Current drawn into a digital voltmeter

Hi all,

I'm asking how I can know the current drawn into an autoscaling voltmeter.

And in general , how I can know the input impedence of any DIGITAL type meter.

Thanks in advance

It all depends on the particular meter and the full scale setting. They usually run 1 to 100 Mohm or so.

May be you can do like that :
a. Measure let say batter voltage .
b. Then connect 1 MOhm resistor serially to multimeter and measure voltage again .
c. Using the got value from first and second measure and resistor value calculate input impedance of voltmeter as below :

Rmult = 1MOhm * V2/(V1 - V2)

The suggested method is simple and easy but that Rmult will be cahnged at every voltage scale as the meter will switch automatically - an autoscaling one.

As I know , the input stage is an OP-AMP stage and an attenuator. Using autoscaling makes it difficult to get a real full scale value.

Most digital multimeters have an input resistance of 10 Mohm, in all voltage ranges.

The old Fluke 8060 series had two low DC ranges (2 V and 0.2 V full scale) that had 1 Gohm guaranteed and 10 Gohm most of the time.

There was another Fluke model that I cannot remember now that had the 100 M ohm in the higher voltage scales.

All of these impedances are shunted by 100 pF of capacitance which drags the impedance down below 1 M in the audio range.

Also, using a high impedance meter is not always a good thing. When measuring power circuits you can get fooled with a high impedance meter. I once chased my tail for an hour with a wall mounted power supply. The strip extension socket measured full AC voltage, the output of the supply measured full voltage, but the battery charger it powered showed no charging of the battery. What was going on was the on-off switch on the power strip had been put to off. There was enough open circuit switch capacitance for the meter to measure full voltage. The DC supply was just a transformer-rectifier-shunt capacitor. There was enough leakage current to charge the capacitor. had I used an analog meter with a 50 microamp draw, I would not have been fooled.

The input impedance should be almost the same.
If the autoranging meter is in mV mode and suddenly you apply some 100V to it, the device should not burn through. Thats why in most Volmeters are decade resistor deviders.

look here: **broken link removed**

The input impedance is on the whole mostly 10MOhms.
In this example the series resistors have the values like
9MOhms ---- 900kOhms --- 90kOhms --- 9kOhms --- 900 Ohms and so on.
The last resistor is connected to ground, the open side of the 9MOhms to the input signal.

by measuring between on connection of these resistors to ground you get a decadic resistor devider.

e.g. between 9M and 900k to ground the voltage is divided by 10.
next measuring between 900k and 90k to ground is divided by 100.
and so on.

but for the input signal the impedance is almost constant 10MOhms.