Why latching current

Notify me when new comments are added. Perfect Reply. Latching Current. Holding Current. It is related with turn on process of SCR or thyristor. It is related to turn off process. Minimum current above which gate losses its control. So the SCR prevents conducting. The difference between latching current and holding current is discussed below. Latching current can be defined as it is the least amount of anode current which is necessary to supply from the anode terminal to the cathode terminal to activate the SCR after detaching the gate terminal.

This is allied with the turned-off method. This is allied with turned on a method. This current is always below the latching current. This is around two to three times above the holding current. The SCR will be deactivated once the anode supply decreases to below 5mA for the particular holding current rating mA within the datasheet.

The holding current value, as well as latching current value, is stable. Latching Current and Holding Current Ratio Generally, latching currents are higher than the holding currents used for high rating thyristors. But they can drop to 0. Usually, the 20A thyristor used in this is BT and the ratio of this is 1. Consequently, if the total number is in use, then it can be taken like 2 at 25 degrees centigrade.

SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation makes them suitable for use in medium- to high-voltage AC power control applications, such as lamp dimming, power regulators and motor control. The basic working principle in the SCR is that as the triggering or the biasing is applied at the terminal gate then the conduction begins. As it is a unidirectional device the current will be in a single direction.

When the anode current is reduced below the level of the holding current, the SCR turns off. The voltage reverses every half cycle in an ac circuit, so that an SCR in the line would be reverse biased every negative cycle and would turn off.

Consider the below figure and observe that, until the time td, the SCR is in forward blocking mode, so the anode current is a small leakage current. When the gate signal is applied at 90 percent of Ig then the gate current is reached to 0. Once the SCR is on, the switch can be released and remain open as the action of the SCR sustains the current flow through the device and hence the load.

The resistor R1 connects the gate to the supply via the switch.