Saturday, October 3, 2015

12 Volt LEDs for a Boat

I needed some new lights in some places on the boat. Many of the LED light strips and and clusters these days use either 12 volts or an AC adapter for 12 volts. Some even say "for car".

However on closer inspection most of these have only a current limiting resistor in series with sets of 3 LEDs. Thus they are suitable for ONLY 12 volts and below. Boats and cars are nominally 12 volts. That means they are NOT 12.0 volts. My boat runs from 12.4 to 14.4 volts depending on the state of charge of the batteries, the sun on the solor panels and whether the engine/alternator is running.

The advertised long life of LEDs is dependent on ideal conditions. The 2 main areas of abuse LEDs suffer are over temperature and over voltage.

In the tropics, the only thing I can do about the heat is by choosing how and where they are installed. They should get natural cooling of have a cooling fan.

For over voltage, what I have discovered is the LM2940T-12.0 low dropout voltage regulator. The ones I bought worked perfectly in my tests with my LEDs.

My LEDs use 20 to 40 mA each. I varied the voltage from 10.0 volts to 18.0 volts. Below 12.0 volts, the voltage out at the LED was the same as the input voltage. Above 12.0 volts the voltage stayed between 11.9 and 12.1 on my cheap multi-meter.

I used the LM2940 with no additional circuitry. Just soldered the input positive wire to Vin ground to GND of both the regulator and the LED and Vout to positive of the LED.

So far, so good.
My Chinese LED Light Tube with the LM2940 soldered inside the end. The legs straddling the end LED.

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