Handyman Tip:

How to Use a Digital Multi-Meter

A digital multi-meter is a very handy tool to have in your tool box. If you own an analog multi-meter you probably already know how to use it. But, if this is your first foray into using an electrical multi-meter, I am going to concentrate on the use of a digital multi-meter as it would be difficult to even find an analog multi-meter for sale anywhere today.

When you first look at the digital multi-meter, it looks quite overwhelming. The fact that the meter can test for AC voltage, DC voltage, circuit resistance, line continuity, circuit current and solid state device testing is pretty impressive. Let’s look at the most common uses of the multi-meter.

AC Voltage: Alternating current voltage is the voltage available at any electrical outlet in your home. This is the voltage that powers your two prong (three w/ground) devices and appliances. The switch on the multi-meter for testing AC voltage will be the letter “V” with “ac” next to it or “V” with “~” next to, or above the letter. Also you may have two or three options in this mode “0-200 VAC” or “200 – 750 Vac”. Your meter may not have any options; this is a “self-ranging” meter. NOTE: Always check your meter for proper operation before running a test. Go to a known live outlet, one that is in use and is operating an appliance. Unplug the appliance cord and insert one meter lead into each slot on the outlet. With the meter turned on and in the proper position “Vac”, the reading on the meter should be between “115 volts and 125 volts”. Any reading within this parameter is OK as each house circuit has small variances. With the knowledge that the meter is working properly, you can proceed to test questionable outlets. If you get a “0.0 Vac” reading, you have a dead outlet. If you get a “115 Vac – 125 Vac” the outlet is live and OK. If you get any other reading, it is time to call a professional. You can also use the meter in this position to check light sockets. Touch one of the probes to the center contact at the base of the socket; touch the other probe to the threaded part of the socket. Do you have voltage? Switch the light switch and check again. If you know you have voltage at the outlet but no voltage after operating the light switch, there is a problem with the light fixture wiring.

DC Voltage: Direct current voltage is the voltage found in batteries, or from adaptors used to take the place of batteries in portable devices. The switch position on the multi-meter will be the letter “V” and “dc” or “V” and a line with several dots below it. In this position there may be a couple of voltage options. Choose the position based upon the voltage you are testing for, ex. 9 volt battery. Select the voltage closest to 9 volts, but not less than 9 volts. This will give you the most accurate reading. Touch one probe to one battery post, the other to the second battery post. It is important that the black wire goes to the “-“negative battery post and the red wire goes to the “+” positive battery post. (This does not matter when checking AC voltage). You can also check adapters used to replace batteries. Read the voltage rating on the adapter. A typical description of the adapter voltage will read: 120 VAC, 3 VDC (or 6VDC, 9VDC, 12VDC). When checking this adaptor you may not see any indications of polarity (+ or -), typically the adaptor plug is a “barrel with a hole in the center”. The “Barrel” is typically (-) negative and the “hole” is (+) positive. You can also use the digital multi-meter to check your car battery or any devices that can be plugged into your car cigarette lighter socket. All modern automobiles have a negative ground chassis (the metal frame and body parts are the negative connection), the red wire (typically) is the positive connection.

Resistance/Continuity: The resistance position is indicated by “Ω” the omega sign. This position has many settings (unless your meter is self-ranging). Resistance to electron flow is measured in “Ω” OHMS, you will probably not be using this function to measure resistance as much as you will use it to check “continuity” (whether a circuit is broken or complete). Your meter may have a separate audible circuit used for this purpose, it will probably be identified by a picture of a speaker or concentric expanding curved lines “)))”. To use the resistance reading, make sure the circuit is not live. This could damage the meter. A circuit is an unbroken connection from one point to another. A good example is a light bulb. The circuit is from the center base contact, thru the filament and back to the screw base. When voltage is applied to this circuit the bulb lights up due to the resistance in the filament. When the filament burns out the circuit is interrupted and the bulb fails to light up. By using your multi-meter in the resistance/continuity position you can test the bulbs circuit. Touch one probe to the center contact and the other probe to the screw base. If the bulb is good you will have a reading of “000.00” or all zeros (NOTE: You may see other numbers after the period “000.3”, this is not an indication of a defective bulb). If the circuit is broken you will get a large resistance reading or in some cases “O.L”, this indicates a bad bulb. If you have an audible continuity position you will hear a beep if the bulb is good, nothing if it is bad.

Current: This is a measure in “AMPS” as to how much current is flowing in a circuit. To use this feature means to break the circuit and put the meter into the circuit to complete the circuit. This is not a reading that a novice will need so it is better left to a professional.

Other Positions: These are typically for professionals to test solid state devices such as diodes, transistor, bridge rectifiers, etc…

Basically you will use your digital multi-meter to check voltages, both alternating current (house electrical), direct current (batteries, battery adaptors, automobile battery and circuits) and resistance/continuity (whether a circuit is complete or broken ie. a light bulb or electrical wire).

NOTE: Digital multi-meter use a battery as a power source, always check the battery before use.