Application examples

AN302

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Application examples

The importance of the holding current is highlighted by the following application examples.

1.1

Example 1: light dimmer

Figure 2.

Dimmer with interference suppression filter (coil and capacitor)

Figure 3.

Current in the dimmer TRIAC

In the dimmer circuit of Figure 2 the interference suppression filter can produce oscillations.

If the minimum current during these oscillations is higher than the holding current, that is, if

blocked.

It is possible, if the coil is the incorrect type or of poor quality, that the oscillation is

insufficiently damped and the TRIAC current falls below the holding current. This results in

untimely blocking of the TRIAC. However, it is turned on again at the next gate current pulse;

but the oscillations again prevent continuous conduction and the lamp flickers. Hence this is

known as the flicker effect.

How to prevent the flicker effect

The flicker effect can be prevented by using an appropriate interference suppression filter

which does not produce extensive oscillations, and then by choosing a TRIAC with a lower

holding current.

IG

RC phase-shift network

220V

IT

L

C

Triac

Load

Interference suppression

LC filter

IT

t

IH

IO