Question

What is holding current and latching current in SCR? Draw the gate characteristics of an SCR and explain the characteristics. What are the different signals which can be used for turning on SCR by gate control? compare them.

Answer 1

Holding current:

Holding current is the minimum current flow in SCR to hold SCR in on state. If the anode current fall below holding current SCR will turn off. And that voltage is known as holding voltage. Holding current is less than latching current. The holding current is in the range of 8 to 10 milliamperes.

Latching current:

Latching current is for turning on the SCR. Latching current is the current which is required to turn on the SCR and latch the SCR in conduction mode. Latching current is in the range of 10 to 15 milliamperes.

Gate characteristics of SCR:

I_{g (min)} and V_{g (min)} is the minimum current and voltage below which the SCR will not turn on. Ig (max) and Vg (max) is the maximum current and voltage above which SCR will damage. Means the shaded area is the safe operating area for SCR. For turning on SCR the gate current should be greater than Ig (min) but less than Ig (max) similarly, the gate voltage should be greater than Vg (min) but less than Vg (max).

Turning on SCR by gate control:

The turning on of SCR using gate control is the most commonly used method. The current is applied between gate and cathode of SCR. That current may be AC, DC or pulsating. Once the SCR start conducting because of positive gate current, the gate current is no longer needed to keep the SCR in conduction mode. The gate signal should not be applied if the SCR is in reverse bias condition. The gate terminal should be more positive with respect to a cathode for turning on of SCR.

The three different types of gate control signals are:

1. DC gate signal

2. AC gate signal

3. Pulsed gate signal

1.DC gate signal: In DC gate signal triggering the DC voltage of proper magnitude and polarity is applied between gate and cathode. In DC gate signal triggering the gate, power losses are more.

2.AC gate signal: In AC gate signal triggering the AC voltage is applied between gate and cathode. This method is used for fast turning on of SCR. Due to the positive half cycle of AC at a particular point of that positive half cycle, the SCR will be turn on.

3.Pulsed gate signal: In pulsed gate signal triggering the pulse signal waveform is applied to the gate of SCR. In pulsed gate signal triggering the gate, power losses are less. In this type of triggering the pulse transformer is used. 

Answer 2

(i) Holding Current: Holding current is defined as the minimum anode current required to maintain conducting SCR in the on-state. 

(ii) Latching Current: Latching current is defined as the minimum anode current required to maintain the SCR in the on-state immediately after the SCR has been turned on and the gate signal has been removed.