DIY Basic Analog Ohmmeter Circuit

By John Papiewski

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.

Ohm's Law

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.

Meter

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

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.

Resistors

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.

Completed Circuit

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.

About the Author

Chicago native J.T. Barett has a Bachelor of Science in physics from Northeastern Illinois University and has been writing since 1991. He has contributed to "Foresight Update," a nanotechnology newsletter from the Foresight Institute. He also contributed to the book, "Nanotechnology: Molecular Speculations on Global Abundance."