Working out the rating of a unmarked lamp

[Kenny.s]

Hi,

This is probably very simple so please excuse me for being dim, no pun intended

My car has an adjustable centre differental, it is adjusted via a potentiometer which varies the input to an electromagnet, the amount of "lock" is indicated via this display unit, the lights come on one at a time.

**broken link removed**

The lamp which is off the board has blown, there are no markings on it or any of the others. Is there any way to identify the voltage and wattage of it by testing the others?

Also if this can be done would it be possible to change these for led's or would more knowledge of the rest of the circuit be required?

Many thanks

Kenny

 

[Dr.EM]

Being automotive I would assume they are 12v, and by thier size perhaps 50ma each. To test them, I would just fix a voltmeter/multimeter accross one of the working lamps terminals and measure the voltage whilst it is lit. Then, connect up a the bulb to a supply of that voltage and use the multimeter to measure the current through it; multiply voltage by current to give watts. Once you know the voltage and current, changing them for LEDs seems a good idea and should't cause any issues.

 

[Hero999]

Just be sure to disconnect one of the leads from the bulb and connect the multimeter in series with it to measure the current.

 

[Kenny.s]

I'm ok with measuring voltages and current, just want to make sure i was on the right track before i had a go at it. Replacement parts for this aren't easy to get hold of and therfore expensive.

Assuming they are 12v then the ballast resistor required for the led's should i decide to fit them shouldnt make any difference as the supply will just see the same load as before?

 

[ThermalRunaway]

Also, changing them for LEDs will of course involve a series current limiting resistor, probably around 800 to 600 ohms.

I'm interested in this centre differntial of yours though. Can I ask, what vehicle has this feature fitted? There are many four-wheel-drive vehicles that feature locking Centre Differentials (mine does infact), but I've never seen one where you vary the amount of lock. On mine you either lock the centre diff or you don't, and I can't immediately see any advantage to being able to vary the amount of lock?

Brian

 

[Kenny.s]

It's a 95 Impreza type Ra.
The torque split front to back is constant but the centre differental is magneticaly controlled as apposed to the normal viscious type. By increasing the current through the magnet the slip between front and rear axles is reduced until they are eventually locked. As you can imagine it has quite a big effect on the cars handling characteristics, fully open the car is quite tail happy, as lock is added it becomes more understeery and tends to go into nice four wheel drifts but once pointing in the right direction you are able to put down a lot more power.

A very brief overview but i hope that makes some sense?

Kenny

 

[ThermalRunaway]

Ah right now you've explained that it's fitted to a high performance 4-wheel drive car I can see why it's useful. I couldn't see any advantage straight away because I was thinking in terms of my vehicle - a Land Rover Discovery, where the ability to vary the amount of lock would have no application whatsoever

Let us know how you get on with changing the bulbs over to LEDs.

Brian

 

[Kenny.s]

I agree that it probably wouldn't have much use on an offroad vehicle but it definately makes for good fun on my car

Ill hook the battery back up tomorrow and see what voltages i get.

Cheers

Kenny

 

[Dr.EM]

If it is indeed 12v, then you could use 12v LEDs (if space is at a premium).They may fit into the holders with no modification.

 

[Kenny.s]

They dont need to fit into the holders, infact it would be better for what i indend doing if i could use panel mounted led's.

 

[audioguru]

The amount of lock on the center differential of the off-road truck determines how many times it rolls over and over when cornering, doesn't it?

 

[ThermalRunaway]

Actually, I'm not sure mine builds up enough speed to roll over in a corner

Brian

 

[audioguru]

Actually, I'm not sure mine builds up enough speed to roll over in a corner

Brian

Hee, hee. Dumb, heavy, gas-gussling trucks just roll the edge of their tires and wheels over and over when cornering, don't they?

 

[Kenny.s]

I went out this morning and connected the battery back up, turns out they are 12volt after all, havent check the current yet but they're probably going to be pretty small so the 12 volt led's should work fine as i doubt they'll draw more than the lamps do.

Kenny

 

[RODALCO]

Most LED's are rated for 20 mA continuous.
Take U car as 14 Volts dc when alternator is running.
So a resistor of 680 Ω ½ Watt is ok for a red LED. (1.7V)
and 560 Ω ok for a Blue LED.
To be on the safe side put a 1kΩ 1 Watt resistor in series. the LED will run at about 15 mA and will have a indefinte life

 

[Dr.EM]

You might be best getting a diffused indicator, a lot of the clear package ones have quite a glare if viewed straight on and could distract your driving. Blue LEDs are often too piercing, diffused models are rare. Best bet is diffused red if you can't find diffused blue (or don't want blue). Alternatively, a diffusing panel in front of the indicators might work.

 

[Kenny.s]

To make it easier to tell the position of the diff i had planned to use different colours,

Red = Fully locked
Orange ...% locked
Orange ...% locked
Yellow ...% locked
Green ...% locked
Green = Fully open

But i hear what you are saying about using diffused led's, i wanted to try and use fairly low intensity ones if possible for that reason.

Kenny

 

[ThermalRunaway]

You will definately need series current limiting resistors Kenny, you can't just do without them. A bulb will simply draw as much current as it needs to operate, but an LED needs the current limiting resistor in order to operate within it's specifications. If you don't include a current limiting resistor, LEDs won't last very long at all.

Alternatively, you can buy "12V" LEDs which are infact normal 2V LEDs with a current limiting resistor suitable for operation at 12V built in to the LED assembly.

Brian

 

[Kenny.s]

Yeh dont worry i knew that

I've been having a look for some nice looking panel mount led's but so far i havent found any that are available in the 4 colours i'd like to use. Any suggestions?

Dont mind having to use normal type led's and fit an external resistor, there's plenty of room to mount this wee pcb and then have flying leads up to where the led's will be mounted. Could either mount the resistor at the pcb end or put it in the flying lead and then shrinkwrap it support and insulate it.


Kenny

 

[ThermalRunaway]

I'd just buy the LEDs on their own and then use panel mounting bezels like these **broken link removed**

That way you can buy exactly the type of LED you want so that it looks the best!

It probably would be easier to put the resistors in-line and then shrink wrap them as you suggested.

Take a pic of the final result for us!!

Brian