October 23, 1997

Two-wire dc lamp dimmer replaces rheostats

Kevan O'Meara, KO Systems, Chatsworth, CA

Originally for use in commercial trucks, a low-cost dc lamp dimmer (Figure 1) can control more than 100W of incandescent panel lighting and replaces a high-power rheostat, whose heating darkens the instrument panel. The circuit has several unique advantages: A two-wire connection means no change to existing wire harnesses, high efficiency means minimal heat sinking, and a typical circuit costs less than the rheostat it replaces.

A 555 timer generates a PWM drive signal to a power FET. Bootstrapping the entire control circuit across the FET using D₁ allows an n-channel FET to make two-wire operation possible. Operating the PWM at a low switching frequency of about 120 Hz and shaping the rise and fall time minimize EMI without using any filter components.

The optional components in the dotted box add short-circuit protection and may be unnecessary in many applications. You should size the FET's on-resistance to the load (potentially saving some cost), but make sure the FET can handle the lamp's start-up current, which is typically 10 times the steady-state current. All parts are generic and noncritical: 10%, 1/8W resistors and ±30%, 50V capacitors work fine. (DI #2102)

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