T&T: DIY LEDs

[Jim Fuller]

I looked at a couple of datasheets for 1W LEDs. With a nominal shutdown
voltage of about 2.5 volts your 3.5V LEDs will be dark when your batteries
are down to a mere 10VDC. Just when you need lights in the engine room you
won't have any, at least not the LEDs. LEDs need dynamic load regulation
which means a switching regulator, not only for efficiency but dynamic
range, you know the dynamic range our boats put on their electrical systems:
9VDC to 16VDC.

Here is one of the datasheets I looked up:

http://www.vishay.com/docs/83229/83229.pdf

Let me point out the Forward Current vs Forward Voltage curve in Fig 3.,
that shows 0 current at 2.5 volts per LED and at 4 volts you are over spec
on current. Also, the ambient temp curve shows that derating begins at 70
degrees C.

Old timers on the list should remember some animated mail-trails on the
subject from the first half of the decade.

Capt. Jim
MISS EM
Santa Cruz, CA



-----Original Message-----
From: trawlers-and-trawlering-bounces at lists.samurai.com
[mailto:trawlers-and-trawlering-bounces at lists.samurai.com] On Behalf Of
jonas
Sent: Saturday, May 30, 2009 11:37 AM
To: trawlers-and-trawlering at lists.samurai.com
Subject: T&T: DIY LEDs


Just went through a bunch of thinking about putting in LED lighting in my
engine room. 3 of 4 incandescent have either burned out or the contact
spring has given up. I need to do something and since access is a pain and
getting harder as I grow horizontally, I would like to come up with a
solution that eliminates ever having to do it again.

I looked at the voltages needed by the 1W star LEDs (3.4 to 3.8V) and the
need for regulation and overvoltage protection. It looked like 3 LEDs in
series would not allow enough head room in a 12v system for simple linear
regulation and 2 LEDs in series wasted too much power. The hostile voltages
in a boat's 12v system made me nervous about running without regulation. I
looked at regulated power supplies and they seemed too pricey.

Browsing ebay with keywords (a problem solving technique I use) came up with
a potential solution. There are a bunch of universal car DC laptop adapters
out there. They are switching regulators, so they are efficient, can handle
a bunch of watts, can handle overvoltages and spikes, and are pretty cheap
in comparison. Some of them come with switches that allow you to select
anything from 15v to 24V as the output. I am going to use one of these to
drive 3 strings of a resistor and 4 LEDs in series. I figure 12 1W LEDs
should brighten up the engines nicely and use less power than the current
40W setup. I would choose a resistor voltage combo to minimze the power lost
in the resistor and possibly use the voltage switches as a dimmer if needed.
Anyone see any problems with this strategy? If this works out OK, I would
look at using the same strategy for cabin lighting.

Since I don't want to attach 12 lights in the engine room ceiling with
screws due to access issues, I am thinking of using industrial strength
Velcro to attach the individual LED/heatsink assemblies as well as the
wiring to the ceiling. The adhesive creates the bond, and the Velcro makes
it easier to service in the future if needed. Since each light will only
weight a couple of ounces it should be pretty secure. Has anyone used Velcro
in an engine room and are there any issues I should be aware of?

JonasB
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