- Capacitance Meter Definition: A capacitance meter is a device used to measure the capacitance of discrete capacitors.
- Working Principle: It operates based on the proportional relationship between capacitance and a time constant.
- Measurement Method: Capacitance is measured using a 555 timer by calculating the time period of oscillations.
- Role of 555 Timer: The 555 timer acts as an astable multivibrator, with its frequency determined by the unknown capacitance (CX).
- Practical Use: Proper shielding of leads is necessary to avoid instability in low capacity measurements.
A capacitance meter is defined as a device used to measure the capacitance of discrete capacitors. It operates on the direct relationship between capacitance and a time constant.
The measurement method rests on that relation, so start with a simple RC circuit supplied by a voltage VIN (shown below).
During the charging period of the capacitor, the voltage across it at any instant is
The time needed to charge the capacitor to exactly 63.2 percent of the total input voltage
is termed the time constant, denoted ‘τ’.
Now assume a capacitor that charges from a constant current source and discharges through a resistor of fixed resistance. To measure its capacitance we can implement a 555 timer together with a few digital test instruments. The straightforward way to obtain the value is to measure the time period of the oscillations. By selecting a suitable charging resistance, the reading can be obtained directly in nanofarads or microfarads.
Compared with some other techniques, this arrangement measures large electrolytic capacitors, with values up to tens of thousands of microfarads.
If the capacitor under test leaks, this method reads a smaller value than the true capacitance. The result is also an efficient indicator of how the test capacitor will behave in most bypass and timing circuits. The block diagram of a basic digital capacitance meter built around the 555 timer IC is shown below.
The 555 timer in the circuit works as an astable multivibrator, and its frequency is governed by the unknown capacitance value (CX). The multivibrator output feeds a digital counter that measures the length of each square-wave cycle.
The cycle length of the square wave formed by the 555 timer can be calculated using the formula:
The counter resets at the peak value of the charging curve, when a clock of 100 kHz pulses is made ON and routed to the counter. After the discharge part of the cycle completes, the display updates and the capacitor value can be read out directly. For the display to show the capacitance value simply and correctly, the selection of charging currents and the reference frequency should suit the measurement range.
For measurements of small capacitance, keep the leads short and properly shielded, because 50 Hz mains hum can introduce minor instability.
