Instrument light dimmer
Instrument light dimmer
A few instrument-light dimming
circuits have been published here,
usually using a 2N3055 power
transistor. However, this circuit is
much more rugged. The
potentiometer will never burn out
since little current passes through it.
More importantly, the regulator shuts
itself down above 125ºC so even if a
post light socket gets shorted nothing
will melt down or burn out. This
circuit is easy to build, uses common
parts (see www.digikey.com), and is
much cheaper than any of the prebuilt commercial units.
+12V
+
Input
10µf
To instrument
lights
Output
1K
LM1084
+
47µf
Adj
Top view
Part Discription
5A Low Dropout Regulator, TO-220
10K Potentiometer, with switch
Heatsink, Dual TO-220, 15W
1K resistor, ¼ watt
10µF electrolytic capacitor, 25V
47µF electrolytic capacitor, 16V
10K
Digikey Part No.
LM1084IT-ADJ-ND
CT2227-ND
294-1082-ND
1.0KQBK-ND
P813-ND
P5137-ND
Cost
$2.14
$3.48
$2.13
$0.06
$0.30
$0.21
If you notice a sudden uncommanded
dimming of the instrument lights, this
means that the regulator is nearing its thermal limit and is an indication that you need a larger heat sink. The
heat sink shown here can handle a small lighting system - up to about 2 amps. Above that you will need a
bigger heat sink. The switch on the 12V input is part of the potentiometer. It’s included because even though the
lights would be dark at the dimmest setting, appreciable power would be wasted. When you rotate the knob all
the way towards the dim setting, you will hear a click as it switches off. Of the potentiometer’s three terminals,
one end terminal remains unused. If the switch turns off as you rotate the knob to the full bright position then
you have connected to the wrong end terminal. If your panel already has an on/off switch for the instrument
lights then can save a little by buying a potentiometer without the built-in switch.
I used red, black, and green for the wires to +12V, ground, and the instrument lights respectively. Typically you will want
to build two of these, one for the instrument lights and the other for the radio lights. I used a separate heat sink for each,
but you could use a single large heat sink instead. You can’t see the 1K resistor in the picture because it is under the
heat shrink tubing at one of the potentiometer terminals. You can’t see the capacitors because I got lazy and left them
off. Theoretically that could cause the circuit to oscillate, although it is working fine in my airplane. You should be able to
use the same circuit in a 24V airplane. Just change the 1K resistor to 470 ohms and double the capacitor voltage
ratings. One final installation warning: Don’t mount the heat sink on anything connected to ground. The regulator case is
connected to the regulator output, so if the heat sink touches ground, the instrument lights are shorted out.
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