Using A Digital Multi Meter (Dmm)
Tesy your home cicuitry using a Digital Multi-meter. It's easier than you think.
A digital multi-meter(DMM) can be a handy device to have around the house. You can use it to check your outlets, verify a blown fuse, check extension cords, or even test a light bulb.
The first thing you need to do is familiarize yourself with the various settings available on your DMM. There are usually 5 basic settings for a simple DMM. They are as follows:
AC Voltage Wave symbol
DC Voltage The Letter's 'DC'
Resistance Designated with the Greek
letter Omega, it looks
like Kilroy's head.
Current The letter 'A'
Continuity Usually has a symbol that
looks like a speaker.
Make sure you know which is which. It is always best to look to your owner's manual to be certain of the proper settings. A mistake in this area could damage your meter.
The next step is to understand what kind of circuits you might be dealing with. All household electricity (outlets) is AC Voltage. Everything in your house that plugs into the wall also runs on AC Voltage. This includes your light bulbs. All batteries and all things that run on batteries, including your car, run on DC Voltage.
All DMM's have two wires, one is Red and the other is Black, that either come directly out of the unit or plug into the unit. The wires generally end in metal probes. The Black wire is considered your Ground wire, the Red is your 'Hot' wire. For the DMM's that require you to plug in the wires, the plugs are always labeled. The 'Ground' socket will have what looks like an upside down Christmas tree next to it. This is where you place the Black wire. The Red or 'Hot' wire goes into the socket that has AC and DC Voltage ranges labeled next to it.
Now that your Digital Multi-Meter(DMM) is correctly assembled, it's time to put it to use. AC Voltage is the most dangerous type of electricity, so you must always be careful when checking household circuits. The following are some safety tips for all types of electricity.
Always be aware of what you are touching. Even bumping the back of your hand across a circuit can cause injury.
Keep your hands away from the tips of the probes. This helps prevent accidental contact.
Never touch two wires simultaneously with one probe.
Don't let the probe tips touch each other when testing a circuit.
Never work with electricity in a wet environment or when you or your clothing are wet.
Follow these rules and you should have a safe experience testing your circuits.
Now, household circuits have three wires, unless you are in an older house, in which case you will find most outlets have only two slots. The three slots in a modern home outlet are called the hot, neutral and ground. The slots are short, long and round respectively. For an older home, understand that the left slot is neutral and the right slot is hot.
Short Slot - Hot
Long Slot - Neutral
Round Slot - Ground
The following are the voltages you should find (+/- 10%) between the slots:
Hot -> Neutral = 120VAC +/-10%
Hot -> Ground = 120VAC +/-10%
Neutral -> Ground = 0 - 1.0 VAC +/-10%
To test these voltages you simply set the DMM to AC Voltage and remember that the red probe is the 'hot' wire. So, in the first test, the red probe goes in the hot slot and the black probe goes into the neutral slot. The neutral to ground test can be done with either probe in either slot. Any fluctuation greater than 10% on these readings will indicate a problem with either your wiring or the outlet itself.
DC Voltage is far safer to work with, though this in no way implies you can ignore the safety rules. When testing a DC circuit, such as in your car, the black probe can usually be fixed into what is called a chassis ground. The time this can come in handy is when checking your lamps or your fuse box. Simply find a way to clamp the black probe to a piece of bare metal in your car, and then you are free to check voltages with the red probe. The basic voltage of most cars is 12 VoltsDC. Keep in mind not all voltages are present unless the ignition is in the 'Accessory Position'.
You can use your DMM to check your batteries as well. All basic batteries carry a charge of 1.5VDC. The nub at the top is your positive or 'hot' contact, thus this is where you will place the red probe. The black probe then goes to the bottom of the battery. Any reading less than 1.0VDC indicates a bad battery.
Let's move on to the next item, resistance. This is a setting you will most likely never use, unless it is also your continuity setting, which would be indicated by the aforementioned speaker symbol. The same goes for the current setting. This is used to check the current in a circuit and, unless done correctly, can cause damage and/or injury. There aren't many non-specialized reasons to be checking resistance or current in your home.
Continuity, however, will prove itself to be the most useful setting on your meter. This is used to check for open or shorted circuits. The position of the probes is not important for this setting and won't be discussed. You will find this is the best way to check fuses, extension cords, light bulbs and even track wires. Most meters will beep while in continuity mode and the probes are on a complete circuit. To check that the DMM is working properly, simply cross the tips of your probes and make sure the DMM beeps, indicating the closed circuit of the touching probes.
For fuses, simply place the probes over either end of the fuse, AFTER REMOVING IT FROM THE CIRCUIT. If your meter beeps, the fuse is good.
Foe extension cords, place the probes in the following test patterns.
Hot -> Neutral
Hot -> Ground
Neutral -> Ground
If the meter beeps at any of these times, the cord is shorted out and must be thrown away. You can use this same method to check for shorts in your car, or your phone lines, or virtually any circuit. Just make sure the CIRCUIT IS POWERED DOWN BEFORE YOU CHECK CONTINUITY. Also remember that in your car, all grounds are shorted together into a chassis ground. So be certain before you declare a short.
To check a light bulb, place one probe on the tip of the base, and the other on the side where the threads are. If your DMM beeps, the bulb is good.
Remember these rules:
Always be sure to keep your skin away from any circuit.
Never test continuity on a live circuit.
Never cross a single probe across two contacts.
Don't go beyond your abilities. If you are the least bit uncertain, then turn to the services of a licensed electrician.