Do You Know the Internal Resistance of Your DMM?

       The voltmeter and ammeter functions of the DMM are two very useful functions for technicians. And they're generally quite accurate. One of the determinants of accuracy in a meter is its internal resistance. Ideally, a voltmeter would have an internal resistance of infinite ohms. Otherwise, the resistance of the meter, in parallel with the resistance between the points across which the voltage is being measured, causes the resistance to become lower, thus distorting the reading. A well-designed DMM has a very high internal resistance on the voltage function. An ideal ammeter would have an internal resistance of zero ohms. Any departure from that ideal introduces some inaccuracy into the readings taken by the meter.

       Since an ammeter is inserted in series with the circuit being measured, any ammeter resistance would add to the total circuit resistance, thus reducing the circuit current. A well-designed DMM has a very low resistance on the ammeter function.

       Did you know that you can measure the internal resistance of your DMM? Here's how: Measuring the resistance of the voltmeter function To measure the internal resistance of the voltmeter function, construct the circuit of Figure S1. The voltage read by the voltmeter in this circuit is not only the voltage across the series combination of the voltage source and the 1M Ohm resistor, it is also the voltage across itself. And because this is a simple series circuit, the current that flows through the resistor is also the current that flows through the meter.

       The equation for the current is: I =(VS - VM)/R

If we know the current (I) and the voltage (VM),
using Ohm's law, we can calculate RM :
RM = VM /I =VM /[(VS - VM)/R] = RV M/(VS - VM)

Thanks to Electronic Servicing & Technology Magazine for providing this article.