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Autoranging Digital
Capacitance Meter


BY DAVID H. DAGE


Autoranges from l pF to l µF and from 1 µF to 4000 µF.
Updates readings automatically.

THE DIGITAL- READOUT capaci-tance meter described here is a most useful instrument when one has to determine values of unmarked capacitors or those with unknown codes, or when checking the tolerances of marked compo-nents. Its autorange function greatly simplifies what would ordinarily be a measurement chore without this feature. Moreover, the meter's accuracy of over 1% (dependent on the tolerances of a few passive components from 1 pF to 4000 ~LF enhances its utility. The project is easy on the budget, too, as low-cost 7400 series logic and 555 timer IC's are used throughout.
To operate, simply turn on the unit, connect a capacitor to the test terminals, and read the cligital value displayed for any capacitor up to 1 ~LF Switching a mode switch from nF to ~LF extends the autorange function to 4000 ~LF and beyond, limited only by the leakage characteristics of the test capacitor.

How it Works. Traditionally, capacitance has been measured on an ac bridge by balancing known components against the reactance of an unknown capacitance at a given, fixed frequency.

 

However, instruments are now appearing which employ a different method to determine capacitance they measure time.

Here's how. Mathernatically, the voltage across a capacitor discharging through a resistor in a simple RC network can be expressed by the equation:

VC = VO (1 - e - t/RC)

where VO is the voltage across the capacitor when fully charged, R the resistance in ohms, C the capacitance in farads, t the time in seconds, and e the exponential constant or base for natural logarithms (approximately equal to 2.718). If we let a capacitor that has charged to a known voltage discharge through a fixed, stable resistance to some given voltage, the discharge time will be directly proportional to the component's capacitance, which then can be readily cletermined. The meter clescribed here employs this method of measurement, which readily lends itself to use with a cligital readout and eliminates null adjustments. As shown in Fig. 1, the capacitance to be measured is charged through RA and RB. When the voltage across the capacitor equals VREF comparator A sets the flip-flop, turning on the transistor.

 

The capacitor then discharges through RA until the voltage across it drops to one-half VREF At this point, comparator B resets the flip-flop, which in turn cuts off the transistor.

The capacitor then starts to charge up to VREF, and the cycle is repeated. A reference oscillator output at a fixed frequency is gated by the flip-flop output signal. The gated reference pulses are counte5 by a digital counter, decoded, and displayed directly as capacitance. The two comparators, flip-flop, transistor, reference voltage sources, and an output driver are all contained in one package-the common 555 timer IC.
The meter's autorange circuit functions during a single capacitor discharge cycle. If the three-clecade counter overflows, the reference frequency input is automatically clivided by ten. Simultaneously, the clecimal point in the digital display is shifted one position to the right. If necessary, the process is repeated once


Popular Electronics Febr. 1978