Posted by By Darrell Nicholson at 12:15PM - Comments: (1)
One of my projects this summer will be to help a friend install new interior lights on an Endeavour 42 that he is restoring, so recently, I've been digging through our many articles on interior marine lighting. While fluorescent lights are an efficient way to illuminate wide areas like the galley, he is also interested in using dimmable LEDs for cabin lighting. Our January 2009 article on LED lights featured a good cross-section of what is available, although only one of the lights we tested for that report was dimmable. Dimming LEDs isn’t as simple as dimming an incandescent bulb. Several companies like Imtra Marine Products offer stand-alone dimming solutions, but they aren’t cheap. For my friend’s project, I was drawn to an inexpensive do-it-yourself upgrade described by reader Tom Wetherbee in the 2009 article, in which he retrofitted a standard, brass berth light with an LED Sensibulb and a dimmer for little more than the price of the bulb itself.
I’m reposting Tom’s article below, along with his illustration below.
“The new Sensibulbs are so bright that I wanted to install a dimmer. With the dimmer described below, I am able to get half the light while using only one-sixth the power. (Editor’s note: Many LEDs are not dimmable. In our comparison, only Imtra’s GU4-21 bulbs were dimmable with this device.)
"Sailor’s Solutions offers a mini-dimmer for $23. Being cheap, I elected to build my own for about $5. The dimmer is a simple circuit and is easy for anyone with a little soldering experience to make.
“As shown on the schematic in the PDF and the photo above, my dimmer circuit consists of three components: a transistor, a linear potentiometer with integral switch, and a self-reseting PTC over-current protection device. The protection device acts as a circuit breaker in the event of a short circuit. The switch on the potentiometer is used as a power switch. The potentiometer provides a variable voltage to the base of the transistor. The transistor serves as the power device and passes the voltage to the Sensibulb. This type of device is called a voltage-follower or emitter-follower circuit.
“Turning the dimmer knob clockwise clicks the switch on at the dimmest setting. Turning further increases the brightness. If you prefer to switch on full brightness and then dim, reverse the plus and minus wires connecting the potentiometer. Having the switch integrated with the dimmer makes it simple to use.
“The parts and sources I used are listed below, but you can substitute similar components. If you use a different potentiometer, make sure it will fit. If the shaft on the potentiometer is too long, it can be easily shortened with a hack saw. Snipping off or folding back the lugs on the potentiometer will give you a little more room.
"Once you have the parts, use the large photo above to guide placement, and the schematic below for electrical connections.
“I chose to solder the transistor and PTC protection device directly to the potentiometer. This saves space and makes for good vibration resistance.
“The transistor requires a small heat sink. By gluing it to the side of the potentiometer, you can use the case of the potentiometer. I used heat sink epoxy (amorphous gray material in photo) for better heat transfer, but ordinary epoxy is adequate.
“The older Sensibulbs, as shown in the schematic, have three wires. For 12-volt operation, the red and white wires are connected. If you have a newer Sensibulb, you can ignore the white wire in the schematic.
"Small pieces of insulation stripped from scrap wire can be used to insulate the module’s leads and keep them from shorting. Also, leave slack in the wires for the lamp to swivel and rotate.
“Finally, be sure to test the dimmer before installing it into the lamp, just in case you have a faulty component or bad connection.
286620 1/8W 10K linear potentiometer w/switch, $2.35
162499 1/4”-inch shaft knob, $1.85
RXEF185-ND Polyswitch PTC protection device, 53˘
P2N2222AGOS-ND transistor, 36˘