Testing voltage follows a simple process. For those of you new to electrical testing, we asked our resident Pro how they might teach an apprentice how to use a voltmeter or multimeter. Once you know how to use a multimeter, you can move on to troubleshooting and fixing electrical problems, verifying power at outlets, checking continuity, and more.
Quick Article Summary
- Practically speaking, voltmeters and multimeters are the same
- 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 as voltmeters but also test current, resistance, and continuity. Sometimes, with the right probes and sensors, multimeters also capture additional information like temperature.
Going back to the voltmeter, or voltage meter, it measures the difference in electrical potential between two nodes of an electrical circuit. Admittedly, that sounds pretty complicated and technical. However, 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. Digital voltmeters also exist that simply give you a digital readout instead of using a dial. 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 uses 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 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, letting you take a measurement without damaging the tool. If the tool shows no range settings, your voltmeter likely uses an auto-ranging feature.
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 runs around 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 up to 12.6V DC. An alternator typically charges the vehicle’s 12V battery at ~14V.
Editor’s Note: When testing power tool batteries, a voltmeter shows you that 18V and 20V Max battery packs put out the exact same voltage.
Inserting the Multimeter Test Leads
Your multimeter includes at least two test leads, one red and one black. Each has a probe on one end and a plastic-covered metal jack on the other. The latter insert into the appropriate colored slots on your multimeter.
The black-colored jack will always plug into the port labeled “COM” (common). When measuring voltage, the red jack will plug into the hole labeled with a V.
See below for measuring current (amps).
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 during use on live circuits.
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 around 120V. However, if you get an overload reading (“OL” or “1”), you’ll need to raise the range on your multimeter.
This procedure is also pretty simple. On a battery, you’ll have the multimeter set to read DC voltage (V-). Touch the black lead of the voltage tester 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.
Measuring Current (Amps)
You will notice as many as two other spots on a multimeter for measuring current (amps). There are two primary things to understand when measuring current.
- You always measure current in series with a load
- You need to set the multimeter to the proper setting, including having the probes inserted into the proper ports
Take care to follow the instructions for the maximum amount of current the meter can handle. If you don’t, or you connect to the wrong port, you can blow the internal fuse or even fry the meter.
How Most People Fry Their Multimeter
Most of the times we’ve heard about people frying their multimeter, they measure current with no load. That means they set their meter to Amps, plug in the red probe to the spot marked 10A…and then promptly stick the probe tips into an outlet or otherwise in parallel with a high-current circuit.
That blows your meter. You just applied a load of 0-ohms to a high-current power source. Always measure current in series with the load. That means you place the meter in series with the hot leg.
Voltage is a sort of “potential” measurement. It’s what’s available. Current doesn’t actually exist until you create a load for it. What you never want to do is create a load with zero resistance (your meter).
Wrapping Things Up
Clear as mud? Good. If you remember nothing else, it’s that you typically only measure Voltage directly across an outlet. 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!