How to Build an Ohm Meter

An Ohm meter uses the basic electronics principle of Ohm's Law to indicate the resistance portion of a circuit. Ohm's Law states Voltage equals Current times Resistance. (V=IR). This means Resistance equals Voltage divided by Current. (R= V/I).
Any three-variable equation where two of the variables are known will deliver the third.
For an Ohm meter, a known battery Voltage is applied across the unknown resistance and is measured by an Amp meter. Given the known Voltage and measured Current, this yields the unknown Resistance value.

Things You'll Need

• Resistor variety (10 ohm, 500 ohm, 8200 ohm for example)
• Amp meter with clip-type leads
• One pair of clip leads (usually a red and a black one, with clips at all ends)
• 6 volt battery

Instructions

How to Build an Ohm Meter

1. Connect (clip) the positive (+) battery lead to one metal lead of the resistor (it doesn't matter which lead).

2. Connect the positive (+) lead of the Amp meter to the other resistor lead.

3. Connect the negative (-) Amp meter lead to the negative (-) battery terminal.

4. Record the Amp meter reading, then remove any one clip to break the circuit.

5. Given an approximately accurate fixed 6 Volts from the battery and your measured current reading, use Ohm's Law to determine the resistance value. R= 6 Volts divided by (measured) current.
   Ex: If the measured current is 0.6 Amps (600 mA), then R= 6 Volts / 0.6 Amps = 10 Ohms.

6. Repeat steps with other resistors to verify Ohm's Law and your readings.

Tips & Warnings

• Today, it is difficult to find electronic meters which are not Digital MultiMeters (DMM) or older Volt-Ohm-Milliammeters (VOM) types. In each case, internal settings will convert Amp meters to Ohm meters for you. Common DMMs may be found in discount hardware stores for under $10 and will give a digital resistance readout.

• Commercial resistors will usually have tolerances no less than 1% and batteries may vary from their stated values. Each of these variations may add uncertainty to the final calculated resistance value. Verifying the voltage (with Volt meter) of the battery is the best way to derive an accurate resistance reading.

References

• Sample of inexpensive DMM