How to Use DMMs
- 1). Look at the front of a digital multimeter and make note of the four positions for plugging in the leads. On the Fluke 87 series, for example, they are marked for amperes, milliamps, or microamps, common and volts/ohm/diode test. Be certain that the leads are in the correct plug for the test operation.
- 2). One of the most common uses for DMMs is to measure voltage. Rotate the selector to either AC or DC volts and attach the leads to the circuit. Some meters will automatically set the proper range while others will have a range switch. Displays may include a bar graph as well as digits to show the voltage, and include a separate dial position for millivolts. Increment ranges manually by pushing the "range" button.
- 3). Rotate the dial selector to the ohms position to read resistance. The meter reads resistance by applying a small voltage to the probe tips. Be certain the circuit under test is not powered up and attach the probes. Advanced meters will give an audible warning if voltage is detected in the circuit. Inexpensive meters will not, and may be damaged if they remain attached.
- 4). Rotate the dial selector to the ampere setting to read current. Plug one test lead into the appropriate jack for amps or milliamps. Again, an advanced meter will provide an audible warning if it's configured improperly. An inexpensive meter will not do so. Remove power from the circuit under test and open a test point. Connect the meter in series and re-apply power. The display will read out in amperes or milliamperes.
- 5). Use the "frequency" setting to read frequencies accurately up to 200 KHz. While this is far too low for reading a computer clock directly, it may suffice to detect the presence of a divided-down clock signal used for peripheral circuits. Digital multimeters can detect the frequencies of pulse width modulators used in power supplies and automotive control systems. With the Fluke 87, simply rotate the selector to AC and push the "freq" button. Other advanced multimeters may have this function under a menu.
- 6). Use the duty cycle setting to read the on and off time of square waves or detect distortion on sine waves. Duty cycle is a sub-routine under the frequency setting. If the signal is a sine wave and the duty cycle varies from 50 percent, the signal is likely distorted.
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