[Healeys] reflector brake light-NIFTY UPGRADE-

[Alex]

Ken, LEDs will work with your positive ground car, as long as they're 
isolated from the chassis, and / or the cathode of the diode is attached to 
the negative side of the system (-12-V). If your LED assembly has two leads 
coming from it, connect the cathode to -12-V and attach the anode lead to 
ground (+).

Otherwise, you may have to modify the housing if the design already has the 
leads attached. That assumes you can get to the wires in your housing. I 
suspect you can.

Also, there should be a series-limiting resistor in the LED assembly, to 
prevent too much current from flowing. this may already be built into your 
kit.

Single LEDs operate at about 2-V, so the resistor must drop roughly 12-V. 
The value of the resistor is determined by how much current the LED draws 
(from its spec sheet). Using Ohm's Law, R = E/I, where E is voltage and I is 
current.

For example, say your LED draws 20-mA (0.02-Amperes). If you need to drop 
10-V, R = E / I = 10 / 0.02 = 500 ohms.

The power is found by P = I x E, so 0.02 x 10 = 200-milliwatts. That means a 
half-watt carbon 470-ohm resistor is all that's needed in series with the 
LED. It's color code would be yellow-violet-brown.

 I have LEDs installed on my positive ground MG-TC this way. Two LEDs serve 
as running lights (series resistors keep them dimmer than my brake light 
LEDs, which have lower-value series-resistors, and are therefore brighter).

My LEDs are arrays of more than one diode, so I had to calculate my series 
resistors based on the current spec of the array.

== Alex in Maine
     "The Blue Mainie," 1960 Austin Healey 3000 BT7
     "Conkling," 1946 MG TC #1321
     Former owner 1957 A-H 100-6, 1967 A-H BJ8,
     1965 MG Midget
     http://users.adelphia.net/~alexmm/ai2q.htm


----- Original Message ----- 
From: "Ken Wignall" <ourxke at hotmail.com>
To: <healeys at autox.team.net>
Sent: Saturday, September 15, 2007 2:46 PM
Subject: Re: [Healeys] reflector brake light-NIFTY UPGRADE-


>I read about the upgrade a lister posted recently to use LED technology and
> install  very high intensity bulbs, that last forever and never get hot, 
> into
> the rear reflectors of my 61 BT7. I also ordered the bayonet pigtailed 
> sockets
> along with them. Great idea except for one little glitch-They only work on
> negative ground!  ...................