LED Tail Light Bulbs

Slightly different question - does anybody do plug in replacement LED instrument lights? (would seem a very sensible place to put them, given the tedium of getting the dash off to replace bulbs)

cheers, clive

This sort of thing?

I'm not an electronics expert; how do LEDs react to an adjustable brightness control?

Yes, like that. Unfortunately not the sort I have, but a good try :-).

Good point. If it was just reducing the voltage, and the LED was a simple LED/resistor combo (which these probably are), then it would work fine. But if it worked by introducing resistance, and the current draw of the lamps had changed, it would all behave a bit differently. However it wouldn't fail like flourescents do.

cheers, clive

Bulbs are a comprimise between brightness for a given power consumption and longevity. I suspect dash bulbs are very long life models.

They're not legal. They don't give the same spread of light as tungsten so may be as bright square on, but not to the sides.

The legal verdict (as in "they aren't") or the aesthetic verdict ?

Why? A resistor in line with an LED is there to reduce the current, not the voltage. All you're doing in effect is adding another resistor in series.

We've had this discussion wrt getting 12v out of 24v, remember? A simple potential divider is sensitive to current drawn. If the dimmer is a potential divider driving a proper voltage regulator, then the LED thing will work fine - the led current will be roughly proportional to the voltage (give or take the approximately fixed voltage drop across the LED). If the dimmer is a simple resistor in series with the rest of the lamps, it will behave differently, and if it's a simple potential divider supplying the lamps, it'll be different again. Of course for the latter two cases, they'd also be sensitive to bulbs blowing.

cheers, clive

Ah...you're talking about a more complex system than I was envisaging. I'm seriously out of date with modern car systems.

(P.S. The lack of insult in a reply denotes a serious question, not a piss take)

TBH I don't know what arrangements are used for instrument light dimmers - it could be a simple variable resistor as you suggest in series with all the lights, with all the flaws which go with that. Changing the current drawn by the bulbs (ie the resistance) changes the behaviour of the dimmer - half brightness would move to a different position, and potentially it may not dim as much as before. Eg the resistor required for a 0.05 to 0.1A current range on 12V (assuming constant load resistance) is 0-120 ohms. Put in a

20mA LED, which has say a 500 ohm resistor to make it work off 12V (2v volt drop on the LED), and the existing dimmer resistor now only varies the current between 20mA and 16mA - ie the dimming is much less.

(Mine feels like it's a bit more sophisticated than that (ie it's not a wirewound pot) - but I don't know what that means in practice.)

I guessed :-) Apart from a mild pisstake at the beginning, I gave you a serious answer :-)

cheers, clive

It is on the Capper. On some of the trucks I drive, it's a button you press and release when the brightness is what you want, suggesting some more complicated electrickery. Same result but one justifies a higher wage for the designer.

I'm trying to change, honest.

Tee hee. I can see one advantage - only one digital input, the rest solid state and thus theoretically less likely to go wrong, mechanical bits being natural failure modes n'all. It might even be cheaper, especially if the electronics are built into something which is already there. But on the whole it does sound a bit like overkill :-)

cheers, clive

Both, really. I was actually looking at some which have LEDs on the back (same as those in the example), and clustered around the sides giving loads of light to the sides. Probably not still legal...

Asthetically? Yeah good point. I'm not into dodgy light mods etc., but I was wondering if these things had any value.

You can of course dim an LED by increasing the series resistance. The problem is the value of the pot for the original bulbs is too low for LEDs. If you change all the panel lamps for LEDs you might be able to find a suitable value one which will fit, or use the original and add some electronics to get the correct law.

I've been playing with those too and they again don't give the same level of even light.

You can buy a lot of ordinary E marked bulbs for the cost of just one of these.

LEDs as you probably know are very bright and so would help when braking, in theory.......

They are also directional. Meaning that the light emited will be pointed straight out of the bulb and very little out of the sides. On a normal bulb, light emited from the sides would reflect on the lamp-holder and iluminate the whole of that segment of the lamp-holder. I dont think that this would happen with an LED brake light. It would be very bright though.

Also, if the bulb is not pointed straight back, or people werent looking straight at the bulb then the effect would be lost. Steve

I don't think the brightness of LEDs over incandescent bulbs or otherwise would make any discernable difference to a car's braking whatsoever.

Quite the reverse: incandescent take more current therefore more from the alternator which will aid braking.

;-)