Can LEDs be dimmed ?

cm64 · Apr 10, 2008

Not quite a flashlight question, more LED electronics but I'm not sure where to ask ! If there is a better forum can a Mod. please move.
I want to replace the courtesy lights in a car that holds the lights on for a period after the door has been closed & then dims the light instead of just switching then off.
What will happen if I replace the festoon globes with a LED ?
I guess I could just try it but I don't really want to buy the globes & then find they don't work properly.

Jarl · Apr 10, 2008

Depends on a lot of things. Are you putting in an aftermarket LED module, or just poking in some LED's and hoping for the best?

If it's the latter, it won't work. Chances are that it's running on 12V (ish) off the car battery, which would destroy most LED's. Then it's probably PWM dimming (wiki it if you don't understand) used to dim the globes- this is fine with a filament, because of the short response time, but if you use an LED the PWM might be visually detectable, so you'd have flickering lights as they dim.

ColorMaker · Apr 10, 2008

Jarl said:
Depends on a lot of things. Are you putting in an aftermarket LED module, or just poking in some LED's and hoping for the best?

If it's the latter, it won't work. Chances are that it's running on 12V (ish) off the car battery, which would destroy most LED's. Then it's probably PWM dimming (wiki it if you don't understand) used to dim the globes- this is fine with a filament, because of the short response time, but if you use an LED the PWM might be visually detectable, so you'd have flickering lights as they dim.

It is possible to use LED's in place of the filiment bulb. Series 3 LED's and use a load resistor. If you know the Vf (foward voltage) and you know the AMPS you can use this formula
(Vf x 3) -12.5 * .022 = resistor value

IE
(2.3 x 3) -12.5 * .022 = 151 ohm

(Vf x 3) is the total vlotage drop over 3 LED's
-12.5 is the battery voltage
*.022 is 20ma Multiply by desired current to get resistor value.

You will need to know the wattage so you dont burn out your resistor

Raymond · Apr 10, 2008

something seems off in that formula. It appears that when you want a higher current, you'd need to put in a higher value resistor.

Marduke · Apr 10, 2008

Depending on the bulb, there are a number of aftermarket LED modules available which are meant for use in cars, and that run on 12v. If you care to take a picture of the bulb, there is a good chance there is already a direct replacement for it.

TigerhawkT3 · Apr 10, 2008

ColorMaker said:
It is possible to use LED's in place of the filiment bulb. Series 3 LED's and use a load resistor. If you know the Vf (foward voltage) and you know the AMPS you can use this formula
(Vf x 3) -12.5 * .022 = resistor value

IE
(2.3 x 3) -12.5 * .022 = 151 ohm

(Vf x 3) is the total vlotage drop over 3 LED's
-12.5 is the battery voltage
*.022 is 20ma Multiply by desired current to get resistor value.

You will need to know the wattage so you dont burn out your resistor

First, I don't know of any LEDs that will make much useful light at 2.3V. At least 2.4V-2.5V is probably more like it, and if you want 20mA from a 5mm, you'll need at least 3.2V or so. If you're using power LEDs (Crees, etc.), 3V-3.3V would be reasonably bright, with 2.5V-2.7V or so being a little more subtle. Note that these are just voltages for particular currents.

Next, V=IR, or R=V/I. What you've got is VR=I, or R=I/V. You'll notice that that's inverted.

To get what resistance you need: (voltage you need to drop) / (current the emitters will pull) = resistor value. To find the wattage for the resistor, it's (voltage the resistor is dropping) * (current the emitters will pull).

I believe that the dome light would dim down by simply ramping down the voltage, instead of by PWM. If that's the case, LEDs would work fine.

If you want, you can also just replace each 6V bulb with two Crees/Seouls in series and each 12V bulb with four Crees/Seouls in series. Make sure you give them a reasonable heatsink.

cm64 · Apr 11, 2008

These are the globes I plan on using. They are a direct replacement for the incandescent globes, from the feedback they seem very good. They must have the load resistor built into the bulb.
http://www.dealextreme.com/details.dx/sku.9092

B@rt · Apr 11, 2008

Yes, you can replace them directly without harming them, but as said before, they might strobe a bit during the dimming phase.

