In meter applications, the same meter movement can be used for the voltmeter, ohmmeter, or ammeter. The difference is, the internal components are arranged and wired in different circuits. The circuit selection switch chooses the mode function of the multimeter and the clamp-on ammeter. Figure 2-1 shows a typical multimeter and clamp-on ammeter.
Fig. 2-1. Multimeter and clamp-on ammeter.
In Fig. 2-2 the basic circuitry of a multimeter in the ac voltage mode is shown. Resistor A is in series with the meter movement. The very sensitive movement is designed for a maximum load of three (3) volts. The resistor is sized to drop high voltages below the meter limit. The actual voltage read is proportional to the dropping resistor. Most meters have several circuits that can be used at different voltages. They range from zero (0) volts into the thousands of volts.
Fig. 2-2. Basic circuitry of the multimeter.
To check a branch circuit at the entrance panel, remove cover carefully as shown in Fig. 2-3. Be very careful not to cause an arcing by touching the cover against load wires. In Fig. 2-4 you see the procedure. First, remember, electricity travels faster than a blink of an eye. Considering the high cost of multimeters, caution should be used.
Fig. 2-3. Circuit breaker panel with cover removed. Entrance cables are being indicated going into the main circuit breaker.
Fig. 2-4. Meter hookup for testing voltage and amperage.
Always check meter first to see that it is set for ac voltage at its highest scale. A good habit is to place the meter’s selection switch to the off position when you finish using it. If it doesn’t have an off switch leave it set on the highest ac scale when you put it away.
Place meter in parallel with load circuit. Some test leads have alligator clips at their ends; this type requires the load circuit be turned off when hooking up to the circuit. This eliminates the possibility of you receiving an electrical shock. If you have the pin-type leads, the circuit may be left on.
Move selector switch to a lower voltage scale until needle deflection is mid-scale on meter face.
NOTE—Many technicians have damaged a meter when they were in a hurry and didn’t adhere to this procedure.
Figure 2-5 shows an ohmmeter with its components. The ohmmeter uses only a small dc voltage through a load and is read when dropped through a resistor. This dropped voltage is equal in ohms resistance dc.
Fig. 2-5. Schematic of typical ohmmeter circuit.
The ohmmeter is usually the second tool used in troubleshooting after determining the correct voltage exists across the load.
All ac or dc loads have resistance. In practical applications, the line voltage to the circuit is turned off. Pull out internal fuses and the ohmmeter should read zero ohms. When testing a coil, or resistor, remove one side of the component electrically from the circuit. Place the ohmmeter across the component and measure the resistance. By disconnecting the component from the circuit, you eliminate the possibility of a voltage from a parallel circuit.
Alternating current coils and motors have less dc resistance than dc coils and motors; the reason for this is that an ac load equals dc ohms resistance plus inductive or capacitive reactance. Reactance is the ability of a load to create its own opposition to current when voltage is applied.
A typical multimeter being used as an ohmmeter is shown in Fig. 2-6.
Fig. 2-6. Typical multimeter being used as an ohmmeter.
IMPORTANT—Make sure there is no line voltage on the load circuit.
Place the meter in parallel with the load.
Start with the highest resistance range and work down each scale until the needle is close to the center of the meter face, if possible. Check to see if you have continuity to the load ground, case, or shell. When the load you are checking is of low resistance, most of the battery voltage is dropped across the meter drop resistor.
When the load resistance is high, most of the battery voltage is dropped across the load. If you have continuity from the load ground, case, or shell, you have a short circuit.
Remember, there must not be any line voltage connected to the meter. In the ohms mode, the drop resistors’ values are too small to conduct line voltage. Some have fuse protection to prevent damage to the meter if you happen to make a mistake by placing line voltage though the meter when in the ohms mode, some do not!
A clamp-on ammeter is used for testing ac only. A single conductor is placed inside the jaw of the meter. With the circuit loaded, start at the highest amperage scale. Watch the needle deflection and keep lowering the scale until the current reading is near mid-scale. Figure 2-7 shows the basic wiring that accomplishes this operation. Remember the electrical device must be operating and causing a power demand. The power need not be turned off to hook up a clamp-on meter. If bare wires, conductors, or bus bars are being tested, jaws of the meter should be well insulated and extreme caution should be used. In Fig. 2-8 a typical clamp-on ammeter is shown. These meters are important tools in helping a technician to determine if an electrical device is performing according to the manufacturer’s specifications. Most manufacturers install a data plate on their equipment that gives all the vital information to the service technician. Usually the first thing listed on the plate is the model and serial number of the device. These become very important at times when a parts replacement might be necessary. Other information given on plate are the voltage and amperage for operating the unit. Abbreviations such as F.L.A. (full load amps) are used on the plate; L.R.A., (locked rotor amps) is a split second current demand to move an armature, or rotor, from inertia into a rotational movement.
Fig. 2-7. Basic circuitry of a clamp-on ammeter.
Fig. 2-8. Typical clamp-on ammeter. F—Moveable and fixed jaws. E—Conductor. I—Opens fixed jaw. D—Fixed jaw. G—Meter movement needle. H—Scale selector switch.
CAUTION—Do not wire an in-line ammeter in parallel with load. The applied line voltage to the low resistance of the meter will cause very high current flow inside the meter. This will destroy the meter by fire and/or explosion. An ammeter must be wired in series with load. The meter should be set at highest scale first, then readjusted. Multimeters such as the one in Fig. 2-1 ‘A’ and ‘B’ can be used for dc in-line ammeters.
The in-line ammeter has one or more shunt resistors that are rated from nearly zero (0) ohms to five (5) ohms, with high wattage. They are wired in parallel with the meter movement. The meter can be wired in series either before or after the load. The shunt resistor(s) conducts all the current/voltage to the load, except for a small trickle current/voltage that will deflect the ammeter needle. A basic wire diagram explains this in Fig. 2-9. To use an in-line ammeter, the line and load must be de-energized. One line wire must be opened and the meter connected in series with the load at this point.
Fig. 2-9. In-line ammeter.
On dc the meter negative terminal must connect to the most negative side of the circuit, the side with the least potential. On initial start up, use a momentary voltage to observe the needle deflection. If leads are wired backwards, the needle will try to deflect the wrong way, which can result in permanent damage to the meter. If the leads are backwards, reverse the connections to achieve proper meter operation. This only holds true for dc ammeters. The ac ammeters are not sensitive to polarity.
In the modern system of measurement units, any of the following prefixes can be attached to the basic unit to increase or decrease its size.
For example:
1 milliampere equals 0.001 ampere. In symbol form write 1 mA.
1 kilohm equals 1,000 ohms. In symbol form write 1 kΩ
100 kilowatts equals 100,000 watts. In symbol form write 100 kW.
1 megohm equals 1,000,000 ohms, written symbolically as 1 MΩ
5 megahertz equals 5,000,000 hertz, written symbolically as 5 MHz.
The amount of resistance in dc components can often be calculated by Ohm’s Law. Here are some typical components with the resistance you can expect to find in them.
Capacitors are used in ac circuits since they do not pass direct current. It is possible to test a capacitor with an ohmmeter using the following procedure.
Set selector switch at highest value. Place probes on capacitor terminals and observe meter needle deflection. The needle will slowly return to infinite resistance. The needle swing and then dropping is due to placing a charge into the capacitor. If there is no deflection of the needle, the capacitor is defective. If the needle deflects but doesn’t return to the infinity position, the capacitor is defective. This is covered in more detail in Chapter 4.
Remember when checking the operating current of an electrical device, the amperage should not exceed the limit set by the manufacturer’s recommendations found on the data plate.
