# Dimmer and LED Help Request

Discussion in 'LEDs and Optoelectronics' started by sall, Jan 29, 2010.

1. ### sall

51
0
Jan 29, 2010
Hello,

New here. Nice community you all have! Hoping you can help me out with a project.

I have an HVAC circuit board for my vehicle. It has several 5 and 3mm filament twist-lock style bulbs. I would like to convert these filament bulbs to LEDs. I need help figuring out how this is going to work. The bulbs are dimmable via potentiometer on the dash.

I already know how I am going to get the leds on the board. This example is ugly but will work, mine will not be as haggard.

Here is a picture of the HVAC board:

Here is the back side with a closer look of the twist-lock sockets. (bulbs removed)

Here is a diagram of the dimmer in pdf form.

Dimmer Control

Instrument Cluster Illumination lamps

So, my question is what value resistor do I need for each LED. Also, how well will this work with the factory dimmer? The dimmer is what throws me off otherwise I could easily decide what value resistor with a simple equation.

BTW the leds to replace the filaments are rated forward voltage: 3.2-3.6v @ 20mA.

Thanks for reading and I hope to hear any and all suggestions.

Sall

2. ### Resqueline

2,848
2
Jul 31, 2009
Welcome to our place Sall!
Yes, it's turning out quite nice here, thank you.
The dimmer modules seems to simply be emitter-follower buffers, putting out a non load sensitive 6-14V.
So you just calculate the resistors for up to 25-30mA at 14V. Keep the resistors away from the LED's btw..
If you manage to put two LED's in series in each bulb holder you'll have the effect that the dimmer can dim them down to zero light.

Last edited: Jan 29, 2010
3. ### sall

51
0
Jan 29, 2010
Thank you very much for quick reply.

I suppose I may have looked to much into it and it was just the simple equation.

R=(14.4v-3.6v) / .02

I have a very accurate voltage meter as part of the driver information center that tells me the status of the charging system. Optimal range for my vehicle is 14.9v. I do however see 15.3 at times. Don't worry this normal, I am an active member at ACNA. Which is forum dedicated specifically to Oldsmobile Auroras.

So, I take it I need to use the 15.3v as my base.

R = (15.3v-3.6v) / .02

R = 585

Round up to common value of 685R and 1/2w. Would this be correct? Or do you think going with 15.3v is overkill?

Also, I like the idea of two LEDs in series unfortunately there is not enough space. let's just there was though. How would I determine what resistor to use for 2 in a series per bulb base? I may be able to figure this out space wise if this will achieve a greater range of dimness/brightness.

Also, the LEDs that are factory on the board (can see pictured in first photo of the board). I would to change them as well. Would I be right in assuming I can just remove them and add a 5mm LED in its place. I believe they receive 5v at all times. Each LED has a SMD resistor valued at 147 ohms. This is why I believe they only see 5v. Upon looking at the diagrams if my 5v statement is wrong, please explain. If I am right the 147 ohm is actually overkill but these lights stare me right in the face basically so too bright would be bad thing.

TIA

Last edited: Jan 29, 2010
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,482
2,830
Jan 21, 2010
I'd use 560 ohms -- closer to what you require and the LEDs probably have a max forward current > 20 mA anyway.

The resistors will dissipate 0.5W max, so you really need a 1W resistor.

As for replacing the existing LEDs, you *should* be able to replace a blue LED with a white one without any problems. However, if these are a higher current LED, you might have issues (or the replacement LED might).

Another issue with replacing lamps with LEDs is that the LEDs will probably be focussed into a narrow beam whereas the existing lamps probably are not. For a similar purpose to yours I once solved that issue by roughing up the surface of the LED with some fine sandpaper.

5. ### Resqueline

2,848
2
Jul 31, 2009
15.3V is not more than it needs to be in very cold climates, but it's a Volt & a half too much at summer temps - giving the battery a hard time then.

I second all what steve says, but observe that modern "1/4W" size metal film resistors actually handles 0.7W.

The 20mA spec on the LED's is supposed to give them something like 100000 hours lifetime. A current of about 100mA (like some cheap flashlights use) will reduce the lifetime to a few hundred hours. A car will only last for something like a couple of thousand hours so you don't need to be afraid to let your LED's see 30mA (plus you have the dimmer).
The dimmer module outputs also always loose 0.6V of the battery voltage.

Series LED's: 14.7V-(3.6V*2)/0.02A=375 Ohms. Use 360 Ohms.
You can file or grind the body of LED's to half it's original width before getting close to the internals, if that helps fitting them.

It seems sensible that the original (red?) LED's are running on 20mA from 5V. Try to estimate their beamspread & brightness.

6. ### sall

51
0
Jan 29, 2010
Nice. You guys are great! Thanks for the input thus far.

The original 5mm LEDs are green. In the mid 90's I hear white was too expensive to implement (atleast for GM). Could be wrong on that. I will be replacing the green with blue to match the rest of my lighting.

