How to protect LEDs in automotive applications?

I am currently in the process of converting my car's dome light to use two 5W white luxeons. Due to limited space and budget, I'll be using only resistors to drop the voltage. I know that there'll be transient voltage spike, reverse voltage surge and load dump which may damage/shorten the lifespan of the LEDs. I've searched thru the web for a few days but couldn't find any page with simple explanations on how to protect the LEDs from the spikes etc. etc.

Can someone show me or provide a link which give simple explanations that layman like me can understand? Best if wiring diagrams are provided too. Thanks a millions.

Have you seen geores80's 1W dome and map light modules? 5Ws seem like they would generate too much heat if left continuosly on. You mentoned limited space, I would think heatsinking would be an issue with 5watters.



I've got one of his dome light modules in mine. Works great and I recommend them. You don't have to worry about different voltages/brightnesses when using the dome when the car is on (14v) and off (12v) when using only resistors to make your own.

I know you mentioned doing this on a budget but his modules cost less than one or two 5Watters (dome $25 - 1x1W-HD, map $40 - 2x1W-HD). For what they consist of, they're a real bargain. Here's the link.

The descriptions of all of the spikes and surges and hazards of automotive power are pretty nasty, but it's not all that hard to protect your circuitry (usually).

The first job is to protect from reversed battery connections. A series power diode does the trick, usually. A basic 1N4004 should work, but you might also consider a fast recovery diode to protect against negative spikes. Something like a MURS360 works fine on some of my designs.

For spikes, you might also consider a suitable zener to catch the worst stuff. For a 12v system, a 24v zener should catch the worst stuff. Typically, we expect that a 12v battery can be anywhere from 9v to 18v, with spikes of around +/- 200v.

Since the battery voltage can vary so much, some sort of regulation is usually needed. In your case, probably a simple current regulator (only need a sense resistor, a power transistor, a small transistor, and a bias resistor).

To help keep the high frequency stuff out, a ferrite or two, combined with some caps, usually works. I've been pretty happy with some of the surface mount ferrites from Murata.

I design stuff for earthmoving equipment, which is subject to battery power as defined by SAE J1113. Automotive specs might be a bit gentler, but for home projects, it never hurts to have a bit extra protection.

good luck, /ubbthreads/images/graemlins/wink2.gif
Steve K.

Hello,

Here's an idea for some good LED protection...

Protecting against positive spikes and surges:
Say you normally would use a 470 ohm resistor to drive a white
LED at 20ma from the car 12v system. Divide this value (470)
in half, which gives you 235 ohms. Purchase two 220 ohm resistors
and connect them in series, then connect them in series with the
LED anode. Connect the loose end of the resistor string to the +12v
system. Connect a 9v zener cathode to the junction of
the two resistors. Connect the 9v zener anode to system ground.
Connect a 0.1uf capacitor right across the LED leads.
Connect the LED cathode to system ground.

Protecting against negative spikes:
A diode has capacitance which for spikes looks like
a low resistance, so dont bother connecting a diode in series
with the LED but instead connect a 1N4003 diode in anti-parallel
to the LED. This means diode cathode to LED anode, diode anode
to LED cathode.


Al