As commercial ohmmeters are cheap and easy to find, anyone who wants a
good instrument for his own use will not build one. But you can build a simple
ohmmeter to demonstrate how it works or study how the big ones are made.
The simple ohmmeter described here can easily be made from a milliammeter or microammeter, and can be used in experiments, demonstrations and
many other applications.
Our ohmmeter is also a voltmeter. And since this ohmmeter can measure a
large range of voltages and resistances we can say that it is, in practice, a
multimeter. Voltages can be measured In the range between 0and 12 volts
(the range used to power almost all circuits in this book), and resistances
between 0and infinite.
lf using a2OO uA microammeter, the sensitivity will be 5,000 ohms per volt.
This value is typical of several commercialtypes of common multimeters.
But you can use any other ammeter with a full-scale value ranging from 100
uA to 1 mA. Simply change Rl to achieve best performance.
The schematic of the experimentalohmmeter is shown in Figure 1. Note that
we use only four components that can easily be housed in a small plastic box.
Layout is shown in Figute 2. Components mounting is not critical, and you
can replace the banana jacks by probes or any other method to make external connections.
Probes are connected to the external banana jacks depending on the measurement you intend to make. To measure voltage, the red probe is connected to J1 and the black probe is connected to J2. To measure resistance,
connect the red probe to J1 and the black probe to J3. Put the probes together and adjust P1 to get an indication of zero at the instrument scale.
By using a series of known resistors, it is possible to calibrate the meter scale
to read ohms directly. Start from 0 (with Jl and J2 shorted), and note the
meter readings when lk,2k, etc, reslstors are connected across Jl and J3.
After calibrating the device to read ohms, use a variable power supply to
calibrate the voltage scale.
Position of the polarized components, such As the ammeter and power supply (cells), must be observed.
Parts List - Experimental Ohmmeter
81 -3V-twoMcells
R1 - 10,000 ohm, 114W,5% resistor
Pl - 47,000 ohm, 1l4W trimmer potentiometer
M1 - 0-200 uA microammeter - see text
J1-3 - banana jacks
No comments:
Post a Comment