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.
A simplified schematic of an ohmmeter is shown in figure 8-137. E is a source of EMF; R1 is a variable resistor used to zero the meter; R2 is a fixed resistor used to limit the current in the meter movement; and A and B are test terminals across which the resistance to be measured is placed
If A and B are connected together (short circuited), the meter, the battery, and resistors R1 and R2 form a simple series circuit. With R1 adjusted so that the total resistance in the circuit is 4,500 ohms, the current through the meter is 1 ma. and the needle deflects full scale. Since there is no resistance between A and B, this position of the needle is labeled zero (figure 8-138). If a resistance equal to 4,500 ohms is placed between terminals A and B, the total resistance is 9,000 ohms and the current is 0.5 ma.
This causes the needle to deflect half scale. This half scale reading, labeled 4.5 K ohms, is equal to the internal resistance of the meter, in this instance 4,500 ohms. If a resistance of 9,000 ohms is placed between terminals A and B, the needle deflects one-third scale. Resistances of 13.5 K and 1.5 K placed between terminals A and B will cause a deflection of one-fourth and three-fourths scale, respectively
If terminals A and B are not connected (open circuited), no current flows
and the needle does not move. The left side of the scale is, therefore, labeled infinity to indicate an infinite resistance.
A typical ohmmeter scale is shown in figure 8-138. Note that the scale is not linear and is crowded at the high resistance end. For this reason, it is good practice to use an ohmmeter range in which the readings are not too far from mid scale. A good rule is to use a range in which the reading obtained does not exceed ten times, or is not less than one-tenth, the mid scale reading. The useful range of the scale shown is, by this rule, from 450 ohms to 45,000 ohms
Most ohmmeters have more than one scale. Additional scales are made possible by using various values of limiting resistors and battery voltages. Some ohmmeters have a special scale called a low ohm scale for reading low resistances. A shunt-type ohmmeter circuit is used for this scale.
Shunt-type ohmmeters are used to measure small values of resistance. In the circuit shown in figure 8-139, E (voltage) is applied across a limiting resistor R and a meter movement in series. Resistance and battery values are chosen so that the meter movement deflects full scale when terminals A and B are open. When the terminals are short circuited, the meter reads zero; the short circuit conducts all the current around the meter. The unknown resistance Rx is placed between terminals A and B in parallel with the meter movement. The smaller the resistance value being measured, the less current flows through the meter movement.
The value of the limiting resistor R is usually made large compared to the resistance of the meter movement. This keeps the current drawn from the battery practically constant. Thus, the value of Rx determines how much of this constant current flows through the meter and how much through Rx.
Note that in a shunt-type ohmmeter, current is always flowing from the battery through the meter movement and the limiting resistor. Therefore, when using an ohmmeter with a low ohm scale, do not leave the switch in low ohm position.
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.