Ohmmeter Scale

Figure 149 shows a typical analog ohmmeter scale. Between zero and infinity (∞), the scale is marked to indicate various resistor values. Because the values decrease from left to right, this scale is often called a back-off scale.

In the case of the example given, assume that a certain ohmmeter uses a 50μA, 1000Ωm eter movement and has an internal 1.5 volt battery. A current of 50μA produces a full-scale deflection when the test leads are shorted. To have 50μA, the total ohmmeter resistance is 1.5 V/50μA = 30kΩ.T herefore, since the coil resistance is 1kΩ, the variable zero adj ustment resistor must be set to 30kΩ – 1kΩ = 29kΩ.

Now consider that a 120kΩ resistor is connected to the ohmmeter leads. Combined with the 30kΩ internal resistance, the total R is 150kΩ. The current is 1.5 V/150kΩ = 10μA, which is 20% of the full scale current and which appears on the scale as shown in Figure 149.

Now consider further that a 120kΩ resistor is connected to the ohmmeter leads. This will result in a current of 1.5V/75kΩ = 10μA, which is 40% of the full scale current and which is marked on the scale as shown. Additional calculations of this type show that the scale is nonlinear. It is more compressed toward the left side than the right side. The center scale point corresponds to the internal meter resistance of 30kΩ. The reason is as follows:

With 30kΩ connected to the leads, the current is 1.5V/60kΩ = 25μA, which is half of the full scale current of 50μA.