High Series Resistance with Micro Ammeter
- A very high resistance of the value of a few hundreds of megaohms is connected in series with a micrometer for measurement of High dc voltages as shown in Figure.
- A large celebrated resistance R is connected in series to the moving coil micrometer. The current I flowing the R is calibrated and voltage to be measured is given as
V = IR
- The voltage of meter is only few ohms compared to the large value of series resistance (few hundreds ohm) and therefore voltage drop in meter is considered negligible.
- A protective device with suitable resistance is connected parallel to the meter so as to protect the meter in case the resistance R fails.
- These protective devices can be paper gap , a neon glow tube or a gener diode.
- The ohmic value of series resistor (R) is chosen such that a current of 1 – 10 mA is allowed for full scale deflection.
- The resistance is constructed from a large number of wire wound resistors in series. The otg drop in each resistor element is chosen to avoid surface flashover and discharges.
- The material for resistance elements is usually carbon-alloy with temperature coefficient less than 10-4/o C.
- Carbon and other metallic film resistors are also used.
The following are some of the limitations in design of series resistors :
a) Effect of temperature and long time stability.
b) Dissipation of power and source loading
c) Voltage dependance of resistive elements.
d) Sensitivity to mechanical stresses.