High-efficiency Dome Lamp Dimmer

There are times when a little light inside the car would greatly assist one of the passengers but thedome light is too bright for safe driving. The dimmer circuit in fig. 1 may be added to an existingdome light or included with a new passenger spot lamp.

High-efficiency Dome Lamp Dimmer Circuit Diagram:

Dimmer Circuit Diagram

The upper op-amp generates a 700 Hz sawtooth waveform which is compared to a setpointvoltage by the lower op-amp. When the sawtooth voltage is above the setpoint, the transistorsturn on supplying current to the bulb. The setting of the potentiometer determines the width of thepulses sent to the lamp and therefore the average voltage. The lamp is dim when the potentiometeris set near the higher voltage. Since the TIP32 switches on and off instead of simply droppingthe voltage like a power rheostat, the power it dissipates remains low and a heat sink is notnecessary.

Many autos run power to lamps with only one wire using the car body for the return current pathso the dimmer must interrupt the positive lead as shown. Simply cut the wire leading to the lampand connect the lamp end to the collector of the TIP32 and connect the battery end to the circuitpower input. Run an additional ground wire to the auto chassis from the circuit. This groundwire will not carry much current and may be a smaller gauge.


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