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On Introductory Circuit Analysis of Boylestad I found out the following about the relation of internal resistance of a multimeter with the scale choosen on the instrument.

  • Analog voltmeter: internal resistance does depend on the scale chosen
  • Analog ammeter: internal resistance does depend on the scale chosen
  • Digital voltmeter: internal resistance does not depend on the scale chosen

Is all of this true? How can the internal resistance of the digital voltmeter be independent on the scale chosen? And is it the same for the digital ammeter too (i.e. also in that can internal resistance does not depend on the scale choosen)?

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    \$\begingroup\$ Any meter can be designed one way or the other. There are no hard and fast dependencies here. If the analogue voltmeter has a battery it can perform just the same as a digital one (except for the display movement). \$\endgroup\$
    – Andy aka
    Commented Jan 19, 2017 at 12:38
  • \$\begingroup\$ Many DMM's have two ammeter connection points, one rated for 10A and one rated for smaller currents (possibly 400 mA). The 10A connection point has much lower series resistance. In my experience, the scale setting does not change the series resistance on a DMM. It only depends on which connection you use. I don't know much about analog meters. \$\endgroup\$
    – user57037
    Commented Jan 19, 2017 at 16:32
  • \$\begingroup\$ I have never come across a situation where the current flowing into a DMM in voltmeter mode created a problem. You can just pretend the current is zero most of the time. But the series resistance in ammeter mode can definitely cause problems if you are testing low-voltage devices. The voltage drop across the meter can be appreciable, and can definitely affect the way low-voltage circuits behave. Generally it is better to find some other way to measure current. For example using an existing component (fuse or ferrite bead) as a shunt. \$\endgroup\$
    – user57037
    Commented Jan 19, 2017 at 16:39

1 Answer 1

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It's more a function of whether the meter has an amplifier before it.

In the moving coil meter, the meter reading is directly proportional to the current flowing through it. These generally do not have an amplifier, so that current has to be provided by the thing being measured. Therefore the higher the input voltage, the higher the resistance placed in series.

My first moving coil meter was '20k/V', that is it needed a series resistance of 20kohms for every volt on the scale, which indicated that the meter read full scale for 50uA. As range switching is done by a mechanical switch, high voltage withstanding is not a problem, and input resistors are simply switched as required.

You can put an amplifier in front of a moving coil meter, but it's generally not done.

As a digital meter requires electronics and a battery, the amplifier sort of comes for free. It's most convenient there to have a high input resistance like 10Mohms, and switch the ranges by shunting the ADC input with various value resistors. This arrangement allows all the switches to stay at low voltage, even if the input voltage is 1000V. If a 10k shunt was used, then the input voltage would be 1000x the ADC voltage for full scale.

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