How to use a multimeter to test batteries, phone jacks, cables and more
I stopped by the hardware store the other day to pick up one of those door stopper thingies that you install in a hinge to keep the doorknob from slamming into a wall. We have them all over the house but one was brass colored where my wife wanted it to be white, so there I was, looking for something I was trying to describe by waving my hands in the air and babbling incomprehensibly.
Sure, go ahead and laugh. I know now that it's called a "Hinge Pin Door Stop" but I didn't know that then, so I had to wave my hands around a bit.
Anyway, while I was doing that a woman came into the store and seemed so anxious and distracted that I told the clerk to go ahead and help her while I tried to find what I wanted. Before I could even start doing that, she blurted out a tale of woe about some electrical thing and a seemingly quite critical need to find out why it wasn't working. Her husband had sent her out to buy a "continuity tester".
The clerk shook his head sorrowfully and said he didn't think they had anything like that.
I had to intervene. "Sure you do", I said, "She needs a multimeter".
So we walked over to where the multimeters were and, sure enough, they had several models. I explained that the meter was a great thing to have around the house for testing all sorts of things and helped her pick out a basic model. I showed her how to find the "Ohms" setting for continuity testing and she left happy.
The clerk also realized that he did understand what I wanted and led me to them. I picked out the white one that my wife wanted and I was on my way home too.
I see all sorts of articles around the web that tell you how to use a voltmeter to test an electrical outlet. They usually make sure to warn you how dangerous electricity can be and will caution you about which voltmeter probe to stick where and being very sure not to touch bare metal and all that.
Why would you ever do that? If you need to test an electrical outlet, plug a lamp into it!
Sure, if you are a handy guy and have a basic understanding of home appliance stuff, maybe you'd be poking your meter inside a wall switch now and then, but most homeowners aren't going to do that and they probably shouldn't anyway: I've seen "handy" guys knocked flat on their butt because they were careless with home electricity. They were "lucky" - people get killed messing with stuff they have no business messing with.
However, some of the stuff in your home is low voltage: telephone jacks, doorbells, thermostat wiring. You can get a little tingle from that kind of stuff, and you can also damage things if you are really unlucky, but it would be very unlikely that you'd end up on the evening news because of it.
Even better, a lot of electrical things can be tested with power turned off and it gets much harder to kill yourself when there is no power attached.
Though it is not impossible. A classic example is the old CRT tubes that TV's and computer monitors used to use. Those puppies can store tremendous amounts of voltage even after being unplugged. We don't see a lot of those anymore, but your microwave oven can be every bit as dangerous. Your average homeowner should not be messing with stuff like that.
You can buy a perfectly good home use multimeter for around ten dollars. If you go poking around Amazon, you'll find them at that level, but you'll also find them much higher - even hundreds of dollars higher. Those more expensive ones are either better quality or they may have special features for testing specific things.
If you need to test specific things like network jacks and cables, you'll need those tools. For testing things around the house, probably not.
Oh, they do make things for around the house testing. Here's a phone jack tester, for one. That doesn't do anything that you couldn't do with a standard cheap multimeter, but you woud have to learn or already know a bit about phone wiring to use the meter. Specialized tools like that save you from having to learn that stuff.
Here's another type of specialized tester. These are for RS232 ports - the old serial ports. Almost nobody has those any more because they've been replaced by USB - and of course you can find testers for that as well.
Again, most or all of the tests these things do can be done with a multimeter. You might need some basic knowledge about how the thing you are testing is supposed to work, but it's not usually difficult to find that kind of thing on the internet.
There are also many things you can test without knowing any details at all. You'll need to understand the basic functions of the meter and a little bit about electrical circuits. I'm going to talk a little bit about both these things here and your meter almost certainly came with a little instruction booklet that tells you how to use it.
Once again, the Internet is also chock-full of tips ranging from the most basic to very advanced. These are wonderful times we live in aren't they?
A little ingenuity goes a long way
Sometimes the hardest part of testing is getting stuff hooked up so that you can test it. As I said, alligator clips are helpful.
Shortly after moving into our new home we had a phone jack problem. It turned out to be a combination of a short and some mis-wiring but most of my tools and stuff were still packed away and the things I really needed - alligator clips - were not turning up when I needed them.
I improvised using these molly bolts. The screws on the top gave me a place to hold down the wires I needed to test and the little slots gave me a place to jam in the meter test probes so I didn't have to hold them in place.
Rube Goldberg? Sure. But I got it done, didn't I?
Basic Meter features
Your meter basically can test three things: A/C voltage, DC voltage, and continuity. When you are shopping for a home meter, those are the three things to look for.
It may not say anything about continuity - that's what confused that clerk at my hardware store. He knew that the meters tested some mysterious thing called "Ohms", but he didn't know that means they can test continuity. What Ohms actually measures is resistance, which is something useful to measure now and then, but having very low resistance means you have good continuity (at least for the kinds of things you might be playing with around the house).
Your meter will come with cables or probes. It's nice to have alligator clips too - these are handy for clipping on to wires that you want to test.
Let's talk about those test cables, shall we? One of the places a manufacturer can save money is right there. Those cables have thick metal ends, but the insulated wire that connects those ends is very thin. If you cut one open, you might be surprised at just how thin it is - especially if it came with a very cheap meter.
Cheap test probes like that do tend to break with use. You won't know that it's broken, so that's the very first thing we will learn how to test: are the meter cables good?
