How to Use a Multimeter

Contributors: Nate

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Measuring Voltage

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To start, let’s measure voltage on a AA battery: Plug the black probe into COM and the red probe into mAVΩ. Set the multimeter to “2V” in the DC (direct current) range. Almost all portable electronics use direct current), not alternating current. Connect the black probe to the battery’s ground or ‘-’ and the red probe to power or ‘+’. Squeeze the probes with a little pressure against the positive and negative terminals of the AA battery. If you’ve got a fresh battery, you should see around 1.5V on the display!

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Use the V with a straight line to measure DC Voltage

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Use the V with a wavy line to measure AC Voltage

If you’re measuring DC voltage (such as a battery or a sensor hooked up to an Arduino) you want to set the knob where the V has a straight line. AC voltage (like what comes out of the wall) can be dangerous so we rarely need to use the AC voltage setting (V with a wavy line next to it). If you’re messing with AC, we recommend you get a non-contact tester rather than use a digital multimeter.

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What happens if you switch the red and black probes? The reading on the multimeter is simply negative. Nothing bad happens! The multimeter measures voltage in relation to the common probe. How much voltage is there on the ‘+’ of the battery compared to common or the negative pin? 1.5V. If we switch the probes, we define ‘+’ as the common or zero point. How much voltage is there on the ‘-’ of the battery compared to our new zero? -1.5V!

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Now let’s measure voltage on a breadboard or on a device: This is a very useful step to make sure the circuit you are working on is powered up correctly. If your project should be at 5V but is less than 4.5V or greater than 5.5V, this would quickly give you an indication that something is wrong and you may need to check your power connections or the wiring of your circuit.

Set the knob to “20V” in the DC range (the DC Voltage range has a V with a straight line next to it). Multimeters are generally not autoranging. You have to set the multimeter to a range that it can measure. For example, 2V measures voltages up to 2 volts. 20V measures voltages up to 20 volts. So if you’ve measuring a 12V battery, use the 20V setting. 5V system? Use the 20V setting. If you set it incorrectly, you will probably see the meter screen change and then read ‘1’.

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With some force (imagine poking a fork into a piece of cooked meat), push the probes onto two exposed pieces of metal. One probe should contact a GND connection. One probe to the VCC or 5V connection.

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This is the meter trying to tell you that it is overloaded or out-of-range. Whatever you’re trying to read is too much for that particular setting. Try changing the multimeter knob to a different range.

Selection Knob

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Why does the meter knob read 20V and not 10V? If you’re looking to measure a voltage less than 20V, you turn to the 20V setting. This will allow you to read from 2.00 to 19.99.

The first digit on many multimeters is only able to display a ‘1’ so the ranges are limited to 19.99 instead of 99.99. Hence the 20V max range instead of 99V max range.

Warning: In general, stick to DC circuits (the settings on the multimeter with straight lines, not curvy lines). Most multimeters can measure AC (alternating current) systems, but AC circuits can be dangerous. A wall outlet with AC or ‘main voltage’ is the stuff that can zap you pretty good. VERY carefully respect AC. If you need to check to see if an outlet is ‘on’ then use a AC tester. Really the only times we’ve needed to measure AC are when we’ve got an outlet that is acting funny (is it really at 110V?), or if we’re trying to control a heater (such as a hot plate). Go slow and double check everything before you test an AC circuit.


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