Measuring Instruments--part 4

The Ohmmeter

The ohmmeter is used to measure resistance. The common volt- ohm milliammeter (VOM) contains an ohmmeter. The ohmmeter has the only scale on a VOM that is nonlinear. The scale numbers increase in value as they progress from right to left. There are two basic types of analog ohmmeters-the series and the shunt. The series ohmmeter is used to measure high values of resistance, and the shunt type is used to measure low values of resistance.

Regardless of the type used, the meter must provide its own power source to measure resistance. The power is provided by batteries located inside the instrument.

The Series Ohmmeter:

A schematic for a basic series ohmmeter is shown above. It’s assumed that the meter movement has a resistance of 1000 ohms and requires a current of 50 microamperes to deflect the meter full scale. The power source will be a 3-volt battery. R1, a fixed resistor with a value of 54 kilohms, is connected in series with the meter movement, and R2, a variable resistor with a value of 10 kilohms, is connected in series with the meter and R1.

These resistance values were chosen to ensure there would be enough resistance in the circuit to limit the current flow through the meter movement to 50 microamperes. If Ohm's law is used to calculate the resistance needed (3 V/0.000050A = 60,000 V), it will be seen that a value of 60 kilohms is needed.

This circuit contains a total of 65,000 ohms=(1000 ohm [meter] + 54,000 ohm + 10,000 V). The circuit resistance can be changed by adjusting the variable resistor to a value as low as 55,000 ohms, however, to compensate for the battery as it ages and becomes weaker.

When resistance is to be measured, the meter must first be zeroed. This is done with the ohms-adjust control, the variable resistor located on the front of the meter. To zero the meter, connect the leads and turn the ohms-adjust knob until the meter indicates zero at the far right end of the scale. When the leads are separated, the meter will again indicate infinity resistance at the left side of the scale. When the leads are connected across a resistance, the meter will again go up the scale. Be cause resistance has been added to the circuit, less than 50 microamperes of current will flow and the meter will indicate some value other than zero.

+++++ shows a meter indicating a resistance of 150 ohms, assuming the range setting is R31.

+++++ Adjusting the ohmmeter to zero.

+++++ Reading the ohmmeter.

Ohmmeters can have different range settings such as R31, R3100, R31000, or R310,000. These different scales can be obtained by adding different values of resistance in the meter circuit and resetting the meter to zero. An ohmmeter should always be readjusted to zero when the scale is changed. On the R31 setting, the resistance is measured straight off the resistance scale located at the top of the meter. If the range is set for R31000, however, the reading must be multiplied by 1000. The ohmmeter reading would be indicating a resistance of 150,000 ohms if the range had been set for R31000. Notice that the ohmmeter scale is read backward from the other scales. Zero ohms is located on the far right side of the scale, and maximum ohms is located at the far left side. It generally takes a little time and practice to read the ohmmeter properly.

Shunt-Type Ohmmeters

The shunt-type ohmmeter is used for measuring low values of resistance. It operates on the same basic principle as an ammeter shunt. When using a shunt-type ohmmeter, place the unknown value of resistance in parallel with the meter movement. This placement causes part of the circuit current to bypass the meter.

+++++ Shunt-type ohmmeter. Unknown resistor; Meter movement.

Digital Meters

Digital Ohmmeters: Digital ohmmeters display the resistance in figures instead of using a meter movement. When using a digital ohmmeter, care must be taken to notice the scale indication on the meter. For example, most digital meters will display a K on the scale to indicate kilohms or an M to indicate megohms (kilo means 1000 and mega means 1,000,000). If the meter is showing a resistance of 0.200 kilohms, it means 0.200 x 1000, or 200 ohms. If the meter indicates 1.65 megohms, it means 1.65 x 1,000,000, or 1,650,000 ohms.

Appearance is not the only difference between analog and digital ohm meters. Their operating principle is different also. Analog meters operate by measuring the amount of current change in the circuit when an unknown value of resistance is added. Digital ohmmeters measure resistance by measuring the amount of voltage drop across an unknown resistance. In the circuit shown a constant-current generator is used to supply a known amount of current to a resistor, R3. It will be assumed that the amount of current supplied is 1 milliampere. The voltage dropped across the resistor is proportional to the resistance of the resistor and the amount of current flow. For example, assume the value of the unknown resistor is 4700 ohms. The voltmeter would indicate a drop of 4.7 volts when 1 milliampere of current flowed through the resistor. The scale factor of the ohmmeter can be changed by changing the amount of current flow through the resistor. Digital ohmmeters generally exhibit an accuracy of about 1%.

The ohmmeter, whether digital or analog, must never be connected to a circuit when the power is turned on. Because the ohmmeter uses its own internal power supply, it has a very low operating voltage. Connecting a meter to power when it’s set in the ohms position will probably damage or destroy the meter.