Plug the red cable into the red jack on the meter and plug the black cable into the black jack. If they aren't colored, put the black into the "+" or "Positive" jack and the red into the "-" or "Negative".
Honestly, it doesn't matter if you get that wrong - you won't hurt the meter if you are testing continuity. However, certain instructions for testing more complicated things will tell you to put the red or the black in a specific place and while it may not matter if you have them in backwards, it could, so get in the habit of doing it right.
If you look closely at the picture at right, you'll see that I have alligator clips hooked up to my meter. That's because the original probes that came with it were cheap and they broke a year or so after I bought this. I improvised by finding some studs that fit the meter nicely and they let me use these clips that I already had. The "black" one is actually green, but electricity doesn't care what color the insulation is, so this works fine for me.
If you notice, the meter's dial is turned to the RX1K position and there is a little symbol beside it that might remind you of a pair of headphones. That's the Ohms symbol, and that's where we want the meter set to measure resistance.
If you now touch the other ends of the meter probes together, the meter dial should move to the right. A digital meter will change from displaying infinite resistance (or possibly just a "1" or "0L") to zero resistance.
If it does not move, you either have bad meter probes or you forgot to put a battery in the meter.
Yes, a battery. The meter needs a battery to measure continuity.
There's another thing about cheap meters: the battery might not be easy to get at. Mine needs a very tiny screwdriver to get the case open. One saving grace is that it does use a standard AA battery.
If the meter did move but didn't move all the way to the right (or if a digital meter didn't indicate 0 Ohms or something very close), the meter's battery is probably old and weak.
Great news: you can test the battery and you probably don't even need any battery in the meter to do that.
Testing the battery
To test a AA battery, you'd set your meter to DC Volts. If it has multiple choices, choose the lowest - on my meter, that's 15 Volts (it just says "15" on the dial).
Touch the red probe to the bottom of the battery - the flat end. Touch the black probe to the top - the little bump end. Your meter should move or display how many volts the battery can produce.
If it's much less than 1.2 volts, the battery is weak. If you have an old analog meter like mine, it might be hard to read the scale that accurately - compare it to another new battery to get an idea of how weak it is.
Here's the thing about testing batteries: except for the 9 volts, which are easy to test with alligator clips, testing batteries is a clumsy task. It's not easy to hold the probes in place, especially with a small battery. This is one place where a specialized battery tester can be nice to have.
There's another way to test a nine volt battery - touch it to your tongue. A fresh 9V battery will give you a pretty sharp bite and a weak one will produce barely a tingle. The less wet your tongue is, the less the bite. I would not recommend doing this often, but for an emergency where you strongly suspect a dead battery, this is quick.
One of the more common things that you might test around the house is a lamp. Usually that's the bulb, of course, but if you know you have a good bulb, you can test the lamp switch.
Here you see my meter hooked up to the lamp plug and set for continuity testing. Turning on the switch should cause a reading on the meter. What if it doesn't?
If it doesn't, either the switch is no good or you have a loose broken wire somewhere.
Let's assume you feel confident about changing the switch. It's not hard to do - it might take a little finger strength and some basic tools, but you can find well illustrated instructions and even videos on the Internet.
If you are going to do that, you might as well test the wires while you are at it. You might even be able to do that before you get to the point of dismantling the lamp entirely. If you can get at the wires and take one off the switch (while leaving the lamp unplugged and hooked up to the meter as shown here), you can touch the loose wire to the other wire and the meter should show 0 Ohms or good continuity. If it doesn't, there's a break in the wires somewhere.
Another fairly simple thing to test is phone jacks. Again, you can easily find Internet articles that show you how to isolate whether the problem is inside or outside the house, but there is one very simple thing that comes up in older homes that you don't even need a meter for.
That's corrosion. You can see it if your eyes are good by just by shining a flashlight into the jack. I had very old jacks in our first home and after twenty years living there, we started having odd phone troubles. When I peeked into a jack that would kill every phone in the house whenever anything was plugged into it, I could actually see green crud across the little wires. I replaced every jack in the house after that discovery; all of them showed at least some signs of corrosion.
You also expect to see DC voltage in that jack. There can be AC "ring' voltage but testing that is more difficult.
So how are you going to get those thick probes onto those tiny little wires?
Inside a phone jack
You don't need to. If you take the jack off the wall, the wires are neatly separated and exposed for you.
In this picture, I added another set of wires so I could extend out to somewhere more convenient - this jack was buried at the back of a tiny closet. Adding those other wires let me work without squatting on my knees in a dark corner where I couldn't see the meter without a flashlight.
As to what to look for, put the "+" probe on the green wire in the jack and touch the other to red - you should see about 48 volts DC. If you see nothing, try it the other way - maybe it is mis-wired or you plugged the probes in backwards. See Land Line Telephone Troubleshooting for the Homeowner for more details, but if you aren't finding voltage anywhere, that jack is not connected to the phone company (or the phone company is down and out right now).
It's usually easy enough to test any kind of computer cable for continuity. The only hard part may be getting at the metal parts. Given how cheap these things usually are, I'll swap in another cable almost automatically - if that works, I might test the bad cable just out of curiosity, but really, what's the point? You are going to throw it away, so why bother?
If the issue is not continuity, you will be looking for voltage, and for that you need to know what to look for and where. The Internet is your resource, of course.
A useful tool
As you can see, an inexpensive voltmeter can be a useful tool to have and it isn't all that difficult to understand its basic use. I hope this article has helped you understand that.