I barely know anything about electronics, but perhaps a small electrolythic capacitor might smooth it out. :thinking: :shrug: I'm sure someone who actually knows what he is talking about will respond.

ifor powell · Apr 11, 2008

I have put somthing similar in my camper van which has a dimming feature for the cutracy lights. For the ones I have the dimming dose not work, as it gose to dimm I get an low volume audible whistling from the led module and at some point during the dimm it switches off. I have had them in for 6 mounths and they are still working so I don't thjink I am doing any permanent harm.

Ifor

cm64 · Apr 11, 2008

Thanks for all the replies.
I will go ahead & get the LED festoon globes & try them. I guess that the globes have a simple resistor to drop the +12v car voltage to around 3v for the LED. I doubt that there is any sort of driver circuit in the globes. The dimmer circuit holds the voltage up for 10-40 sec after the car door is closed & then fades away instead of just switching off. I think the circuit will work with the LED but there will be no gradual fade as with a normal globe. As the voltage drops down from 12v, when it reaches 3v the LED will switch off with no dimming, which is still OK.
I will try report back when I have installed them.

TigerhawkT3 · Apr 11, 2008

cm64 said:
Thanks for all the replies.
I will go ahead & get the LED festoon globes & try them. I guess that the globes have a simple resistor to drop the +12v car voltage to around 3v for the LED. I doubt that there is any sort of driver circuit in the globes. The dimmer circuit holds the voltage up for 10-40 sec after the car door is closed & then fades away instead of just switching off. I think the circuit will work with the LED but there will be no gradual fade as with a normal globe. As the voltage drops down from 12v, when it reaches 3v the LED will switch off with no dimming, which is still OK.
I will try report back when I have installed them.

Nope. The LEDs won't just "switch off" below 3V. They'll get progressively dimmer. The operating voltage for an LED is not a fixed value, applicable for all currents, but rather a voltage range that corresponds to a current range. Cree XR-Es, for example, need around 2.4V for 50mA or less, 3V for 150mA, a little under 3.2V for 350mA, and 3.5V or so for 1.2A.

ColorMaker · Apr 11, 2008

TigerhawkT3 said:
First, I don't know of any LEDs that will make much useful light at 2.3V. At least 2.4V-2.5V is probably more like it, and if you want 20mA from a 5mm, you'll need at least 3.2V or so. If you're using power LEDs (Crees, etc.), 3V-3.3V would be reasonably bright, with 2.5V-2.7V or so being a little more subtle. Note that these are just voltages for particular currents.

Next, V=IR, or R=V/I. What you've got is VR=I, or R=I/V. You'll notice that that's inverted.

To get what resistance you need: (voltage you need to drop) / (current the emitters will pull) = resistor value. To find the wattage for the resistor, it's (voltage the resistor is dropping) * (current the emitters will pull).

I believe that the dome light would dim down by simply ramping down the voltage, instead of by PWM. If that's the case, LEDs would work fine.

If you want, you can also just replace each 6V bulb with two Crees/Seouls in series and each 12V bulb with four Crees/Seouls in series. Make sure you give them a reasonable heatsink.

ERROR
(Vf * ) -13.5 / .022
Divide the votlage by the desired current to get the resister

cm64 · Apr 12, 2008

Thanks TigerhawkT3, thats the info I needed. Seems like it will work as with a incandesant globe.

TigerhawkT3 · Apr 12, 2008

ColorMaker said:
ERROR
(Vf * ) -13.5 / .022
Divide the votlage by the desired current to get the resister

Yes, that's what I said.

Glad I could help, cm64. :thumbsup:

TorchBoy · Apr 12, 2008

ColorMaker said:
ERROR
(Vf * ) -13.5 / .022

Yes, looks like an error to me. I'd believe this:

Code:

( 13.5 - Vf * 3 ) / 0.022

cm64 · Apr 13, 2008

Oops, forgot to mention that the courtesy light dimmer that i have installed in the car is this one ...
http://www.jaycar.com.au/productView.asp?ID=KC5392

cm64 · Apr 27, 2008

Replacement LED festoon globes installed & the dimming circuit works very nicely. Dome light stays on after the door is closed & the after 12 sec (adjustable) it dims to off.

Can LEDs be dimmed ?

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