Resistors and LED's - replacing incandecent bulbs

[aceventura]

Resistors and LED's - replacing incandescent bulbs

One of the globes that lights the LCD screen for my climate is blown, and I am having a lot a trouble chasing another down that would suffice, so I thought I would try and use an LED.

Doing a bit of searching (on this site and others) I found that you have to solder a resistor in series with the LED to cut the voltage down to a safe level for the LED. I am happy enough with this.

My questions - please correct me if I am misguided:

The working bulb has a 14 Ohm resistance when cold, which is slightly less than a 1.2W wedge bulb I have. Is it reasonable to assume that the CC bulb is ~ 1W?

A 3.4 V LED @ 20mA will require about at 430 Ohm resistor (Super Bright LEDs - LED Info), which would would make the circuit (resistor + LED) about 0.3 W. I assume that running at a lower power will not damge the CC module.

Thanks for any comments

 

[muttley383]

Remember that voltage level with the engine running is about 14v. Using your calculator that would give a resistor value of 560 ohm for a led with a 3.4v forward voltage drop.

 

[aceventura]

Remember that voltage level with the engine running is about 14v. Using your calculator that would give a resistor value of 560 ohm for a led with a 3.4v forward voltage drop.

Good point - Thanks.

Does anyone have any feeling for the distribution of light given off by white LEDs?

Specifically, I can get hold of 3mm white LEDs with a 30 degree view easily. If I replace the two normal bulbs above the LCD screen, will the screen have two bright strips below the LEDs, or will the light diffuse ok?

Should I put 3 above the screen, or will that be too bright?

 

[Mike S]

Does that screen dim with the dash control?, meaning a lower voltage to the globe when dimmed, I'm not sure your LED will like that too much.

 

[greenacc]

I replaced the globe behind the stock stereo screen with a very bright green LED and resistor. Now ya have to wear sunnies at night cos its so bright. Anyway, point is, the light from LED's is often more directional than globes, and it can look like a bright spot rather than broad light.
There's another formula u can use, think i got it from dick smith website or catalogue?

 

[commsirac]

One of the globes that lights the LCD screen for my climate is blown, and I am having a lot a trouble chasing another down that would suffice, so I thought I would try and use an LED.

Doing a bit of searching (on this site and others) I found that you have to solder a resistor in series with the LED to cut the voltage down to a safe level for the LED. I am happy enough with this.

My questions - please correct me if I am misguided:

The working bulb has a 14 Ohm resistance when cold, which is slightly less than a 1.2W wedge bulb I have. Is it reasonable to assume that the CC bulb is ~ 1W?

A 3.4 V LED @ 20mA will require about at 430 Ohm resistor (Super Bright LEDs - LED Info), which would would make the circuit (resistor + LED) about 0.3 W. I assume that running at a lower power will not damge the CC module.

Thanks for any comments

No, I think leds are full of trouble here, especially if you want it to dim with the same character as the other lamps in the dash.
The working resistance of a tungsten light globe is many times higher than the cold resistance. Possibly the dash lights are operated at low levels, but even for mild light 2000-3000C, the filament resistance increases ~15 fold, so the 1W estimate is roughly correct : P = voltage squared/resistance= 14^2/(14*15) =~1W

 

[aceventura]

Thanks guys - great comments:thumbsup:

Does that screen dim with the dash control?, meaning a lower voltage to the globe when dimmed, I'm not sure your LED will like that too much.

I think it does dim with the dash control - I should rethink the LEDs

Now ya have to wear sunnies at night cos its so bright. Anyway, point is, the light from LED's is often more directional than globes, and it can look like a bright spot rather than broad light.

This is what I thought!

The working resistance of a tungsten light globe is many times higher than the cold resistance. Possibly the dash lights are operated at low levels, but even for mild light 2000-3000C, the filament resistance increases ~15 fold, so the 1W estimate is roughly correct : P = voltage squared/resistance= 14^2/(14*15) =~1W

I looked for something like this everywhere! I wanted to know what the resistance increase of a hot bulb was. I could meausure the voltage ok, but I couldn't break the circuit to measure the current in this case - Thanks.

 

[commsirac]

I looked for something like this everywhere! I wanted to know what the resistance increase of a hot bulb was. I could meausure the voltage ok, but I couldn't break the circuit to measure the current in this case - Thanks.

Whilst I had a good idea that the resistance of tungsten(common incandescent light globe) was much higher when heated, needed some exact info: this is what at the top of the list when I searched for: (tungsten temperature resistance)

Resistivity of Tungsten

 

[cewing]

JUst a quick thought on the subject, at work i use LEDs for use in lighting systems(installed and left for the long haul) so we run them at 10 mA, rather than the reccomend 15-20. Little difference in output, yet they'll last much longer

 

[aceventura]

Ok, thanks for the input everyone. I found a 12V, 50mA (0.6W) bulb that does the job. Turns out the screen doesn't dim, it is always on so I could have used an LED. If you were to use an LED you would have to find a wide viewing angle one I guess.

For those interested, I got it at Dick Smith (Cat. No. - P8140). It is called a wheat grain bulb - It is not identical, but I think it does the job quite well (see attached images - LHS replaced).

Also there is another thread that lists a Holden part number for these bulbs - it isn't correct.

BTW ring ahead and get them to find the bulb if you can. I went to a few outlets trying to get the part, but the components sections are a shambles:bang: