Designing of Analog Voltmeter

[pranjul]

What is the basic impact of resistance value in analog voltmeter.
and how this value is calculated.

 

[tronitech]

Ideally you want infinite impedance with a voltmeter but this doesn't happen and any analog voltmeter will have an impact on the circuit under test when inserted. A low end analog voltmeter will have a typical quoted impedance of 2kΩ/V, so if you are on the 50V DC range you will introduce a resistance of 100kΩ when the meter is inserted into the figure. Not a problem perhaps when measuring a power supply rail but if you're measuring the voltage across a 1MΩ resistor in some circuit or other then the analog voltmeter will put 100kΩ across it and have a large impact that needs to be taken into consideration. Hope that helps?

 

[Reloadron]

The Ω/V value of an analog DC meter movement is the reciprocal of the full scale current. The meter movement is driven by the current flowing through it. Note the attached images to get an idea with a focus on the old Simpson 270 meter. The full scale current for just the meter movement is 50uA so if we take 1 / 50uA we get 20,000 Ω/V which is the sensitivity of the meter movement. That is the relationship between full scale current and Ω/V.

Most analog meters have either their full scale current or their Ω/V on the meter face somewhere, I say most as not all meter movements have that information on their meter face.

Obviously to reduce loading effect when the meter is placed in the circuit the higher the Ω/Volt rating the better. So in the attached images the old Simpson 269 is the superior meter.
Does that help?

Ron

 

[Nigel Goodwin]

You also need to be VERY aware that digital meters have a similar effect - and in some aspects even worse.

Digital meters commonly have a fixed input of 10 megaohms, and while this is fine on low voltage ranges, on high ones it actually presents more of a load than an analogue one.