I often meet handy people, both men and women, who do not go near to anything electrical because they are frightened of electricity. It is good to have a strong respect for electricity. It is also good to know how to make basic electrical measurements around the home. A multi-meter is the tool every home ought have. They eliminate the guess work from so many things and can save a lot of money.
A suitable multi-meter for occasional use around the home and automobile can be had for less than $ 5. This link is to a very inexpensive meter that includes some functions found only on meters costing twenty times more just a few years ago. More expensive meters do not necessarily have more features. They give greater accuracy for professional work, and they are more rugged in their construction and durability.
Step 1: Will I electrocute myself?
The black probe is normally associated with the ground or negative terminal. The red probe is normally associated with the "hot" or positive terminal. For household uses illustrated in this Instructable you can ignore all of that and use either probe on any terminal.
Step 2: So, what can a multi-meter do?
(This meter has an ON/OFF slider switch. Many meters turn on when the round selector switch is moved to any setting range. OFF is at the top of the selector dial on those units. Meters have internal batteries. Turn the meter OFF when finished to save the batteries. Many meters switch themselves off automtically after a few minutes.)
See the second photo. Touch each end of the battery with a probe. Ideally, the red probe touches the "+" end of the battery and the black probe touches the "-" end of the battery, but in practice it does not matter for this purpose. A minus (-) sign will appear in front of the numbers if the red and black wires are reversed when testing the battery. You are interested in only the numbers. A 1.5 volt battery is "dead" when it registers 1 volt or less. If a device does not work, but the 1.5 volt battery registers (for example) 1.38 volts, a new battery will not make it work. There is another problem to be solved. A 9 volt battery is "dead" when it falls to read about 7.5 to 8 volts. But, when a battery is "dead" also depends on the application. I use a 9 volt alkaline battery in a wireless microphone. I have learned by experience that a battery with a voltage below 8.5 volts will not last through a one hour church service.
Shown in the photo are several types of common batteries: a 3 volt camera battery, a 9 volt "radio" battery used in smoke detectors and other things, a button battery used in watches and other instruments, and a common AA battery. In this test the AA battery registers 1.59 volts, which is new in condition.
There are dedicated battery testers you can buy, but a multi-meter costs about the same and does so much more. Some multi-meters come with a battery test function built in. Such a function adds a resistance to test the battery under a load similar to what might be encountered in a device. Here is a way you can add a battery test function to a meter you already own.
Step 3: Your car battery
Step 4: Corrosion?
See the text boxes in the second photo. The newer side post battery terminals are now more common than the old-style shown in the first photo. Much of the terminal is covered with insulation. One meter probe touches the terminal's bare metal bolt. The other probe can touch a straight pin pushed through the cable's insulation.
(The first photo is from Bing Images.)
Step 5: Light bulbs
If the bulb tests good, some further checks on the lamp socket are necessary. See the second photo. Set the meter to the 400 volt AC selection. Touch one probe to the side of the bulb socket. Touch the other to the tip at the bottom of the socket. The meter should read about 120 volts. (Note: I live in the USA where most electrical outlets operate on 120 volts. Heavy duty appliance circuits [electric stoves, clothes dryers] use 230 volts. In many other parts of the world the standard voltage in household electrical systems is 230 to 240 volts. Adjust the readings you expect according to the normal voltage provided in your locale.)
You may find the expected voltage in the bulb socket and also the bulb are good, but the lamp still does not light. If the socket is older, the contact tip at the bottom of the socket may have lost its springiness and it may not make dependable contact with the center tip of the bulb. Use a popsicle stick to lift the tip. If you can be sure there is no electrical power to the socket, you may use a screwdriver. Then screw the bulb into the lamp socket and it should work. In time you may want to have the lamp socket replaced, but it can work this way for a long, long time.
