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@hotmail.com>
To: <healeys@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! ...................
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