Use an Ohmmeter

An ohmmeter is an electronic device which measures resistance in an electronic component or circuit. It is composed of a ruled scale with an indicator needle or digital display, a range selector, and two probes. This article will explain the basic operation of one.

Steps

1. Disconnect completely and/or turn off all power to the circuit you are testing. You must have a completely dead wire or circuit in order to ensure accuracy in measurement, as well as your own safety. Your ohmmeter will supply the voltage and current for your circuit so NO other power is necessary. As the instructions for our Blue Point volt/ohmmeter state, testing a powered circuit can "cause damage to the meter, circuit, and *you*."
2. Select an ohmmeter suitable for your project. Analog ohmmeters are very basic and inexpensive, and usually range from 0-10 to 0-10,000 ohms, digital devices may have similar ranges or "auto-range", reading the resistance of your device or circuit and selecting the correct range automatically.
3. Check the ohmmeter to see if it has a battery. If you just bought an ohmmeter, the battery may have come pre-installed in your unit, or packaged separately with instructions for installing it.
4. Plug your test leads into the sockets on your meter. For multi-functional meters, you will see a "common", or negative plug, and a "positive" plug. These may also be colored red (+) and black (-).
5. Zero your meter if it is equipped with a zeroing dial. Notice that the scale reads in the reverse direction of most conventional measuring scales, that is, less resistance is to the right, and more resistance is to the left. Zero resistance should be observed when your probes are connected directly to each other, and you can adjust this by holding them together and turning the "adjust" dial until the needle on the scales is at zero ohms.
6. Choose the circuit or electrical device you want to test. For practice, you can use almost anything which conducts electricity, from a piece of aluminum foil to a pencil mark on a sheet of paper. To get an idea of the accuracy of your readings, buy a few different resistors from an electronics supplier, or some other device with a known resistance value.
7. Touch one probe to one end of a circuit, the other to the other end, and note the reading on the instrument. If you bought a 1000 ohm resistor, you can place a probe on each conductor on the resistor, and select the 1000 or 10,000 ohm range, then read the meter to see if indeed it reads 1,000 ohms.
8. Isolate components in a hard wired electrical circuit to test them individually. If you are reading the ohms on a resistor in a printed circuit board, you will have to unsolder or unpin the resistor to assure you are not getting a false reading through another path in the circuit.
9. Read the resistance of a run of wire or a branch of a circuit to see if there is a short or open break in the circuit. If you read "infinite ohms", there is no path for the electrical current to follow, and in simple terms, this suggests a burned out component somewhere in the circuit, or a broken conductor. Because many circuits contain "gate" devices (transistors or semiconductors), diodes, and capacitors, however, you may not read continuity even when the complete circuit is intact, which makes it difficult to test complete circuits with only an ohmmeter.
10. Turn the ohmmeter off when not in use. Occasionally the test leads will become shorted while the device is stored, draining the battery.

Video

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Tips

• If you are investing in an ohmmeter for general use, buy a good quality "multimeter" (multi-tester), which can test other electrical values such as voltage and amperage.
• Become familiar with electrical and electronic terminology and circuit board schematics, as well as reading schematic (or wiring) diagrams.
• It's good to know that even though a resistor says it's 1000 ohms it could be off by up to 150 ohms either way. Smaller resistors will be off by smaller amounts, larger ones by larger amounts.
• Try a variety of different experiments with electrical conductivity. Draw a line on a sheet of paper with a graphite pencil, and touch the probes to each end, you should find it does, indeed, conduct an electrical current.
• To learn the ranges of your ohmmeter, buy a variety of different resistors and test each one according to its specified resistance. Don't listen to this web site

Warnings

• When testing an electronic circuit, be sure it is not live. This means unplugging it, 'and discharging any capacitors in the circuit. Televisions and other electronics with cathode ray tubes and microwave ovens can have capacitors charged with thousands of volts even when unplugged. Such items are fatally dangerous and should be handled professionally.

Things You'll Need

• Ohmmeter.
• Sample conductors or resistors to practice with.

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Sources and Citations

• http://www.allaboutcircuits.com/vol_6/chpt_2/2.html

See also

• Calculate Wattage
• Read a Digital Ohm Meter
• Calculate Kilowatt Hours
• Measure Amperage
• Read Axial Lead Resistors
• Read a Capacitor
• Calculate Kilowatts Used by Light Bulbs
• Measure Inductance
• Test a Four Terminal Relay
• Test a Heating Element

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