Thyristor Operated Dimmer, Electronic Dimmer

Dimmers

Different lamps/luminaires produce light with constant brightness once they are switched on.
For the following spots, the variation of brightness of the lamps is required.

For (i) Photo studios (ii) Stage (iii) Drama theatres (iv) Decorations (v) Various functions.

The device for controlling the brightness is a "Dimmer".
Dimmer is an electrical and or electronics device. It basically changes the current or voltage which is the cause of varying brightness of luminaries.

Let us study the circuit arrangements and operation of these dimmers.
1. Variable Resistance Type Dimmer
1.1 Resistance Type Dimmer

1.2 Salt Water Dimmer

2. Dimmer Transformer
2.1 Autotransformer Dimmer
2.2 Two Winding Transformer Dimmer

3. Electronic Dimmer

Electronic dimmers are very compact and handy.
These circuits make use of solid state devices like SCRs, TRIAC, DIAC.
These are used to control the intensity of lamps.
In comparison to resistance control dimmer, the power loss in this system is negligible,

These are two types of circuits possible hence two types are possible

3.1 Thyristor Operated Dimmer

If we compare the power loss in such type dimmer with resistance type dimmer it is negligible.
SCR (or thyristor) is used to control the phase angle i.e. voltage applied to the lamp which controls brightness of the lamp.
The circuit consists of diodes D1, D2, D3, D4, forming bridge rectifier. When rectified output voltage of bridge rectifier increases, capacitor C1 is charged.
The voltage across the capacitor increases till it causes zener diode D5 to conduct. When zener diode conducts, the SCR is turned ON.
The capacitor C1 now discharges through SCR. The SCR is turned OFF when this current comes to zero. The combination of R1 and C1 decides the conduction current of SCR.
Thus voltage applied to the lamp is varied, hence the lamp intensity can be controlled.

Following are waveforms of voltage applied to the lamp.

1st case : Brightness is maximum.
2nd case : Brightness is moderate.
3rd case : Brightness is minimum.

Thus by this dimmer the brightness is controlled from maximum to dimming gradually to minimum

Application of Thyristor Operated Dimmer

3.2 Triac Operates Dimmer