15-08-2011, 10:11 PM
Analogue meter limitations: extending the FSD of a moving-coil meter. Some will know this, some may not. In the latter case, it's worth committing to memory.
To extend the FSD of a voltmeter by using an external series resistance:
Rs = Rm(n - 1)
To extend the FSD of an ammeter by using an external shunt resistance:
Rs = Rm ÷ (n - 1)
where:
Rs = value of series or shunt resistor;
Rm = resistance of the basic movement;
n = number of times you want the existing FSD to be multiplied by.
Example 1: voltmeter.
Existing voltmeter has Rm of 10,000 ohms and FSD of 10 v
We require an FSD of 100 v.
Hence, n = 10
So, Rs = 10,000(10 - 1) = 90,000 ohms: use two 180 k-ohm resistors in parallel.
Example 2: ammeter.
Existing ammeter has Rm of 1,000 ohms and FSD of 1mA
We require an FSD of 1 amp.
Hence, n = 1,000
Here, (n - 1) = 1,000 for all practical purposes: I'll use that figure.
So, Rs = 1,000 ÷ 1,000 = 1 ohm.
Notes.
Sometimes, a quick check on the power dissipated in the 'multiplier resistance' can avoid an unpleasant surprise (usually applicable to shunts for ammeters). In example 2, for all intents and purposes, all of the measured current, 1 amp, flows through the shunt, whose resistance is 1 ohm. Hence, the shunt will dissipate 1 watt. So that 0.25 watt resistor won't do! :s
When very low values of resistance are required, I often use nichrome wire, salvaged for broken elements of old electric fires or from burnt out elements of toasters.
For ammeters or voltmeters, you also really need a reliably calibrated meter to check the resultant FSD: tolerances of resistors - especially for voltmeter multipliers - can introduce significant errors. The use of high ohmic shunts across these multiplying resistors is sometimes a useful way of trimming for the exact value.
Al.
To extend the FSD of a voltmeter by using an external series resistance:
Rs = Rm(n - 1)
To extend the FSD of an ammeter by using an external shunt resistance:
Rs = Rm ÷ (n - 1)
where:
Rs = value of series or shunt resistor;
Rm = resistance of the basic movement;
n = number of times you want the existing FSD to be multiplied by.
Example 1: voltmeter.
Existing voltmeter has Rm of 10,000 ohms and FSD of 10 v
We require an FSD of 100 v.
Hence, n = 10
So, Rs = 10,000(10 - 1) = 90,000 ohms: use two 180 k-ohm resistors in parallel.
Example 2: ammeter.
Existing ammeter has Rm of 1,000 ohms and FSD of 1mA
We require an FSD of 1 amp.
Hence, n = 1,000
Here, (n - 1) = 1,000 for all practical purposes: I'll use that figure.
So, Rs = 1,000 ÷ 1,000 = 1 ohm.
Notes.
Sometimes, a quick check on the power dissipated in the 'multiplier resistance' can avoid an unpleasant surprise (usually applicable to shunts for ammeters). In example 2, for all intents and purposes, all of the measured current, 1 amp, flows through the shunt, whose resistance is 1 ohm. Hence, the shunt will dissipate 1 watt. So that 0.25 watt resistor won't do! :s
When very low values of resistance are required, I often use nichrome wire, salvaged for broken elements of old electric fires or from burnt out elements of toasters.
For ammeters or voltmeters, you also really need a reliably calibrated meter to check the resultant FSD: tolerances of resistors - especially for voltmeter multipliers - can introduce significant errors. The use of high ohmic shunts across these multiplying resistors is sometimes a useful way of trimming for the exact value.
Al.
