American Dental Tech Blog

Monthly Archives: August 2010

  • Tech Tips #25: Multimeters

    Multimeters

    A number of items in the dental office are powered by electricity. Many of those things we’ve examined in past issues of Tech Tips (e.g. sterilizers) have a variety of circuits, switches etc. to control function.

    Now we will introduce the use of multimeters to help troubleshoot electrical problems. We can’t cover every check of every piece of equipment, instead, we’ll just cover basic steps in meter use.

    A multimeter is simply an electrical test meter that will perform several tasks, i.e. voltage tests, both AC and DC, resistance checks, and in some cases, current tests. Basically, there are two types of meters: the analog and the digital. The analog meter has a needle to show readings. Digital meters obviously have a digital readout, are fairly easy to use, and are usually more durable than analog. Some features available are fuse protection, auto-ranging, audible signals and auto off. Ask a knowledgeable friend, or store personnel for help in finding just what you want. As you can see, we are biased toward digital meters and will only cover their use.

    Continuity checks: we use the meter to check bulbs, fuses, switches and some circuit components by checking for lack of interruption in the component or circuit. To do this we will use the ohms scale. The ohms scale is usually identified by the “Ω” symbol.

    So, get a meter and let’s get started. Usually the meter leads will plug in at the bottom of the front of the meter. Instructions normally accompany your meter showing you where each lead goes. Look for one hole with something like V-Ω. This is where you would insert the red lead whenever you are reading volts, whether AC or DC, or ohms. The other lead should be plugged in to the Common hole.

    Now you need to turn the dial on the meter to the appropriate scale, in this case the scale marked with the “Ω” symbol.

    Note: If your meter has the auto ranging feature, your meter is set up when you turn the dial to the desired function. If you do not have this feature, more instruction is needed: Generally, your meter scale in the ohms section will have divisions starting with “2”, i.e.200, 2K, 20K and so on. The 200K scale, for instance, would allow you to read up to 200,000 ohms. For most uses, you will need to use only the 200-ohm scale. Notice other divisions for ACV (AC volts), DCV (DC volts), ACA and DCA (amps). Most of us will never need the amps scales, but only the volts and ohms scales.

    Checking Fuses, Bulbs, etc.

    After setting up your meter to the proper scale (ohms), touch the leads together. The readout should read “000”. Separate the leads and “OL” should appear. Touch the leads to each end of a fuse (to accurately check a fuse, it must be removed from the circuit) if the fuse is good, the reading will be “000”. If your meter has an audible alert, it will sound. If the fuse is defective, the reading will show “open” or “OL”. On a bulb, touch the leads to the tip on the end of the base and to the metal on the side of the base. You can use this simple method to check for an open wire, or power cord or to check a switch, (on-zero, off-open).

    How about an experiment to see how we are doing? Let’s check your car battery. It will be DC voltage, so set up the meter for DC. Make sure the red lead is in the VΩ hole, the black lead is in the common. Apply one lead to each post of your car battery, meter set on DCV at the 20-Volt scale. You should read about 14 volts.

    AC Voltage

    Now you must be careful here. Be certain that your lead only touches one thing, that it is not across another wire, or touching the chassis. To set up the meter for voltage, put the leads as you did for ohms. Turn the dial to VAC (if you are reading common household current, put the dial on 200 VAC). If you are using a meter with auto ranging, you will not need to signify a range. With one lead of the meter on a good ground (look for a ground wire affixed to the component housing, or find a spot on the housing that is bare of paint or debris), then touch the other lead to where voltage should be present. You should read approximately 115VAC. If not, go back in your checking, eventually to the power source and try to find where you lost the power.

    You can check a receptacle in you office: AC volts, 200-volt scale, put one lead into the right slot, the other to the left. Is voltage present? If the ground hole (the round one) is on the bottom, the right slotted hole should be “hot”. Check your ground circuit by going from ground to the hot side (you might have to move the lead in the ground hole to an angle to make contact), if you cannot get a reading here, your ground circuit may be faulty.

    Electrical Shock– Some Tips on How to Avoid It

    Electricity always seeks the shortest path to ground. The human body is a very good conductor, so you must avoid becoming a part of that path.

    110VAC can kill. All arguments aside, more people are killed by 110 volts than any other rated current.

    Never assume. Do not assume that a breaker is off, a wire is not hot, and an item is unplugged. Check it out!

    Wire is a conductor of electricity. Any metal is a conductor, water and wet objects, green tree limbs will conduct, wet ground, graphite fishing rods and golf clubs will conduct, so do not come into contact with any conductor that might be in contact with electricity.

    Be safe – unplug an item or turn breaker off before checking for electrical problems.

    Electronics schools teach technicians to keep one hand in their pockets while troubleshooting, and only one person at a time do the work.
    When changing a fuse, remove power, change the fuse then restore power.

    Just turning a switch off does not remove power. You can still get shocked with the switch off.

    Do not stand in water or place yourself in contact with water pipes or other metal objects when working with electricity.

    Do not remove the ground lug from an electrical plug.

    Whether a piece of equipment is giving trouble or not, always take care to note frayed wires, loose wires, or dried and cracked wires. Watch for burned spots on wires and components, loose or burned outlets, etc.- keep your eyes open.

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