---
title: "Latching and holding current"
subject: "Power Electronics"
chapter: 1
key: "semester_7/power_electronics:latching-holding-current"
source: https://www.bctengineer.com/q/semester_7/power_electronics/latching-holding-current
site: BCT Engineering
---

# Latching and holding current

Power Electronics · Chapter 1: Characteristics and specification of power electronic devices

- Asked in: 2071 Shrawan, 4 marks
- Video: [Latching and holding current of a thyristor](https://www.youtube.com/watch?v=_24_ImQjXao) — •Learning Space•
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_electronics/latching-holding-current
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1

## Question

What is latching current and holding current?

## Short answer

Latching current IL is the minimum anode current, while gate pulse is present, to keep the thyristor ON after gate removal. Holding current IH is the minimum anode current to maintain conduction once ON. IL > IH (typically 2–3 times), so gate pulse must last until anode current exceeds IL.

## Answer

Both are minimum anode currents of a thyristor; latching current concerns turning ON, holding current concerns staying ON.

- **Latching current ($I_L$):** the minimum anode current that must flow through the SCR immediately after it is triggered, while the gate pulse is still present, so that it stays ON after the gate pulse is removed. If the anode current has not reached $I_L$ when the gate pulse ends, the SCR turns OFF again.
- **Holding current ($I_H$):** the minimum anode current that must keep flowing to hold an already conducting SCR in the ON state (gate open). If the anode current falls below $I_H$, the SCR turns OFF and returns to forward blocking.
- $I_L$ is greater than $I_H$, usually $I_L \approx 2$ to $3\,I_H$. Example: for an SCR with $I_L = 40$ mA and $I_H = 15$ mA, the gate pulse must last until the anode current reaches 40 mA, but once ON it stays ON until the current drops below 15 mA.

```text
  iA
   ^         ______________
   |        /              \
 IL|-------/ <- gate pulse   \
 IH|------/---must last till--\----- turns OFF
   |     /    iA reaches IL    \     below IH
 --+----/-----------------------\----> t
      gate
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Latching current is the minimum anode current that:
   A. Must flow after triggering so the SCR stays ON once the gate pulse is removed
   B. Keeps an already conducting SCR ON with gate open
   C. Flows in the SCR during reverse blocking
   D. Is needed to turn the SCR OFF
   Answer: A. If the anode current has not reached I_L when the gate pulse ends, the SCR turns OFF again. (Key point: Latching current)

2. What happens when the anode current of a conducting SCR falls below the holding current?
   A. The SCR remains ON because the gate was applied earlier
   B. The SCR turns OFF and returns to forward blocking
   C. The SCR enters reverse blocking
   D. The SCR breaks down by avalanche
   Answer: B. I_H is the minimum current needed to stay ON; below it the SCR reverts to forward blocking. (Key point: Holding current)

3. Which relation is correct for a typical SCR?
   A. I_H is about 2 to 3 times I_L
   B. I_L equals I_H
   C. I_L is about 2 to 3 times I_H
   D. I_L and I_H are unrelated to each other
   Answer: C. I_L is greater than I_H, usually 2-3 I_H; e.g. 40 mA versus 15 mA. (Key point: Relation between I_L and I_H)

## Related in Power Electronics

- [Thyristor V-I; latching vs holding current](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-latching-vs-holding) — Power Electronics, chapter 1
- [Thyristor V-I, holding/latching; vertical BJT](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-vertical-bjt) — Power Electronics, chapter 1
- [Thyristor V-I, latching/holding, gate signal circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Athyristor-vi-latching-gate-circuit) — Power Electronics, chapter 1