Step 6: Fuses
Sometimes fuses are out in the open where they are visible. Sometimes they are under a cover marked "Fuses." Sometimes they are inside an appliance with no notice of their presence. This is true of some microwave ovens and some television sets. Open the case and look for a small cartridge fuse like the one shown in the photo. The fuse will usually be near where the electrical cord comes into the appliance. You may have thought you need a new microwave, but the real problem may be only a small fuse costing less than a dollar. Sometimes there is a more serious problem that will cause a new fuse to "blow," too. But, more often, the old fuse simply became weak or "blew" because of a momentary electrical surge. If your multi-meter saves you from replacing an appliance that failed due to a blown fuse, your meter will have paid for itself many times over.
It is always good if you can have extra fuses available for whenever one needs replacement. When you need to buy a fuse, take the old fuse with you to the store so it can be matched. Fuses come in a variety of physical sizes and types. It is important to replace a fuse with one exactly like it. Some fuses are available at your local hardware or building supply store. Some, like small cartridge fuses similar to the one shown in the photo, are available only where electronics parts are sold, or on-line. Identifying numbers are imprinted on the shiny metal end caps.
The fine wire in cartridge fuses is often so fine that it is difficult to see with the unaided eye. Some fuses have a solid body that blocks vision. When testing fuses set the multi-meter to CONT. for continuity. Touch the probes to the ends of a good fuse when on this setting and you will hear a shrill tone indicating there is a continuous circuit through the fuse. No sound means a bad fuse. The words "continuity" and "continuous" are purposely related.
WARNING: If you open the case on a television or a microwave, be careful not to touch components other than the fuseholder. There are parts that retain a high voltage electrical charge. It is not likely you would touch these, anyway; but, if you did, they can kill.
Step 7: Your air conditioner unit
Your air conditioning system may have more than one set of fuses protecting it. There may be a set of circuit breakers in your main electrical panel. Check to see that the circuit breaker toggles have not moved to the "off" position due to a sudden overload. Go to the air conditioner unit outside your house. Look for any cable conduits (metal pipes, some flexible). Follow them with your eye and look for any metal boxes that might contain fuses. The fuses will likely be mounted in a fuse block that can be pulled from the box. Use the continuity setting to check the fuses. The first photo shows a control box near our heat pump/air conditioner. It looks like it might contain fuses, but it contains only a disconnect switch.
See the second photo. It shows the inside of the fuse box that controls our air conditioner. The yellow text box contains information on the ON/OFF switch. I overlaid the photo with capital letters as markers. Those in red make a "hot" circuit even when the switch is "off." Keep your hands safely away from these terminals.
Set the meter to the 400 volt AC setting. If you place one probe on A and the other on B, the meter should read about 230 volts. That is true whether the switch in the box is "on" or "off." Place one probe on A and the other on C. The meter should read about 115 volts. Place one probe on B and the other on C. The meter should again read about 115 volts. Readings between F and C or G and C should also each give a reading of about 115 volts when the switch is "on." These readings indicate the fuses are good. With the switch in the "off" position, readings between F and C or G and C should be zero volts.
If you wish to check the fuses without them being electrically charged, move the switch lever to the "off" position. Set the meter to CONT. Touch one probe to D and the other to F. You should hear the meter's shrill chime tone. Now place one probe on E and the other on G. You should hear the tone again. This also indicates the fuses are good. If one of the fuses does not test good, be certain the switch is in the "off" position. With your fingers or a pair of pliers or a wooden stick to pry, remove the bad fuse. Take it with you to a hardware or building supply store and get a replacement. Use a fuse. Do not use a piece of metal, a piece of copper tubing, or a wire with alligator clips to bridge the space for the fuse. In the days when most homes had screw-in plug fuses some people sometimes put a copper penny behind a blown fuse when they did not have a new fuse. Then they often forgot about the penny. Pennies do not burn away like a fuse does when there is an overload. Pennies used this way often caused house fires. There was a frequent saying that, "The words 'In God We Trust' were placed on pennies for the benefit of those who use them as fuses."
Step 8: The electric clothes dryer does not dry.
Dryers operate on two different circuits at two different voltages. The motor that turns the drum works on 115 volts. The heating element works on 230 volts. The electrical outlet behind your dryer that powers the dryer looks like this.
