LED Project in Vehicle: Blown Bulbs & Schematic Questions

[vman1]

Hello all...I have a led project I have been doing in my vehicle...I have a Bose/Clarion radio that has blown incandescent bulbs. I have already done the instrument cluster & climate control with drop-in led's so I decided to try & fix the radio. My problem is that the radio has soldered in incandescent bulbs. I have taken apart the unit and determined(with the help of a schematic) that the unit does have power to those blown bulbs. I would like to solder in led's in those spots if possible...according to the schematic there is 12.6 volts applied to 5 incandescent bulbs. I was thinking that I would be able to solder in 5 led's each with their own resistors(1/4 watt 560 ohm)...Then I thought maybe since they will all be tied in together that if I could find led's rated at 2.5 V then I would not need any resistors at all...Am I processeing this correctly? I don't want to overload the circuit board with led's/resistors so any help would be appreciated!

 

[vk6kro]

LEDs have a small range of voltage where they operate correctly. Below that voltage range, they don't light up at all. Above it, they can take enough current to destroy themselves.

So, they always need a series resistor (or some other current regulator) to limit the current. Putting them in series across a 12 volt source would not be a good idea unless a series resistor was included.

LEDs are more directional than incandescent lamps, so you have to consider how you would mount the LEDs to get the same effect as the lamps. If they just give an indication that something is working, then they are a good substitute as they use a lot less current than a lamp.
If they have to light up the dial of a radio, they may not be able to do it because they are too directional.

Different colour LEDs have different voltages.
The following site gives information about LEDs including these voltages.
http://en.wikipedia.org/wiki/LED

 

[vman1]

vk6kro, Thank you for the reply...I know led's are polarized & I have set up a 12 v power source for that circuit board. I soldered in lead wires that I could just touch with the led to determine the positive flow of voltage(Trust me I have been researching if there is a way to determine (+) flow just by looking at the CB...I could not find the answer!) and after I determine what is positive I will then solder in place. My main question is ...since all the old incandescent bulbs are driven by the 12.6 volts(I assume that means they are in series already) should I wire in an individual resistor with each led or should I find led's with a 2.5 V max(2.5 v X 5 = 12.5 Volts) without resistors. If I use individual resistors with each led will I draw to much current and overload the circuit board. Since I have rigged up a 12 V power source for that board it is easy for me to test. Any thoughts?

 

[vk6kro]

The old lamps would be 12 volt types and they would be wired in parallel.

Your LEDs would take a lot less current than the incandescents, so there is no problem with using them, but they do need resistors. Resistors are very cheap, so just get a packet of them.

Yes, get the polarity right. LEDs have a low reverse breakdown voltage and they can be destroyed by too much reverse voltage.

 

[vman1]

again thank you...If the old incandescents are wired in parallel and I will be using those same spots on the CB: Should I use a resistor with each led in each spot or just one resistor to cover them all?

 

[vk6kro]

You need to give each LED a resistor and then put the LED/resistor combinations in parallel.

As I said, resistors are cheap. However, if you like, you can put two or three LEDs in series and give the combination just one resistor.

If you do this, the resistor values have to be recalculated.

Incidentally, I used the word "directional" above to refer to the pattern of light coming from the LED, not that the LED was a diode and conducted in one direction and not the other. You have to consider the pattern of light coming from the LED to see if it is suitable as a substitute for a lamp.

 

[vman1]

ok since the old bulbs are wired in parallel I will just put an led with a 1/4 watt 560 ohm resistor in each spot that had an incandescent bulb. The "directional" pattern of my led's will be diffused by a clear plastic piece that is part of the radio faceplate. My Led's are rated @ 3.6 V 20 Ma with a millicandella of 2000Mcd & 1 lumen. I realize it has a 30 degree beam but I know that it will be better than 5 blown bulbs. I hope the clear diffuser will disperse the light without a huge hotspot. Anyway thank you for the responses as I will try today to install one led first and test with my power source. I will post pictures when the job is complete.

 

[vman1]

The led project is nearly complete...On the radio there are a few hotspots but I will be putting all the pieces back together tomorrow for a final look. I will try and post pictures of the completed project. Thank you for all the help.