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

Thank you Richard. I started to provide the answer yesterday, but got sidetracked.
Happy days Bob!

Just to clear it up completely, I take it that the LEDs are driven back-biased, then?

Not sure if this is the correct thread to attach to but all this discussion about LED lights has brought a thought to mind.
There has been concern about the visibility of our low cars and I was wondering whether this could be offset a little by attaching rear cycle lights to the top corners of the luggage rack for temporary night time use. Cycle lights are now bright, reliable and long lasting.
I can't find anything in the highway code other than that one shouldn't keep a brake pedal depressed when stationary in traffic although there must surely be some legal guidance, and indeed restrictions, elsewhere as to positioning and use of such devices The advantage of a cycle light to me is that it would be easily removable when parked up.
Just a thought and may turn out to be a silly one.

If you use two of them, someone might think that two bicycles were riding abreast and get angry!

That's true Bob - hadn't thought of that!
Was only thinking of steady mode rather than flashing mode though.

We surely don't want cyclists flashing?

Forward Bob.
If fitted the other way, no current flows through the LED.
It only takes a low voltage to drive current through a diode, so for use at a nominal 12V they need a load resistor (in series) to drop most of the 12V supply (and reduce the currentso the diode does not burn out).
The parallel bypass resistor is so that sufficient current flows for the flasher relay to see a standard incandescent bulb type load. If not, think about when a bulb fails in your indicator circuit - you get a rapid flash (which many fail to notice!). Same effect if you were to fit LEDs with no ballast (assuming you don't have one of those smart flasher relays that will drive either bulb or LED).

Rob, you'd have to go through the Construction and Use regs - light positioning...

So they have a high resistance in the forward bias?

No.
Highest resistance when reverse biased - no current flow.
Forward biased, low resistance, current flows, just like an ordinary diode. But these are small devices, the current flow (power handling capability) is low (unlike big power devices which can handle high current levels).

This is what I can't get my head around. If it is low resistance in use, why use a parallel resistor? (Back to the 1/R scenario)