Set your meter selector to 400 volts AC. Put one probe into the "A" slot. Do not touch the other probe, but insert it into the "B" slot. The fit may not be tight. You may need to wiggle the probes to touch the metal inside and get a reading on the meter. The meter should show 230 volts. "C" is the neutral wire. "D" is the ground wire. Place a probe into "A" and the other probe into "C" or "D" and the reading on the meter should be 115 volts. The same should be true of a reading between "B" and "C" or "D." If you do not get these readings, check to be sure one circuit breaker or one fuse is not blown. There is the possibility that the dryer's heating element could be burned out, but that is not as likely. You can access it from the back of the dryer. Remove a cover over its terminals. Be sure the power to the dryer is disconnected. Use an Ohms setting on your meter and check for an open circuit. Testing an oven element in your stove is very similar. Remove the screws that hold the element in place and pull it from the oven. Do a resistance check on the two terminals.
(The photo is from Bing Images.)
Step 9: Electrical outlets
Step 10: Cords and other things
This toaster works well, and the meter shows a resistance reading due to the heating element inside the toaster. This is normal and good. This test could indicate there is a break in the circuit within the toaster. Further testing would be necessary to determine which part of the circuit is defective. Then a decision would need to be made as to whether the appliance can be repaired and whether it is worth the effort.
Whether it is a toaster, or a phone charger, or a power cord for a computer; cords often fray internally within an inch or two of the plug. This is very understandable. That is the area of the cord that is frequently flexed back and forth. If something, like a phone charger, works some of the time, but not at other times; attach the meter with a resistance scale setting. Gently flex the cord back and forth near the plug end. Make fairly sharp bends. Watch the meter reading to see if it fluctuates between a normal reading and no connection. If it does, ask someone with a soldering iron to cut the cord back and reattach the plug end for you.
Step 11: Lightning and your telephone
During the 1980s I was the pastor of a church in a rural setting. Lightning sometimes struck the utility company's power distribution lines and jumped to telephone lines running into our building. Once our telephone would not work. Twice the phone answering machine would not work. I was able to use my meter to rescue the telephone. I was also able to rescue the answering machine once, but not the second time. After the second strike we added a surge protector and had no problems after that.
See the first photo. The selector is set to the diode checker. Diodes are one way electrical valves and are very sensitive to surges of electrical current, especially with things like lightning. The arrow with a line across the arrow point is the standard symbol for a diode. Not every meter has a diode checker setting. Because diodes are very sensitive to current overloads, the diode checker limits the current in amperes that flows through a diode while testing it.
The second photo shows the circuit board from a telephone. The yellow text box identifies a bank of four diodes. Although diodes can come in many sizes and shapes, those commonly used on circuit boards like this one are small black cylinders about 5/16 inch long and about 5/32 inch in diameter. At one end is a gray band. A wire runs out of each end of the diode.
You may notice the probes do not look anything like the probes you have seen before. This is another accessory I got at Radio Shack. It is a set of clip on probes designed to grasp the small wires of electronic parts to make getting an accurate reading easier.
When testing a diode, you are looking for a relatively high current reading when the red and black leads are attached one way, but a relatively lower reading when the connections are reversed. Note that the red probe is above the black probe in this photo and the reading is 1.585. See the third photo. The black probe is now above the red probe and the reading is one-third the previous reading, which is considerably lower. These readings are the sign of a good diode.
Often a diode can be checked without removing it from the circuit. Sometimes the readings are confusing. Chances are current is feeding through some other electronic component to cause an undependable reading. Then it becomes necessary to desolder one of the diode leads to isolate it from the circuit for accurate readings.
In the lightning incident I mentioned, there were only a couple of diodes. One of them failed its test. For less than a dollar I was able to replace the defective diode and the phone was good ever after. At the time a new phone was about $20.
See the fourth photo. It is a Maglite flashlight that now has one of their LED bulbs. It quit working and I needed to know if the LED was still good or not. I did a diode check with my meter and discovered the switch was the problem.
