Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) LED Side Lights for 2008 Roadster

The forward bias resistance of a stand alone LED is low. The forward voltage drop across them is between 1.8 and 3.3 volts depending on colour. As the voltage drop is fairly constant over a wide current range a higher applied voltage would rapidly increase it, so a series resistance is added as previously mentioned to limit same and avoid burn out. For the sake of the discussion a complete working LED as below has to be considered - sorry it's come out so big



They may also have a conventional diode within for reverse polarity protection or even a tiny bridge rectifier allowing them to be connected in a polarity insensitive manner. I know G4 fitting low voltage DC LED's as used on boats and caravans have such, probably many others including mains ones of course.

Is this correct?

Sorry no. The ballast resistor is external to the sealed LED unit consisting (as a minimum) of LED itself and current limiting resistor.

Typical indicator ballast resistor (6 ohm 50 watt) one for each LED indicator.

I didn't mean the diagram to represent a sealed unit, just the wiring.

Bob,
The reason for the bypass resistor is that the indicator relay (most of them) are sensitive to the current flow.
What happens when a bulb blows? Fast flash. Why? Because the current drawn is lower than with a non-blown bulb.
LEDs draw low current, therefore with a standard relay you get a fast flash. So, to bring the current flow up to "normal" an additional resistor, wired in parallel to the LED, is required...

At last, someone has explained it. The LED has a VERY high resistance when back-biased but also a high resistance when forward-biased. This resistance is generally 10 times higher than an incandescent bulb, so needs a ballast resistor to pull the current up.

Mainly because of the necessary current limiting series resistor.

As reverse bias will kill a bare LED unlike a conventional diode, such properties are academic. Most (but not all) LED units have additional diode protection to avoid reverse polarity damage.