Holding Current and Latching Current with Differences

The thyristor or SCR is a power semiconductor device which is used in power electronic circuits. They work like a bistable switch and it operates from nonconducting to conducting. The designing of thyristors can be done with 3-PN junctions and 4 layers. It includes three terminals namely anode, gate, and cathode. Thyristors are different compare with transistors. Because on-state conduction losses of thyristor are lower & also they have a handling capacity of high power. Whereas in transistors, they have great switching acts, switching speed is high and switching losses are low. This article discusses an overview of holding a current and latching current in SCR and also its differences.

Holding Current and Latching Current in SCR

The difference between a holding current and a latching current in SCR mainly includes what is a latching current, latching current in SCR, what is a holding current, holding current in SCR, its V-I Characteristics, latching current &  holding current ratio and its differences.

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SCR

What is Holding Current?

The holding current for different devices like electronic, electrical, and electromagnetic is the smallest amount current which should flow throughout a circuit to maintain the ‘ON’ state. This can be useful for a single switch otherwise to a complete device. The best example of holding current is within a spark gap.

Generally in basic circuits, whenever the flow of current drops under the holding current, then the circuit will be turned ‘OFF’. But, complex devices and circuits may contain dissimilar delays fixed among the time when the flowing current drops below this level & the time when the device is switched OFF.

A design issue in a circuit is when the flow of current is restored in case a device is switched ON. The threshold current can be defined as the required current to reinstate the circuit to the ‘ON’ state, possibly much better than the holding current.

But, wherever the device is considered to switch ‘ON’ for current restoration & wherever the circuit is working at small differences in the current, then it can reason for flicker when the device cycles ON & OFF.

If flicker is not necessary, then it can be decreased by using capacitors otherwise other circuits. Alternatively, flicker is also employed for measuring little events like in a G-M (Geiger–Müller ) tube.

What is Latching Current?

The latching current is the smallest amount of anode current is required for preserving the thyristor in the ON condition instantly once a thyristor is turned ON then the gate signal has been detached.

This current is connected with the process of turning ON. The worth of this current is around two to three times to that of holding current. The worth of holding current as well as latching current is stable. So it does not be dependent on the magnitude of gate current.

Holding Current in SCR

Holding current in thyristor or SCR can be defined as, the smallest amount of current under which anode current has to drop to enter OFF status. This means, if the holding current value is 5 mA, subsequently thyristor’s anodes current have to turn into less than 5 mA to discontinue performing.

Latching Current in SCR

The minimum current is the Latching current of SCR in forwarding bias which anode current has to achieve to maintain to stay in the mode of forwarding conduction even as gate current is detached. If the anode current value is under this value, then the SCR will not maintain to perform in the direction of forward if the gate current is detached. However when anode current turns into greater than latching current, then the gate terminal losses its power & it may be detached. Finally, the SCR will go on to conduct.

V-I Characteristics

Therefore we know that both the latching current and holding current are two dissimilar quantities. The following diagram shows the V-I characteristics of SCR.

v-i characteristics-of-latching-current-and-holding-current
v-i characteristics-of-latching-current-and-holding-current

In the above VI-characteristics, we can simply observe the latching and holding current of thyristor or SCR and also latching current is in excess of holding current. When the flow of current throughout the SCR is anode current ‘I’ where it falls under holding a current and the current supply will be zero. So the SCR prevents conducting.

Difference between Latching Current and Holding Current

The difference between latching current and holding current is discussed below.

Latching Current

Holding Current

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.

 

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. It doesn’t depend on the magnitude of the gate current.

 

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 0.4 based on the temperature as well as the driving load. Usually, the 20A thyristor used in this is BT152 and the ratio of this is 1.67. Consequently, if the total number is in use, then it can be taken like 2 at 25 degrees centigrade.

Thus, this is all about brief information about latching current and holding current. From the above information, finally, we can conclude that latching current is the highest anode current which is used for maintaining the thyristor turned on instantly once the gate signal is detached. Similarly, the holding current is the lowest anode current which is used for maintaining the thyristor in conducting state. Here is a question for you, what is holding current in TRIAQ?



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