Step 12: Reading amps. (current draw)
First, check the device specifications. Look for a plate or label on the back or bottom of the device. It may tell you the device is designed to draw (for example) 2.3 amps. at 120 volts. Or, it may tell you the device uses 276 Watts at 120 volts. (Watts equals volts multiplied by amps., so divide Watts by volts to determine the proper amps.) If this device were found to draw (for example) 4.5 amps., you would know immediately something is wrong.
Reading amps. is different from reading voltages. Voltage readings are the drop in electrical "pressure" across two points in a circuit, or a whole circuit. The meter is not part of the circuit, but reads what happens across or between two points in the circuit. When reading amps, the meter must become a link in the circuit, just like a link in a chain. See the graphic showing how you can make a sandwich with two conductors and a piece of plastic between them. This sandwich can be placed between two batteries in your device to see what the current draw is. The alternative is to break the circuit by cutting a conductor and connecting the meter to the ends of the cut conductor. You would need to reconnect the cut connector when you are finished. Set the selector for DC amps in the desired range.
You may need to change the holes into which the probes connect on the meter. See the second photo.
Step 13: Shocking, simply shocking!
A few years ago we had a neighbor who avoided having grandchildren visit because anyone who touched the built-in kitchen stove received a mild electrical shock. I set my meter to an AC voltage setting in the 150 volt range and placed one probe on the chrome oven handle and one probe on a sink faucet. My meter told me the electrical current registered about 40 volts AC. It took me some time, guess work, and checking; but I found someone had disconnected the thin green wire that connects the metal frame of the stove to that home's grounding circuit. When I reconnected the wire, the stray voltage disappeared. The green ground wire was located on the back of the oven.
Step 14: Another use from the past
(The photo is from Bing Images.)
Step 15: Auto ranging meters and reading the meter
On my meter the digit "3" indicates an open circuit (= no path for electrical current). If I am using the Ohms (resistance) scale, the digit "3" appears until I attach the probes to the device I am testing. Sometimes the digit "3" remains when attached to a device known to be good. There should be an actual reading, but the reason there is not is that I have the meter set to an incorrect range. The device I am checking may have (for example) a resistance value of 12,500 ohms. If the meter is set to a range too low, perhaps to the 4K range, the digit "3" will appear on the display. If I move the selector to the 40K range the meter will suddenly give the reading I am seeking. I try to remember to begin at a very high range that may not give me as many numbers in the reading as I hope to see. Then I move the selector to lower range settings until I see the number of digits in the reading for the accuracy I need.
Step 16: Accessories and features
Sometimes you need an extra hand. Multi-meters come with a fold out stand on the back of the meter. These usually can also be flipped up to function as a hanger for the meter. This frees both hands for manipulating the probes, especially if the voltages could harm someone.
See the text box in the photo for details on the Hold button. Sometimes you may be able to reach terminals with the probes, but may not be able to read the meter. If you can manage to press the Hold button and release it, the meter will freeze the reading so you can move and see what the meter recorded.
Step 17: User serviceable things
When the batteries grow weak there may be a weak battery indicator on the display screen. You will notice the display numbers grow more weak and difficult to see unless viewed at just the right angle. If the meter will not be used for very long periods of time, it is a good idea to remove the batteries so they do not begin to leak and ruin the metal tabs in the battery compartment. This meter uses three AA batteries. Many meters use one 9 volt battery. Notice that this meter has two fuses. One is a fast blow fuse designed to protect the meter from the wrong setting on the meter's selector switch. I have a few extra fuses in a small plastic bag. The bag fits into the battery compartment. Notice that the fuse specifications are also molded into the plastic case.
Step 18: Additional resources
Do not be cavalier when working with electricity. Take many precautions. Check and double check to be certain a circuit is not live. Many important measurements that tell you about the health of a device can be made without power to the circuit, other than the small batteries inside your meter. Using a multi-meter can help you fix problems safely and quickly without waiting for an expensive service call by a technician.