How do I measure internal resistence of voltmeter without ammeter?

Question

I know how to do it with ammeter, but how can I find internal resistence without ammeter?

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. DC voltmeter impedance measurement technique.

For a DC meter such as a typical digital multimeter you can try the scheme of Figure 1.

• Use a very stable reference voltage. (OK use a PP3 9 V battery.)
• Take a voltage reading from your reference voltage with the meter in question (Figure 1a).
• Use a precision reference resistor. (OK grab a 1M resistor out of your bag of resistors.)
• Take a second voltage reading from your reference (Figure 1b). If R1 is significant relative to the meter the reading will drop.

e.g. If the meter reading falls to half when using a 1M resistor then the resistor must have an input impedance of 1M.

The easiest way to do it is to keep adding or subtracting resistance until you get a reading of half of the battery voltage. That resistance is equal to that of the meter.

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Update

VM2/9V = (R1+Ri)/R1 Is this correct?

No. \$ \frac {meter \ voltage}{total \ voltage} = \frac {meter \ R}{total \ R} \$. So, \$ \frac {VM2}{9} = \frac {Ri}{R1 + Ri} \$.