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EDN Access--10.23.97 Two-wire dc lamp dimmer replaces rheostats

-October 23, 1997


 
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)

Figure 1
A low-cost dc lamp dimmer can control more than 100W of incandescent panel lighting with two wires, high efficiency, and typically lower cost than a high-power rheostat.


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Copyright c 1997 EDN Magazine, EDN Access . EDN is a registered trademark of Reed Properties Inc, used under license. EDN is published by Cahners Publishing Company , a unit of Reed Elsevier Inc.

 

 

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