P1_____________470R Linear Potentiometer R1 to R6________47K 1/2W Cermet or Carbon Trimmers R7______________10K 1/4W Resistor R8_____________100R 1/4W Resistor C1_______________1nF 63V Polyester or Polystyrene Capacitor 5% Tolerance or better C2______________10nF 63V Polyester or Polystyrene Capacitor 5% Tolerance or better C3_____________100nF 63V Polyester Capacitor 5% Tolerance or better C4_______________1µF 63V Polyester Capacitor 5% Tolerance or better C5______________10µF 25V Electrolytic Capacitor (See Notes) C6______________47µF 25V Electrolytic Capacitor IC1____________4093 Quad 2 input Schmitt NAND Gate IC IC2___________78L05 5V 100mA Regulator IC SW1____________2 poles 6 ways Rotary Switch SW2____________SPST Toggle or Slide Switch J1,J2__________1 or 2mm. chassis sockets (See Notes) J3,J4___________4mm Output sockets B1_______________9V PP3 Battery Clip for PP3 Battery
A Capacitance Meter can be an useful tool for the electronics amateur, mainly to measure the value of capacitors obtained after dismantling old radios, PC cards and other electronics appliances.
Good quality capacitors can be found frequently, but often their capacitance value is unknown because the lettering on the case is hardly readable: in some cases being partially or completely erased.
This device can measure capacitors in the 1pF to 22µF range with good accuracy if low tolerance capacitors are used for C1 - C5, and is even more appealing as it requires a common digital or analog Multimeter set to 2V dc voltage range to clearly and cheaply display the unknown capacitor value.
This test tool can be useful also when a more tight value is required for a capacitor: selecting it from a batch will become much easier.
In conjunction with a voltmeter, this circuit gives a direct reading of capacitance. IC1A and IC1B form an oscillator and buffer, the frequency being set by R1 to R6 trimmers and C1 to C5 capacitors switched by SW1A and B in six different ranges. Output goes to IC1D, one of whose inputs is inverted and delayed by the unknown component by time proportional to its value. At the output of IC1D, normally high, a negative-going pulse with a width proportional to the capacitance appears, the duty cycle of the output and, therefore, the average voltage indicating the value of capacitance.
Supply voltage stability is required for accuracy: therefore, a small 5V voltage regulator IC was added.
To calibrate, set the dvm to its 2V range and remove CX, adjusting P1 for a zero reading (a very narrow pulse is present in this condition due to the inherent delay of IC1C). For ranges 2 to 6 (2nF to 20µF) connect a capacitor of the same value and tolerance of C2 to C5 in the CX position, set the frequency switch and adjust R2 to R6 trimmers for reading the correct value on the Multimeter display.
Obviously, this operation must be repeated five times, connecting the correct capacitor in the CX position and adjusting the corresponding Trimmer for each range.
To calibrate the first range (200pF) set the frequency switch in the first position and connect a 100pF low tolerance polystyrene capacitor in the CX position. Then adjust R1 for a reading of 100 on the display.
- P1 must be adjusted for a zero reading (Cx removed) whenever the range is changed.
- A +100% - 20% tolerance value is very common for electrolytic capacitors. Therefore, C5 should be a low tolerance type or a 1µF or 2.2µF polyester capacitor, 5% tolerance or better, can be used in the CX position to calibrate the last range.
- Wiring from the circuit board to J1 and J2 must be kept as short as possible to avoid stray capacitance.
- As described in the Parts List, J1 and J2 can be chassis sockets of 1 or 2mm diameter, or even two short leads ended with crocodile clips, but perhaps the best solution is to use a 2 way spring loaded, lever action, quick connection loudspeaker terminal.
Originally developed for terminating loudspeaker leads, these terminals have found numerous uses in applications requiring quick interconnection between equipment. The spring loaded lever allows wires to be easily inserted into the terminal where they are firmly trapped once the lever is released, giving a reliable connection.
- Total current consumption is 3.5mA.
- This is a modified version of an original circuit by Rae Perälä, Helsinki, Finland.