Series type ohmmeter and Shunt type ohmmeter
OHMMETERS : Two instruments are commonly used to check the continuity or to measure the resistance of a circuit or circuit element. These instruments are the ohmmeter and the megger, or megohmmeter. The ohmmeter is widely used to measure resistance and to check the continuity of electrical circuits and devices. Its range usually extends to a few megohms.
The megger is widely used for measuring insulation resistance, such as the resistance between the windings and the frame of electric machinery, and for measuring the insulation resistance of cables, insulators, and bushings. Its range may extend to more than 1,000 megohms. When measuring very high resistances of this nature, it is not necessary to find the exact value of resistance, but rather to know that the insulation is either above or below a certain standard. When precision measurements are required, some type of bridge circuit is used. Ohmmeters may be of the series or shunt type.
Ohmmeter
1. An ohmmeter is an instrument used to measure resistance and check the continuity of electrical circuits and component. This resistance reading is indicated through a meter movement.
2. The ohmmeter must then have an internal source of voltage to create the necessary current to operate the movement, and also have appropriate ranging resistors to allow desired current to flow through the movement at any given resistance.
3. Two types of schemes are used to design an ohmmeter – series type and shunt type.
4. The series type of ohmmeter is used for measuring relatively high values of resistance, while the shunt type is used for measuring low values of the resistance.
Series type ohmmeter
1. In this Figure1, R1 is the current limiting resistor, R2 is the zero adjust resistor, RX is the unknown resistor, E is the internal battery voltage and Rm is the internal resistance of the d’Arsonval movement. A and B are the output terminals of the ohmmeter across which an unknown resistor is connected.
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Basic series type ohmmeter
2. When RX = 0 (short circuit), R2 is adjusted to get full-scale current through the movement. Then, I = Ifsd. The pointer will be deflected to its maximum position on the scale. Therefore, this full-scale current reading is marked 0 ohms.
3. When RX = ∞ (open circuit), I = 0. The pointer will read zero. Therefore, the zero current reading is marked ∞ ohms.
4. By connecting different values of RX, intermediate values are marked. The overall accuracy of the scale markings depends on the repeating accuracy of the movement and tolerance of the resistors used for calibration. Figure 2 shows a typical scale of the series type ohmmeter. Note that the scale is logarithmic – “expended” at the low end of the scale and “compressed” at the high end to be able to span a wide range from zero to infinite resistance.
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Ohmmeter scales series scale and shunt scale
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series type ohmmeter
Shunt type ohmmeter
1) Figure 3 shows the basic circuit of the shunt-type ohmmeter where movement mechanism is connected parallel to the unknown resistance. In this circuit it is necessary to use a switch, otherwise current will always flow in the movement mechanism.
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Basic shunt-type ohmmeter
2) Resistor Rsh is used to bypass excess current.
3) Let the switch be closed. When RX = 0 (short circuit), the pointer reads zero because full current flows through Rx and no current flows through the meter and Rsh. Therefore, zero current reading is marked 0 ohms.
4) When RX = ∞ (open circuit), no current flows through RX. Resistor R1 is adjusted so that full-scale current flows through the meter. Therefore, maximum current reading is marked ∞ ohms. Comparison of series and shunt ohmmeter scales is shown in Figure 4.
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Ohmmeter scales series scale and shunt scale
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shunt type ohmmeter
Use of the Ohmmeter
The ohmmeter is not as accurate a measuring device as the ammeter or the voltmeter because of the associated circuitry. Thus, resistance values cannot be read with greater than 5 to 10 percent accuracy. While there are instruments which read the resistance of an element with very great accuracy, they usually are more complicated to use.
In addition to measuring the resistance, the ohmmeter is a very useful instrument for checking continuity in a circuit. Often, when troubleshooting electronic circuits or wiring a circuit, visual inspections of all parts of the current path cannot be readily accomplished. Therefore, it is not always apparent whether a circuit is complete or whether current might be flowing in the wrong part of the circuit because of contact with adjacent circuits. The best method of checking a circuit under these conditions is to send a current through the circuit. The ohmmeter is the ideal instrument for checking circuits in this manner. It provides the power and the meter to indicate whether the current is flowing.
Observe the following precautions when using an ohmmeter:
(1) Choose a scale which will contain the resistance of the element to be measured. In general, use a scale in which the reading will fall in the upper half of the scale (near full scale deflection).
(2) Short the leads together and set the meter to read zero ohms by setting the zero adjustment. If the scale is changed, readjust to zero ohms.
(3) Connect the unknown resistance between the test leads and read its resistance from the scale. Never attempt to measure resistance in a circuit while it is connected to a source of voltage. Disconnect at least one end of the element being measured to avoid reading the resistance of parallel paths.
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