Read a Digital Ohm Meter
An ohmmeter's purpose is to measure resistance. A digital ohmmeter is generally combined with an AC/DC voltmeter and ammeter in a digital multimeter. It gives you the answer with no interpretation of where the needle is pointing as in a low cost or an antique multimeter. With a little knowledge about resistance one can achieve very accurate readings.
Contents
Steps
- Obtain a digital multimeter with ohmmeter capabilities in the range of resistance you wish to measure. Most multimeters cover it all except for extremely high (greater than 100 mega ohm or very precise low readings between 0 and 1 ohm, like 0.249 ohm). Many DMMs have auto ranging feature.
- Plug two test leads into the OHM terminals. Sometimes they are separate from voltage or current terminals. Usually there are three terminals. Common, voltage/resistance, and current.
- It is possible to damage some sensitive circuits when taking an ohmmeter reading because there is a voltage from the internal battery applied to the Unit Under Test (UUT). For low ranges it is about 1.5v. For high ranges it could be 30V.
- Turn the meter on. Wait for it to reset itself. Select the resistance function and a range approximating what you expect to measure. Place each probe on both ends of the circuit. For example, if you are checking an electric fan you might put a probe on each terminal of the power cord to see if you get a low resistance when the power switch is on.
- To measure very high resistance (over 100,000 ohms), ensure that you are only reading the Unit Under Test. If you are holding the probe tips and the UUT wires between your fingers the reading will be of the UUT and your body in parallel - ie. less than the UUT really is.
- The accuracy for any digital device is the stated accuracy plus or minus 1 digit. If you are trying to read a 1.0 ohm on a 4 digit multimeter set to the 200 ohm range with a 1% stated accuracy you could be off 10% because of the plus or minus 1 digit. So it is important to use the lowest range that includes the resistance you are measuring.
- To read low resistance accurately, some meters have a relative function. Short the tips of the probes by rubbing them together, and look at the reading. It will be between 0.1 and 0.5 ohm depending on your probes. Now press the relative button. The reading will go to zero. Now when you read the circuit, the reading will represent only the resistance at the probe tips. The probe wire resistance has been zeroed out.
Tips
- Your body is an electrical conductor. You can measure your resistance by setting the meter to its highest range and squeezing a probes between the fingers of each hand. For this reason do not hold the probe tips onto the circuits when you are measuring resistance over 100,000 ohms. You will affect the reading.
- Everything has resistance unless it is at a temp very far below zero. Then the resistance approaches zero (0.000000000), therefore when you are attempting to measure a low resistance accurately, remember that the test leads have resistance and are adding to the UUT resistance. Either account for it mentally or use the meter's zeroing function -- not all meters have this capability.
Warnings
- Never attempt resistance readings on energized circuits. It can damage the ohmmeter and scare you into a heart attack.
- Headphones and other type devices might be permanently magnetized from the DC current flowing through them.
- Always check for parallel paths. In other words, other components in a circuit will affect your reading. To isolate a component may mean de-soldering one of its terminals.
- Very low power semiconductor circuits may be damaged from the voltage on the probes.
Things You'll Need
- A digital multimeter. test probes, and something to read the resistance of.