Someone on the Aurora forum sent their board and twist-lock bulbs away to be replaced with LEDs. Since then their car has been totaled and can provide no info to me. Basically I am going for a look like this:

They seem like they are very bright. Which is something I do not want to distract me. Might just be the camera because I too have hard time getting my cheapo camera to focus correctly on LED lighting.

Last edited: Jan 29, 2010
7. ### Resqueline

2,848
2
Jul 31, 2009
Modern LED's are very bright compared to the 90's so you may want to play with the resistor values to get down to a tolerable level.
Yes, they were hideously expensive (>\$5 each?) and weak. I don't remember exactly when they were introduced. Most "blues" were also more cyan in colour.
It's not only cameras that have a hard time focusing on blue, your eyes have a hard time with it too.
While it looks cool it's not exactly what the army would choose for instrument lighting..
Oh, I have an old Olds' too, but it's a worn & tattered Custom Cruiser from the 80's.

Last edited: Jan 29, 2010
8. ### sall

51
0
Jan 29, 2010
Nice. I like to hear about other Olds still on the road. I can't understand why GM dropped Pontiac now too. I guess they learned nothing from losing loyal Oldsmobile owners.

Any way, we are talking about LEDs, right?

In reference to the factory LEDs... would I not be under powering the replacement blue LEDs with the 470R SMD resistor?

R = (5V-3.6V) / .02A

R = 1.4V / .02A

R = 70

Which would call for an 82R 1/8W resistor. If so, the 147R should cut down on the brightness of LED significantly, correct? I definitely do not want them ultra bright or "super bright" as some suppliers would say. These HVAC LEDs are in my peripheral vision when driving and from what I understand peripheral vision plays a huge role in night vision.

If it does come down to replacing the factory SMD 147R resistors, would you all recommend replacing the SMD resistors with a higher value or adding a resistor to the LED itself?

I do not foresee any problems with the 147R value but I may be wrong in my assumption.

I also plan on using a pot w/ lawn tractor battery which holds a charge of right around 14.4 to test with. This way I can see exactly what resistor value to use that agrees with my eyes. Sound feasible?

Keep 'em coming!

Last edited by a moderator: Jan 29, 2010
9. ### Resqueline

2,848
2
Jul 31, 2009
They've done lots of mistakes in the past, and obviously continue to do so.. Yes, lets not venture there..

The blue LED's will run on ~10mA with the 147 ohm in place so they'll be half as bright as spec'd.
I'd suggest replacing the SMD's if it comes to that - after determining a suitable value, but real cheap LED's might also be weak enough to do the trick.

Testing he LED's (out of the car &) before exposing the board to the stresses of soldering is sound thinking.

10. ### sall

51
0
Jan 29, 2010
I probed one of the twist-lock style bulbs today just out of curiosity to see what the voltage would be with engine running. With charging system at 15.4v at full brightness the twist-lock bulb was seeing 13.3v.

This max voltage by math agrees with what you all have suggested. Which is 560R.Although, I come up with needing a 560R 1/2w.

Just want to make sure before I order the resistors. Anyways, I was thinking of ordering the 1w resistors anyways. Here are the specs of the resistors in mind. 1W Metal Film Resistor 560R 1%. Are these alright?

TIA

Last edited: Jan 31, 2010
11. ### Resqueline

2,848
2
Jul 31, 2009
Loosing 2.1V indicates that there's a Darlington transistor in the dimmer module plus some wiring drop (to be expected).

Yes, using twice the wattage will reduce the temperatures and ensure a long trouble-free life.

12. ### sall

51
0
Jan 29, 2010
Thanks, resistors have been ordered.

I believe I have gathered all the information I need to complete this project. I appreciate all the help I have received. As well as the warm welcome. Like I said a nice community you all have here. Granted you guys have way more knowledge in these topic than do I, I will still troll forum and expand my learning curve. Believe me I will post more questions!

I come across a lot of projects and mods vehicle related. I think of the next one before the current one is complete haha. Next project is actually retrofitting the factory steering wheel controls and the remote for my after market head unit. This way I can use factory steering wheel controls for my after market head unit. Without buying an expensive after market adapter. Oh, it's possible! We'll save that for another thread though!

I will post the results of the current project and pictures when completed. I have about a week to wait before all my materials are received.

Best Regards,

Sall

13. ### sall

51
0
Jan 29, 2010
Hey guys, just wanted to check in and let you all know how the project went.

It went great! I still cannot seam to grasp taking pictures of LEDs. Picture did not turn out that bad.

I ended up grinding each 5mm LED down to a flat surface to extend the viewing range and prevent hot spots. I also used the factory surface mount resistors. I retro fitted resistors to the factory twist-lock LEDs as well for various bulbs around the interior/doors.

Once again, thanks for the help! Let me know what you think!

14. ### Resqueline

2,848
2
Jul 31, 2009
Congratulations on a job well done and thanks for reporting back, that looks great!