Product

 

 

Product Description

The goal of this project, Digital Capacitance Meter, is to make the size as small as possible, lower the cost, maximize the testing range, and minimize the error.  The design of this C meter is to make it simple by using the existing low-cost components that can be found generally in the market.  It is achieved by using a Motorola MC14538 Multi-vibrator, Motorola 74LS145 Decimal Decoder, NS 4511 LED drive, Atmel AT89C2051 Microcontroller, a resister relay, a LED display, and green and red LEDs for unit switching.  The software involved is the MCS-51 assembly language.  The C meter can measure the general capacitors by plugging into the sockets on the IC board.  The time of measuring and output the value is approximately 0.5 second, and the measuring range is from 1pF to 330 uF.  Furthermore, the project is success with the error of +/- 5%.

 

 

Features

l          A digital LED displaying the value of the capacitance.

l          Can measure common capacitance in the range from 1pF~330uF.

l          Require only 9V battery power (1.5Vx4).

l          Switch automatically to suitable unit for capacitance value display.

l          Compact size

 

 

Theory

The main idea for the capacitance meter to work is to measure the pulse length generated by the chip MC14538B.  The pulse width of MC14538B is a constant of C x R.  If R, the resistance, is set to a constant, then we can get the value of C, the capacitance, by measuring the pulse width.  The chip is a dual, retriggerable, resettable monostable multi-vibrator.  It can be triggered from either edge of an input pulse, and produces an accurate output pulse over a wide range of widths.  

 

Performance

Factory value

Measurement value

Percentage error

22uF

22.9uF

4.09%

100uF

103uF

3%

0.331nF

0.320nF

3.3%

1uF

1.05uF

5%

330uF

326uF

1.21%

470uF

501uF

6.59%

As we can see from the about table, the measurement error can be kept within 5% for capacitance value range from 1pF to 330uF. The meter can still measure capacitance value beyond 330uF, but the percentage error increase above 5%.

 

 

Copyright © 2002 RESI Technology Inc. All rights reserve