Constructing your own analog ohmmeter circuit will build your knowledge of electronics and leave you with a useful tool. You’ll learn the principles and practical application of meter movement, Ohm’s Law, and simple direct-current (DC) circuits. You can build a simple ohmmeter in a couple of afternoons with about $30 worth of parts.
One of the first principles an electronics hobbyist, technician or engineer commits to memory, Ohm’s Law, states that the electrical potential across a component, in volts, equals the current flowing through it, in amps, multiplied by its resistance in ohms. Ohm's Law figures prominently in the operation of ohmmeters because the meter doesn’t react to resistance; it reacts to current. You can determine the resistance of a common voltage by reading a current on the meter. As the current goes up on a meter scale, resistance goes down proportionally.
Look for panel meters at electronics and hobbyist outlets. Known as meter movements, theyhave little to no connected electronic parts. Purchase a current meter in the range of about 1 milliamp, meaning that the meter will be suitable for small battery power. Meters in a higher range will read poorly or drain a small battery quickly.
Battery power works well for ohmmeters. You need only a modest DC voltage and current, so you won’t need to build an AC power supply. Use a 9-volt radio battery to run an ohmmeter. Compact and inexpensive, 9-volt batteries put out plenty of power, and the 9-volt clip is secure and easy to solder.
The ohmmeter needs a set of resistors to set the range and zero the scale. An ohmmeter needs to read resistances of 100,000 ohms accurately one minute, and a few hundred ohms the next. A single analog scale cannot resolve this range accurately, so analog designs use range switching. A rotary switch with four or more positions will let you select from a set of range resistors. Use Ohm’s Law to calculate the values of range resistors you’ll need to drive a 1-milliamp meter current with a 9-volt battery.
You'll also need a variable resistor to calibrate, or “zero,” the meter for a given range by getting a meter reading with the ohmmeter’s probe wires shorted together. Adjust the variable resistor until you get a full-scale reading on the meter.
The completed ohmmeter circuit will put the meter in series with a variable resistor, which connects to one side of the battery. Connect the other side of the battery to the rotary switch, which selects one range resistor at a time. Then connect one probe wire to the other side of the meter, and the other to the open side of the range resistors.
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