Car alternator LED flicker

[andrewlapham]

Hi, I have put a string of 4x LED's in series around my key barrel in my car for some much-needed illumination.  basically, the LED's are connected in series with a 120ohm resistor to keep the current limited to around 20mA. this works fine but I've already had to replace one of the LEDs as it died. Every now and then a get some crazy flicker out of the LEDs and was wondering if you could suggest any simple circuits build-out of passive components I could use to stabilize the LED's a bit.
I realize the simple solution would be to only have them on while I'm starting the car at night but I really like the pink glow (They are pink LEDs) and would like them to stay on while I'm driving.
Cheers.

[Jwillis]

If they are like most other LEDs then 20mA is probably at their peak tolerance ,Try 15mA instead.They'll only be slightly dimmer with a lower current. Where are you sourcing the power from.That could be whats causing them to flicker.

[mcovington]

Remember also that the voltage in a car varies a lot -- it is not 12 volts -- it is as low as 11 with a weak battery, or as high as 15 charging.

[andrewlapham]

I've based the source voltage for around 14v as this is the voltage being pushed out by the alternator when the car is running. I have thought about reducing the current down to 10-15mA but still, I just think it needs a bit of smoothing aswell.

[JustMeHere]

Cars are very dirty electrical environments.   For example, you have "load dumps" from your alternator.  The starter will back EMF when it shuts down.

You need to have TVS.  Also,  it is a good idea to use a current regulator over just a simple resistor to run the LEDs.

I haven't done much research on car electrical systems, but I have read a post or two around here.  Think of your car's electrical system as a warzone.

[andrewlapham]

I totally agree with you. on other systems, I have integrated into the car I've used those step-up/step-down buck-boost converters from eBay to handle to vary in voltage and they work a treat. I know that the electrical environment in a car can be like you said a "war zone" I was just after any sort of circuit I can put in place to help drive the LEDs.  I have a parts bin full of components and love the DIY aspect of my projects, simply buying another pre-built buck/boost converter or an led driver board seems like cheating to me, haha. so back to my original question, is there anything anyone could suggest for a small circuit that could at least make these LEDs live happier? even if its just a little rather than just relying on the current limiting resistor?

[Jwillis]

You can run those LED's at a lower voltage as long as it's higher than the total forward voltage of your series LEDs. A simple lm7805 or lm7808 regulator circuit should do the trick.

[den]

Did actually you measure current through LEDs? The thing is - I am not getting 120 Ohms… According to AoE amber LEDs have a forward voltage of around 2 V. To be on a safe side let's assume supply is 15 V. Then you need to drop 7 V on you resistor to get in total 8 V on all your LEDs. For that you would need a R=U/I = 7V/0.02A = 350 Ohm resistor. I would assume this is why one LED already died.

Concerning flicker - maybe a simple capacitor will help? Does it flicker all the time (i think it souldn't) or only when consumers are switched on and off?

[tooki]

Pink LEDs are blue LEDs with a red-orange phosphor, so their Vf will be around 3.3V.

(Remember, pink is not a color found on the EM spectrum, thus by definition it has to be a mix of wavelengths, and thus (excepting RGB LEDs) must be a phosphor-based LED and thus usually blue-based.)

[mikerj]

The problem is that the combined voltage drop over four LEDs leaves very little to be dropped over the resistor, so the LED current changes considerably for small changes in battery voltage.  Re-arrange the LEDs as two sets of two series connected LEDs each with their own current limiting resistor (470 Ohms to give a bit of margin for voltage changes).

This will make the LED current much less sensitive to changes in battery voltage, at the expense of lower efficiency (i.e. higher power dissipation in the resistors) though this is wouldn't be a concern unless you want to keep them powered for long periods without the engine running.