AN302 Datasheet, PDF (1/10 Pages) STMicroelectronics – The purpose of this note is to familiarize

AN302
Application note
Thyristors and TRIACs: holding current - an important parameter
Introduction
The purpose of this note is to familiarize the users of TRIACs or thyristors with the
hypostatic current parameter, I_H, also known as the holding current parameter.
The importance of this parameter is illustrated by some typical examples. Then a description
is given of how to measure it and on its variation with the conditions of use and the
sensitivity of the components.
This application note discusses only the TRIAC; however, the concepts are also valid for
SCRs.
Definition
To keep an electromechanical relay turned on, it is necessary to have a minimum current
circulating in its coil. If the current falls too low the relay would turn off. The same
phenomenon can be observed in a TRIAC. This minimum current which keeps the TRIAC
conducting is called the hypostatic or holding current, I_H. (see Figure 1).
Figure 1 below shows the holding current and the gate current pulse, I_G, which is applied to
the basic TRIAC circuit. After the TRIAC is turned on, a current, I_T, flows through it.When the
TRIAC current falls below the holding current the TRIAC is blocked and requires another
gate pulse before it can turn on again.
Figure 1. Current control
IT
IH
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March 2008
Rev 3
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