Citroen XM Forum

Dimming LEDs

Saw this (enormous) thread today, and was aware there had been a good bit of discussion about LEDs as dash-illumination, and as "long life" substitutes for small tungsten lamps.

Several things to note about LEDs:

1. To the circuit in which they are placed, they look just like a Diode (i.e. they will conduct in one direction only).
2. Their tolerance to reverse current (i.e. being connected the wrong way round) is limited - they will blow.
3. They will emit their maximum designed light output at a specified forward current (If, or Ifwd), which is often around 10-30mA for single 'indicator' LEDs.
4. It is this forward current which is critical. If the LED is spec'd as needing 20mA, that is the maximum that should flow through it, and a series resistor should be provided which will allow just this current to flow. So, for a 12v supply, the series resistor would be (by Ohm's Law), 12/0.020 = 600ohm.
5. The forward resistance of an LED is effectively Zero, so it should never be directly connected to a supply without a series resistor. If you do directly connect it, even to just 5v, it will blow. (Five divided by Zero = Infinity!!)
6. The supply voltage for an LED doesn't matter - it can be 5v, 12v, 15v, 24v, 500v... What does matter is the forward current. Get this right.
7. LEDs can be connected together in series without a problem, like old-style christmas tree lights. Their total "chain" resistance is still almost Zero, so the same rule applies: you need ONE series resistor for the lot, at the correct value. (Of course, if one LED fails, they all go out.)
8. You can wire up LEDs in parallel, but it is usually recommended to limit the number to about five, with one current-limiting resistance for the group.

Dimming:

The output (brightness) of an LED across its designed output range is pretty linear, but LEDs which are to be dimmed require very tightly controlled (stable) circuitry and supply voltages for them to work acceptably - which is why cheap dimming units may prove unsuccessful.
Two approaches:
- Analogue (variable, but stable, current supply)
- PWM (variable pulse-width modulation at a frequency the eye will not see as flicker)
More info here:
http://www.digikey.com/us/en/techzone/l ... n-led.html

For dash-illumination purposes (maybe 20+ LEDs) it is possible to dim the LEDs simply by reducing the current flowing through them, but
(a) it won't be possible to do this with the existing rheostat (for controlling a much higher current tungsten array) - one of dedicated value, or a variable-output DC supply pcb will be needed (together with series resistors for each of/groups of the LEDs), and
(b) although the light output can be brought down from maximum to something lower by this means, the behaviour of the individual LEDs will become a bit erratic as their light output becomes very low. They won't dim to black smoothly; for this you need PWM, where the LED is operated at 'design' light output, but only for very brief periods, which the eye sees as 'dim'.
(c) There is obviously an issue about the direction of light output from an LED. Most of them project light forward (180deg cone), while dash illumination often requires omni-directional/side-output lighting (eg behind a display panel, or between a pair of instruments). Choice of LED design will be important, or additional LEDs may be necessary. An out-of-car in-the-dark bench test will be essential!

Sorry if you knew all this before!

Chris

See also:
http://led.linear1.org/category/led-basics/ - What's possible
http://led.linear1.org/led.wiz - LED arrays and circuits
http://www.rapidonline.com/Electronic-C ... cd-55-2228 - Example of a "warm white" LED; plenty more

mds141 wrote:Is the membrane still intact behind the door card?

Got it in one I think Mark. Water is seeping in from behind the leather on the door. It collects on top of the door bin. New one on me. I just washed it with a hose, so it isn't raining in, but it is getting in. I wonder if it is getting in around the base of the window. My trims have shrunk, so might have something to do with it. Anyway...door card off at some point.
Thanks for your suggestions chaps.


Very interesting Chris about you LED info. For me anyway, I am going to stick to conventional bulbs for the dash display. But I am very happy with the LEDS for the window and seat switches. Best thing is they aren't that much brighter, so don't dazzle, but hopefully will last longer as you can't dim them.

I decided to investigate this curious leak today. I have noticed water collecting on the door bin..coming from under the leather padding. So for the first time for me...not the car... The passenger door card came off. Only casualties were one tab which had been glued back before and the speaker connection disintegrated!

Pics below to what I found. Unfortunately I haven't the foggiest how water is getting in. I can only guess it's via the weather trims on the window, and it is trickling down inside the door and then getting through a gap somewhere. The sticky plastic and foam are firmly fixed to the door, yet as the photos show, there is a drip.
Anyway halfway through, it decided to thunder and hail, so I was trapped in the car. But watched the area that is getting wet to see what happens. It did get a little damp, but not soaking. So to try and stop it from getting into the car, I simply stuck a line of duck tape so the water cannot get in or at least rot the leather or door card. Only time will tell to see if it has worked.

Another thing that was bugging me, ever since I got the car, the release handle trim never fitted flush like the drivers side. It was always half hanging off. So I had a go at fixing that. Oddly there were no broken tabs or clips, just over padded poorly trimmed leather. So I doctored it and hopefully it will all fit back nicely. Gave the leather and wood a a clean and polish up too.
Fixed the speaker and reglued the tab. Reassembly tomorrow. ...hopefully....if it doesn't thunder again!

Also, this morning I had my first petrol station recognition.

" is that your car...don't see many of those now a days"

That water will be coming in past the window guide seal for the fixed quarter glass.

It could be, what's interesting is it just appears in the foam by that bracket for the door handle. It is not running down from the window under the plastic/foam wrap stuff. That is welded to the door! It must be tracking down inside the door. I sat in it watching it like a hawk today in a thunderstorm. Nothing really to report. Foam got a little wet. That's all. ERGH, hate water, gets everywhere.

Am I the only one with a watertight XM?

Yes Mark. You are.

I am getting there...but as with anything, as soon as one is fixed it springs another!

Russ,
While you've got the door trim off, might be worth rigging up a garden sprinkler, and sitting in the car to see what happens. Then again, with current UK weather, any day would do...
Also check the main door-surround seal?
The door/window exterior lip seals do get a bit hard over the years, but even so, water leaking past one of these would, I'd have thought, simply find its way inside the door, not inside the car. Quarter-light could well be the problem.

Don't like the sound of that "watertight" XM...

Oooooer. Don't fancy a fish tank as an interior Chris.

It's not the door trims, or seal. It has to be the weather strip at the base of the windows at fault. Where the drip mark starts on the foam, is where the damp appears. No other signs of it dripping onto it. It definitely is getting in from inside the door cavity, behind the foam. Most odd. Although it looks like there is a cut line around that shape in the foam, the plastic adhesive stuff is firmly stuck to the door with out any tears.

No need for sprinklers tomorrow, the natural one will be on most of the day...