How to Use a Voltmeter: Training the Apprentice
Testing voltage follows a simple process. For those of you new to electrical testing, our resident Pro teaches you how to use a voltmeter to check outlets, appliances, and more.
Quick Article Summary
- Set the mode and (if applicable) the range
- Insert test leads into the tool
- Touch/insert the probe tips to the outlet/switch/device/etc.
Multimeters include the same features but also test current, resistance, etc.). A voltmeter, or voltage meter, measures the difference in electrical potential between two nodes of an electrical circuit. Admittedly, that sounds pretty complicated and technical, but it takes just a few steps to master.
Voltmeter or Multimeter?
First things first, voltmeters typically operate as an analog device. Nearly every Pro troubleshoots by using a multimeter at some point. You can pick up a basic one for less than $20, and they’re a lot more widely available. In this article, let’s assume you want to use a multimeter.
How to Use a Voltmeter: Find Your Setting
Set the Dial
Almost every voltmeter or multimeter has a large dial to set the mode. On this dial, the voltage setting will be denoted with either a V~, which will measure AC (alternating current) voltage or a V- for measuring DC (direct current) voltage. Sometimes manufacturers combine these modes. Household circuits and outlets use AC, while batteries and portable electronics run on DC.
On multimeters with manual range settings, set the dial just above the maximum expected voltage. Many of these measurement tools have a few options marked out for various voltages. This changes the sensitivity of the meter, allowing you to take a measurement without damaging the tool. If the tool shows no rang settings, your voltmeter likely uses an auto-ranging feature to detect the correct voltage.
If your meter lacks an auto-ranging feature, no worries, just set it higher than what you expect the voltage to be. For example, if you plan to test a wall outlet (in the US), which run at 120V, set the meter at 200 V~. If you have no idea what to expect, set the voltage on the meter to the max setting.
Just for your own edification, household batteries typically operate at 9V DC or below, while a fully charged car battery runs at 12.6V DC. When testing power tool batteries, a voltmeter shows you that 18V and 20V Max put out the exact same voltage.
Inserting the Multimeter Test Leads
Your multimeter will come with two test leads, one red and one black. Each has a probe on one end and a plastic-covered metal jack on the other. These insert into the appropriate colored slots on your multimeter.
The black jack will always plug into the port labeled “COM” (common). Since we’re testing voltage, the red jack will plug into the hole labeled with a V. If you don’t have a V marked on your voltmeter, stick it in the hole with the lowest number, or the one marked with mA.
Safety is key when learning how to use a voltage tester. When dealing with electricity, it doesn’t take a whole lot to stop a heart. When touching a live circuit, keep your fingers clear of the metal probes. Also, keep the probes from touching each other.
Jamming Metal Probes Into Hot Outlets
Basically, you test circuits by attaching the leads in parallel. Working from the earlier example of testing wall outlets, take your black (negative) test lead and insert it into the larger vertical slot of your outlet. Most black probes have a retaining bump built onto them so that you stick it in and let go.
Next, you’ll touch the red lead to the positive hole. This will be the smaller vertical hole on a 120V 15A outlet. Check the reading on the meter. You should get a reading of 120V. However, if you get an overload reading (“OL” or “1”), you’ll need to raise the range on your multimeter before it gets damaged.
This procedure is also pretty simple. On a battery, you’ll have the multimeter set to read DC voltage. Touch the black lead to the negative terminal, and the red lead to the positive terminal. If you get no reading on your meter, check to see if your meter has a switch labeled DC+ or DC-. Switch the position if it does. If it doesn’t, reverse the positions of the red and black probes.
Still not getting a reading? Drop the voltage setting by one step until you do.
With the right equipment, you can safely test any circuit around the house or car. What are some of your best practices? Leave our apprentices a tip on how to use a voltmeter below!
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