Question

How can you match the following dividers to the LV measuring instruments

(i) resistive divider (ii) capacitive divider, (iii) resistive-capacitive divider, and (iv) damped capacitive divider?

Answer 1

Generally the mode of connection of low voltage measuring system to the potential divider is very critical because we must get distortion less measurement for fast transient responses. Sometimes if the dividers are not connected properly a large distortions will itself introduced in the L V arm.

i) Resistive Divider

Ans : In case of resistive dividers, The matching is achieved by taking a pure resistance R = Z at the end of the cable. This pure resistance along with surge impedance form an integral part of the cable.In practical applications this signal cables do have the Losses due the skin effect at higher frequencies and makes Z as a complex quantity. Therefore the matching of resistive divider should be done at higher frequencies with a step input

The divider ratio is given by

ii) Capacitive Divider

Ans: In case of capacitive divider , The cables can't be matched completely . Therefore a low ohmic resistance is connected in parallel with capacitor C which will load the Low voltage arm and hence output will get decreased. As we connected the resistance R= Z at the input end of the cable due to which the voltage across the CRO will be same as across capacitance C.

The divider ratio is given by

where C_k = cable capacitance

iii) Resistive - Capacitive divider

Ans: Initial overshoot which appear will either be small or negligible for both small and medium length cables if we take capacitance values very high. Now this error can be neglected and response can be improved if we use Resistive - Capacitive divider .

Generally the L V arms are co-axial and enclosed in the metal box which are solidly grounded. The series resistances used in R-C divider forms an integral part of the divider as in case of resistive divider . And the the resistors  and capacitors  used in L V arm should have very low inductance.

iv) Damped capacitive divider

Ans: Damped capacitive dividers are used for high dynamic behaviour and it can measure full , front, tail impulse voltages ,and switching impulse voltages

It has two component which are damped capacitive divider and external damping resistance

For the damping divider circuit the the reflected wave will see the matching see the resistors R ₃ in parallel with R ₁ and R ₂

The value of matching resistor is given by