+++++ Digital ohmmeters operate by measuring the voltage drop across a resistor when a known amount of current flows through it. Known amount of current Digital Voltmeter; Rx 9 V Current generator

+++++ Digital multimeter.

Digital Multimeters: Digital multimeters have become increasingly popular in the past few years.

The most apparent difference between digital meters and analog meters is that digital meters display their reading in discrete digits instead of with a pointer and scale. Some digital meters have a range switch similar to the range switch used with analog meters. This switch sets the full-range value of the meter. Many digital meters have voltage range settings from 200 millivolts to 2000 volts. The lower ranges are used for accuracy. For example, assume it’s necessary to measure a voltage of 16 volts.

The meter will be able to make a more accurate measurement when set on the 20-volt range than when set on the 2000-volt range.

Some digital meters don’t contain a range setting control. These meters are known as autoranging meters. They contain a function control switch that permits selection of the electrical quantity to be measured, such as AC volts, DC volts, ohms, and so on. When the meter probes are connected to the object to be tested, the meter automatically selects the proper range and displays the value.

Analog meters change scale value by inserting or removing resistance from the meter circuit. The typical resistance of an analog meter is 20,000 ohms per volt for DC and 5000 ohms per volt for AC. If the meter is set for a full-scale value of 60 volts, there will be 1.2 megohms of resistance connected in series with the meter if it’s being used to measure DC (60 ohm + 20,000 ohm/V = 1,200,000 ohm) and 300 kilohms if it’s being used to measure AC (60 ohm x 5000 ohm/v = 300,000 ohm). The impedance of the meter is of little concern if it’s used to measure circuits that are connected to a high-current source.

For example, assume the voltage of a 480-volt panel is to be measured with a multimeter that has a resistance of 5000 ohms per volt. If the meter is set on the 600-volt range, the resistance connected in series with the meter is 3 megohms (600 ohm x 5000 ohm = 3,000,000 ohm). This resistance will permit a current of 160 microamperes to flow in the meter circuit (480 V/3,000,000V = 0.000160A). This 160 microamperes of current is not enough to affect the circuit being tested.

Now assume that this meter is to be used to test a 24-volt circuit that has a current flow of 100 microamperes. If the 60-volt range is used, the meter circuit contains a resistance of 300 kilohms (60 ohm x 5000 ohm = 300,000 ohm). There fore, a current of 80 microamperes will flow when the meter is connected to the circuit (24 V/300,000 ohm = 0.000080 A). The connection of the meter to the circuit has changed the entire circuit operation. This phenomenon is known as the loading effect.

Digital meters don’t have a loading effect. Most digital meters have an input impedance of about 10 megohms on all ranges. The input impedance is the ohmic value used to limit the flow of current through the meter. This impedance is accomplished by using field-effect transistors (FETs) and a voltage divider circuit. A simple schematic for such a circuit is shown.

Notice that the meter input is connected across 10 megohms of resistance regardless of the range setting of the meter. If this meter is used to measure the voltage of the 24-volt circuit, a current of 2.4 microamperes will flow through the meter. This is not enough current to upset the rest of the circuit, and volt age measurements can be made accurately.

+++++Digital voltmeter.

+++++Low-impedance voltage tester. To probes; Plunger; Spring Coil

+++++High-impedance ground paths can produce misleading voltage readings. Voltmeter Load Transformer; Grounded object.

+++++ Wiggy voltage tester.

The Low-Impedance Voltage Tester

Another device used to test voltage is often referred to as a voltage tester. This device does measure voltage, but it does not contain a meter movement or digital display. It contains a coil and a plunger. The coil produces a magnetic field that is proportional to the amount of voltage to which the coil of the tester is connected. The higher the voltage to which the tester is connected, the stronger the magnetic field becomes. The plunger must overcome the force of a spring as it’s drawn into the coil. The plunger acts as a pointer to indicate the amount of voltage to which the tester is connected. The tester has an impedance of approximately 5000 ohms and can generally be used to measure voltages as high as 600 volts. The low-impedance voltage tester has a very large current draw compared with other types of voltmeters and should never be used to test low-power circuits.

The relatively high current draw of the voltage tester can be an advantage when testing certain types of circuits, however, because it’s not susceptible to giving the misleading voltage readings caused by high-impedance ground paths or feedback voltages that affect other types of voltmeters. A transformer is used to supply power to a load. Notice that neither the output side of the transformer nor the load is connected to ground. If a high-impedance voltmeter is used to measure between one side of the transformer and a grounded point, it will most likely indicate some amount of voltage. That is because ground can act as a large capacitor and can permit a small amount of current to flow through the circuit created by the meter. This high-impedance ground path can support only a few microamperes of current flow, but it’s enough to operate the meter movement.

If a voltage tester is used to make the same measurement, it won’t show a voltage because there cannot be enough current flow to attract the plunger.

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