Ohmmeters: Measuring Resistance

Multimeters are current meters, voltmeters, and ohmmeters contained in one case. There are separate current, voltage, and resistance circuits interconnected by switches and they share the same meter movement. A function switch is utilized to choose the type of measurement: position 1 is for de volts and resistance, position 2 is for de and ac current, and position 3 is for ac voltage. The jack on the right, the COMMON jack, is used for all measurements. For every type of measurement, the range switch is set for the correct scale reading. Positions I and 2 are resistance ranges, positions 3 to 6 are current shunt settings, and positions 7 to 9 are voltage multiplier settings. Rectifiers D1 and  D3 are linked for ac current measurements and rectifiers D2  and D3 are used for ac voltage measurements. Fuse FI safeguards the meter movement from overload. Resistors R1, R2, R7, R8, and R9 are switched in and out of the circuit by the range switch. The current shunts, R3 to R6 are only linked into the circuit in position 2 of the function switch. The range switch then chooses portions of the ring shunt.

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How To: Reading a Meter Scale

Whether you install, operate, repair, or design electrical equipment, you must know how to measure and test many types of electrical characteristics. The most imperative of these are electrical current, voltage, and resistance. Additionally, there are various applications that also require the testing and measuring of power, power factor, frequency, impedance, and sensitivity, as well as special component characteristics, such as capacitance and inductance.

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Power Meters

Calculating Power

If you wish to find power dissipated in an electrical load, measure any two of the three basic electrical quantities- current, voltage, and resistance. For example, you will recall that power can be calculated by multiplying voltage by current: P = VI. Therefore, if you use a voltmeter to measure the voltage across a load, and a current meter to measure the current flowing through the load, insert these values into the power equation. Similarly, you can measure current through the load and the resistance of the load, and then calculate power with: P = 12 R. Or you can measure the voltage across the load and use the equation: p = y2; R.

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Digital Meters

Input circuits allow the input voltage to be stepped down by the ranging circuits, which could be a switch or automatic circuits. The ranging circuits also select the proper pulse stream from the clock and divider circuits. The test voltage is amplified and integrated with the gate pulse to produce a ramp voltage that will pass a selected sample of pulses. The number of pulses passed is related to the test voltage. These are shaped and counted, and then decoded to drive the seven-segment displays.

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Amplified Resistance Measurements

The electronic meter is used for resistance measurements in a manner similar to the way it is used for current measurements. Using the same types of amplifier stages as was used for the voltmeters and ammeters, where a 0.5-volt-signal produced a full-scale deflection, resistance circuits can be set up using standard battery source voltages supplying current to standard high­ precision resistance circuits, whose values are known. When the resistor under test is connected into the circuit, it will change the total resistance, resulting in a change in current flow, which produces a signal voltage that is directly related to the resistance under test.

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Amplified Current Measurements

To use the meter movement to make voltage and resistance measurements, the use of Ohm’s law is required so the current flow reading can be interpreted in terms of voltage or resistance. The amplified analog meter differs because the amplifier is voltage sensitive. As explained for amplified voltage measurements, a high input resistance is used with a range switch to tap down the voltage applied to the input of the amplifier stage. Since the amplified meter is a voltage sensitive device, the input circuit used for current measurements must convert the current to corresponding voltage levels and use Ohm’s law to interpret the related current flowing in the circuit under test.

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Amplified Input Voltmeter

The higher the ohms/volt rating of a voltmeter, the less the voltmeter will upset circuit conditions. And the less circuit conditions are upset, the more accurate the reading will be. Most of the higher-end voltmeters and multimeters available now are rated at about 20,000 ohms/volt; more accurate voltmeters are rated at 100,000 ohms/volt. In some of the high-resistance circuits found in some present-day equipment, however, even a meter rated at 20,000 ohms /volt will greatly upset circuit conditions, and result in an incorrect reading. While a 100,000 ohms/voltmeter will give more accurate readings, even more accuracy is needed with some circuits. To overcome this problem, a device with a high ohms/volt rating called an electronic voltmeter was developed.

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Resistance Testing Part 3

Because many insulation breakdowns occur during operation under the stress of high voltages, more dependable tests are performed with a megger (megohm-meter) and with hi-pot (high potential or high-voltage) equipment. The megger is a tiny, portable instrument that can be battery operated or powered by a hand-cranked generator. Meggers supply from 500 to 5000 volts for the insulation test. They measure leakage current, but the readout is calibrated in insulation resistance (ohms), in ranges to 100 and often 2000 megohms (MQ). The megger can measure insulation resistance between wires, between wires and ground, and between wiring and casings. Normal insulation resistance depends on the wire, insulation, length, and rating by the manufacturer. The greater the leakage, the more defective is the insulation. The wire manufacturer’s rating should be the guide.

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Voltage Measurements

With voltage measurements, the total resistance of the voltmeter must be considered. Reading an analog voltmeter scale is like reading an analog ammeter scale. Some multirange voltmeters have only one range marked on the scale and the scale reading must be multiplied by the range switch setting to acquire the accurate voltage reading. Other voltmeters have individual ranges on the scale for every setting of the range switch. When utilizing these meters, ensure that you read the set of values that corresponds to the range switch setting. Many digital meters also have range switches, but they may additionally have a special feature known as autoranging. This means you can either utilize the range switch or let the digital meter set the proper range by itself.

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