SHUNT OHMMETER

The ohmmeter described to this point is known as a series ohmmeter, because the resistance to be measured is in series with the internal resistors and the meter movement of the ohmmeter. Another type of ohmmeter is the SHUNT OHMMETER. In the shunt ohmmeter, the resistance to be measured shunts (is in parallel with) the meter movement of the ohmmeter.

The most obvious way to tell the difference between the series and shunt ohmmeters is by the scale of the meter. Figure 1-34 shows the scale of a series ohmmeter and the scale of a shunt ohmmeter.

Figure 1-34. - Series and shunt ohmmeter scales.

Figure 1-34(A) is the scale of a series ohmmeter. Notice "0" is on the right and "∞" is on the left. Figure 1-34(B) is the scale of a shunt ohmmeter. In the shunt ohmmeter "∞" is on the right and "0" is on the left. A shunt ohmmeter circuit is shown in figure 1-35.

In figure 1-35, R₁ is a rheostat used to adjust the ∞ reading of the meter (full-scale deflection). R₂, R₃, and R₄are used to provide the R X 1, R X 10, and R X 100 ranges. Points A and B represent the meter leads. With no resistance connected between points A and B the meter has full-scale current and indicates ∞. If a resistance is connected between points A and B, it shunts some of the current from the meter movement and the meter movement reacts to this lower current. Since the scale of the meter is marked in ohms, the resistance of the shunting resistor (between points A and B) is indicated. Notice that the switch has an OFF position, as well as positions for R X 1, R X 10, and R X 100. This is provided to stop current flow and prevents the battery from being discharged while the meter is not being used.

Figure 1-35. - A shunt ohmmeter with internal range resistors.

The shunt ohmmeter is connected to the circuit to be measured in the same way the series ohmmeter is connected. The only difference is that on the shunt ohmmeter the ∞ reading is adjusted, while on the series ohmmeter the 0 reading is adjusted. Shunt ohmmeters are not commonly used because they are limited generally to measuring resistances from 5 ohms to 400 ohms. If you use a shunt ohmmeter, be certain to switch it to the OFF position when you are finished using it.

Q.46 What are the two types of ohmmeters?
Q.47 What is the most obvious difference between the two types of ohmmeters?
Q.48 List the four safety precautions observed when using ohmmeters.