T&T: DIY LEDs

[jonas]

Capt Jim, 

I think I solved that problem with the laptop DC adapter. That's why I was
so happy to figure out that I could use a laptop DC adapter to raise the
voltage and clean up the messy, spiky, multi-voltage environment on the
boat. I have not seen their schematics, but from their description, they
appear to be efficient switching DC to DC converters that give me the extra
voltage I need and they can handle 3-4 amps of current, which is a lot more
then I need for my 3 strings of 4 LEDs. Since LEDs are a current driven
devices, trying to control their current with just a voltage source is not a
good idea. As a minimum a resistor is needed in series to keep from burning
up the LEDs. The resistor is chosen based on the current and how much
voltage you need dropped. 4 LEDs at 3.4 V use up 13.6 V, so 1.4 volts needs
to be lost across the resistor at 350 ma. This says about 4 ohms are needed.
The forward drop of the LEDs is not always 3.4V, so I will probaly take some
measurements and tweak as needed.

Jonas

-----Original Message-----
From: Jim Fuller [mailto:jfuller at svinet.com] 
Sent: Monday, June 01, 2009 8:44 PM
To: 'jonas'; trawlers-and-trawlering at lists.samurai.com
Subject: RE: DIY LEDs

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