button switch can be wired in parallel to C1 to discharge it.
- The test probe is now touched onto the charged part.
- The meter will show any charge, its polarity, and its value.
- After use, turn off to save the batteries.