[Dept. of Computer Science]

RC-Meter

Our Department and University.
See my other radios too! (Here is the next one.)

This small instrument is used to measure resistances in the range from 0.1 to 50.000 Ohms and capacitors from 100 pF to 50uF. It distinguishes good capacitors from bad (leaky) ones as well.

Data for this object
Type Wheatstone bridge
Ranges x0.1, x1, x10, x100, x1000 Ohm
x0.1, x1, x10, x100, x1000 nF
Cabinet Wood box with aluminium front, 13.5x12x5 cm.
Controls Range switch, Zero adjust 0.5-50, R/C switch
Produced November 1997

Principle

Resistance measurements are carried out with two resistance voltage dividers; one is formed by the unknown resistance and a known one, the other by the two parts of an (adjustable) potentiometer. The aim of the game is to set the adjustable divider in such the position where the bridge is balanced, that is, there is no voltage between the midpoints of the two voltage dividers. The bridge is fed from a 9V battery and the voltage between the midpoints is indicated by the panel meter. One can switch between five ranges by choosing among five known resistances.

Capacitors are not measured with DC, but with alternating current from an external signal generator; because the panel meter doesn't react to AC, a headphone is connected to seek balance of the bridge by ear. The range switch chooses between five known capacitors in this setting.

A bad (leaky) capacitor has a phase diagram that differs from the phase diagram of the (good) capacitors built into the bridge. As a result, the signal at the midpoint of the voltage divider formed by the capacitors is shifted in phase wrt. the signal fed into the bridge, and it becomes impossible to completely silence the whistle in the headphone. Consequently, leakage of the capacitor is qualitatively indicated by an audible level of residu sound at the best position of the potentiometer.

Schematic diagram

My Radio Workshop

Home-made measuring equipment is a nice addition to the radio workshop. My shop is only small (which forces me to work systematically and only at one radio at the time) and here is a picture:

The white box in the upper right corner is the central heating stove (the red tank below is an expander for the heating installation) and the cupboard in the lower corner contains tools. On top of it are my Oscilloscope and the (home-brew) Isolation transformer. On the workbench are the RC meter and a VOM. The shelf above the workbench houses the Signal Generator and a few boxes of parts.

Gerard Tel, gerard@cs.uu.nl.