[TR] LEDs - a mystery

[Randall]

> Quick trip to my FLAPS as its finally time I get a multimeter of my
> own - so I bought a really good Bosch one that has a diode test position
> among the many tests it is capable of performing.

Unfortunately, that function is designed to test 'normal' diodes, which have
a much lower forward voltage drop than most LEDs do.  Although both are
'diodes', light emitting diodes are optimized for light output rather than
rectifying power or signal.  A typical power or signal diode only takes
around 0.7 volts to get it to start conducting, while an LED may take 2 or 3
volts.  A typical meter puts out less than 2 volts even on 'diode test'.

In addition, it's possible that your replacements have more then one diode
in series inside.  I've not tried the commercial brake/tail/reverse
replacements, but the ones I've built for myself all have several diodes in
series.  They won't pass any current at all below about 8 volts.  (Because
of that, I had to redesign my relay setup to work correctly with the LED
lamps.)

So my suggestion is to quit trying to use the meter to test them.  If they
won't light on the car and an incandescent bulb will, it's probably the
LED's fault.

There are some exceptions, depending on what year TR6 you have (for example,
cars with a single lamp to indicate both left and right turn signal will not
work correctly without modification).  But all of the rear lights should
work if you add load resistors for the turn signals.

BTW, the LEDs I used can pretty easily be destroyed by applying the wrong
polarity.  Again, they are not optimized for being rectifiers, maximum
reverse voltage is less than 12 volts.  Some will take more, but not the
ones I got.

-- Randall