SCR Ratings and Specifications

SCR Specifications and Ratings:

The main specifications of the SCR are its voltage rating and current rating. In this post, let us see various ratings of thyristor.

Voltage Rating:

Peak Invese Voltage (VPIV)
The peak inverse voltage is defined as the maximum voltage which SCR can safely withstand in its OFF state. The applied voltage should never be exceeded under any circumstances.

On State Voltage:
The voltage which appears across the SCR during its ON state is known as its ON state Voltage. The maximum value of voltage which can appear across the SCR during its conducting state is called its maximum on state voltage. Usually it will be 1V to 4V.

Finger Voltage:
The minimum voltage, which is required between the anode and cathode of an SCR to trigger it to conduction mode, is called its finger voltage.

Rate of Rise of Voltage (dV/dt)
The rate at which the voltage across the device rises ( for forward condition) without triggering the device, is known as its rate of rise of voltage.

Voltage Safety Factor:
The normal operating voltage of the SCR is kept well below its peak inverse voltage(VPIV) to avoid puncture of SCR due to uncertain conditions. The operating voltage and peak inverse voltage are related by voltage safety factor Vf
Vf = Peak inverse voltage / ( 2 x RMS value of input voltage)
Normaly Vf value lies between 2 and 2.5

Current Rating:

The current carrying capacity of the device is known as its current rating.
It can be of two types.
1. Continuous
2. Intermittent.

Maximum average ON state current ( Imac):
This is the average value of maximum continuous sinusoidal ON state current with conduction angle 180deg, at frequency 40 to 60Hz, which should not be exceeded even with intensive cooling.

Maximum rms ON-state current: (Imrc)
It is the rms value of the maximum continuous sinusoidal ON state current at the frequency 40 to 60 Hz and conduction angle 180deg, which should not be exceeded even with intensive cooling.

Maximum surge - ON state Current (Imsc)
It is the maximum admissible peak value of a sinusoidal half cycle of tem milliseconds duration at a frequency of 50Hz.

Latching Current (II)
It is the minimum current, which is required to latch the device from its OFF state to its ON state. In other words, it is the minimum current required to trigger the device.

Holding Current (IH)
It is the minimum current required to hold the SCR conducting. In other words, It is the minimum current, below which the device stops conducting and returns to its OFF state.

Gate Current:
The current which is applied to the gate of the device for control purposes is known as gate current.

Minimum Gate Current:
The minimum current required at the gate for triggering the device.

Maximum Gate Current:
The maximum current which can be applied to device safely. Current higher than this will damage the gate terminal.

Gate Power Loss:
The mean power loss, which occurs due to flow of gate current between the gate and the main terminals.

Turn ON time:
The time taken by the device before getting latched from its OFF state to ON state. In other words, it is the time for which the device waits before achieving its full conduction. Usually it will be 150 to 200μsec.

Turn OFF time:
After applying reverse voltage, the device takes a finite time to get switched OFF. This time is called as turn-OFF time of the device. Usually it will be 200μsec.

Rate of rise of current(dI/dt)
The rate at which the current flowing in the device rises is known as its rate of rise (dI/dt) of current.

Read More:

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