24V LED dash lights!!!

[_Walker_]

Solution to the 24volt bulb question!

Ok folks I JUST finished replacing all the burned out internal lighting in my 24V BJ74 with LEDs.
How you ask? Where on earth did I find 24V Wedge , and festoon LEDs?
Well the answer is I didn’t I had been searching high and low for replacement bulbs for the dash lights, heating/AC panel etc… all with VERY little luck. I thought HEY why not go for LEDs, thell last forever and they pull less power, but alas no such thing has been built commercially.
BUT don’t fret I found a solution!!!

Resistors! Now im sure any electronics guy here already figured this one out some time ago, but I recently had the brain flash, tried it out and Presto chango they work like a hot damm,

You will need the following….

(V1-V2)/Current=size of resistor needed

V1 is the system voltage so =24V
V2 is the voltage you want to have so=12V
Current is the current draw from EACH bulb you can figure this out with a multi meter or it may even be published on the website of wherever you buy your bulbs. The site I ended up using has them all published. SUPER BRIGHT LEDS home

From the website I find that mini wedge bulb I ordered to replace the burned out 24 incandescent in the heater/ac panel has a current draw of 12ma =.012A. So the formula should look like this:
(24-12)/.012=1000
So a resistor (or series of resistors) totaling 1000 ohms is what you need to solder/shrink-wrap into the circuit

Anywho just incase someone was looking for an alternative…







EDIT:
Adding Nov 4, 2008
I was starting to run into some troubles over the summer with this installation and found a more reliable method to get the 12V bulbs to work properly....
Go to your local electronics store and buy two types of voltage regulators
a 7812CT, and a 7818CT these things are cheep (less than a buck each where I got them)

you can get data sheets on them here:
L7812CT Datasheet - 500mA positive voltage regulator, 12V from SGS-Thomson Microelectronics

What you want to to is solder the output of the 18CT to the input of the 12CT, and connect both of the grounds to each other+ to a ground on the truck.
Then supply the 18CT input with 24V, and presto you have a regulated 12V off of the output from the 12CT, just be sure your not using standard bulbs because I think this setup is only good for 1 amp, but problem is now solved!!

 

[lumpy70]

Cool. What color of LED's did you use?

Also, can you post a picture of what your soldered resistor looks like in line with the new LED?

Craig.

 

[Tick]

Cool. What color of LED's did you use?

Also, can you post a picture of what your soldered resistor looks like in line with the new LED?

Craig.

x2 please.

 

[_Walker_]

Sorry for the late response folks…
Unfortunately iv already shrink-wrapped everything up so baring tearing it all down again and doing the process a second time were out of luck for pics of things in progress. However Ill explain it as best I can, if people are really snickered I guess I could pull it apart and redo it all. But lets see if this helps first!


The trick here with resistors is they only work to step down voltage when the current is constant, RE why we cant use this same trick to run a radio for example. However a constant draw from say a light… is no problem.


So basically the first thing to do is figure out what bulbs you want to replace. Say for example the instrument backlights. On the 89 BJ74 there are 4 back lights, they are wedge style that plug into twist lock clips. Id suggest first taking apart the dash, then removing the instrument cluster to figure out just what bulbs your going to need. Establish the right size shape etc your going to need, then go online and order the replacement 12V versions, just be sure to get the size/shape/color for what your looking for. –reassemble

Once things arrive your going to have to figure out what size resistors your going to need. The first step in doing this is establishing how many amps your new LEDs are going to draw. This info can in some cases be found from the outfit you bought the lights from (at least from the outfit I got mine.) Conversely if you have a multi meter you can also get it yourself.
After you have the number, lets say for example it was 17 milliamps you can carry on to the next step. If (as in the case above) there are multiple lights on the same circuit, take the 17 and multiply it by the number of bulbs you are installing to get the TOTAL current for all you new LEDs. So in a 4 light system @ 17 ma we get (17x4=68ma) OR 0.068 amps (divide milliamps by 1000 to get amps.)
Then enter that number into the formula from my first post:
(V1-V2)/Current=size of resistor needed

(24v-12v)/0.068amps=176ohms

now get your self to an electrical store. Your going to have a hard time finding a 176ohm resistor so just get a 180ohm job, it doesn’t have to be precise, a different size will yield a different voltage but most LED automotive bulbs are happy anywhere from 8-16V. So basically just get something close.
Your also going to need a soldering iron, and I would recommend shrinkwrap.

Get home and pull the cluster again, but keep it in the truck just flip it over so you can keep the power line plugged in to help find the right wires. You now need to find the POSITIVE line that powers the lights. At least in the 89 you can visually follow the copper tape from the bulbs back to the plug. Use your millimeter to figure out which circuit, and thus line is positive feed for the lights. Turn the lights on and off to make sure you have the right line. After making sure you’v found the right line, follow it back a ways up the cable bundle so you have room to work. I had to pull the tape off the bundle to make it easy to work, (don’t worry just re tape it when your done.) Then……. Cut it. Make sure that was the right line (by noticing that the dash lights no longer illuminate. Then slip the shrink wrap over the line, solder the resistor into place, slide the shrink-wrap over the resistor and heat it up with a lighter etc.

If you then power the system on (with no bulbs installed you will get a reading of 24V…. BECAUSE there is no load on the circuit. If you were to install one of the bulbs you would probably see something like 21V just before that bulb blew. If you added two bulbs you would see 18, etc. etc. etc. so point is plug all the bulbs in first. (often LEDs if they don’t light up at first flip their polarity they MUST be installed the right way or they don’t light.
To do this just turn the lights on and off fast if you see a bulb didn’t light up, flip it and try again, just trying to limit the amount of time the bulbs are running if they are not all installed correctly (re you could blow them if say 3 of the 4 were backwards and you pumped 21V through the single working bulb. After it seems they are all running when you flip the power on and off. Then hook up the multi meter and get the voltage on the circuit, if its somewhere around the 11v,12v,13v your in good shape!

And that’s about it!
PS: you can add resistors together in series IE one after the other after the other, this essentially adds their Ohms ratings together. So 3x180 Ohm resistors all soldered IN LINE would be (180x3=540)

Anywho hope that helps some!



PS those lights are cool white....
http://www.superbrightleds.com/specs/bulb_specs.htm
http://www.superbrightleds.com/cgi-bin/store/commerce.cgi?product=CAR


Also the first style on this page are STELLER for a new dome bulb. on the 89 BJ74 its a 37mmx10mm... get the 6 emiter version you will NOT be disappointed (they work from 8V-30V so just plug and play for 24v & 12V people.
Just one word of caution if your going to do battery work turn the dome to off first. The other day I was reattaching my batts and when I was putting the cable back on I jiggled it a bit so anything trying to get power basically was being intermittently turned on and off really fast.... and apperentally the circuitry didn't like that... so waiting for my replacement at the moment, but REALLLLLLLLLLY worth it.
http://www.superbrightleds.com/festoon.html

 

[daboheme]

You're dropping 12V over that 180R resistor, therefore the power dissipated by the resistor is V^2/R = 0.8Watt if one resistor is used for all four LEDs. The resistor must be able to dissipate that heat. Just want to make sure...