LED light - do I need a limiting resistor?

[l1ttle_d3vil]

I'm setting up a small shift light to use at the drags, basically going to run a light that is switched to earth by an Aux channel on the Link G3. If using an LED, the manual says you MUST wire a 1k current limiting resistor between the Link and LED. The light I've got is a Narva Pilot Lamp / red LED 12v only.

I've been to a couple of auto sparkies today and asked for a current limiting resistor and got told they don't stock them and try Dick Smith.. went there and the girl behind the counter looked at me blankly and said "if you can tell me what kind of resistor it is I can search our website to see if we have it"

Are these known by any other name? Otherwise, can someone give me a product number for them that I can take to Dick Smith or somewhere similar, or point me to the right product on their website - if they have it?

I don't really know what I'm looking for, only going on what information is in the Link manual

Thanks
-Matt

[whynot]

if your LED lamp has 12v written on it then it should by all rights have the limiting resistor already in it. if you are not sure just get a 510 ohm resistor and put it in series with the lamp and chuck it on the battery. if it's very dull then it will have it built in.

[tim_blair]

go to jay car they are WAAAAAAAAAAY more on to it than the idiots at diksmith

itl just be a simple 1k ohm resistor

[RobertC]

a resistor limits current...

All you need is a normal resistor

just go to dse and ask for a 1k ohm resistor. will be like 20cents.
get a wattage of about 0.5w

[l1ttle_d3vil]

if your LED lamp has 12v written on it then it should by all rights have the limiting resistor already in it.

I did wonder that... it is a pretty big LED, not one of the little LEDs like you get on alarms, so could well have a resistor hiding inside. How long would it last if I hook it up to 12v / Earth and it doesn't have a resistor? And would this do any sort of damage to the ECU?

Don't have a jaycar here, just Dick Smith and auto sparkies who don't like working on cars. God this place sounds like a hick town

[whynot]

yeah i wouldnt go hooking it up to the ecu without testing it first, you might fry the output or worse.

If you hook it up to 12v and it doesn't have a resistor in it you will have all of 0.00002 sec of light and then the magic smoke will escape and they haven't figured out how to catch that stuff once it's gotten out

on another note you could try hooking it up to 2x AA batteries. If it lights up bright then it doesn't have a resistor. a red led needs about 2.3 volts and a blue led needs about 3.5 so it would be a good way to check without instantly frying anything

[l1ttle_d3vil]

sweet, hooked it up to two aa batteries and it turns on. I wouldn't say its super bright, but its definitely on. How faint would it be if it did have a resistor, only just noticable or would it not light up at all?

Might just hook it up to 12v and an earth and see what happens, only cost a few dollars...

This is the LED here - the red one - can't find any info on the net about them having/not having resistors.

[Burning Angel]

yea they got the resistor built in

[l1ttle_d3vil]

Mint, thanks for the info guys. Last thing to wire up then the car is ready for the drags...

...might have to go and test that everything is working this weekend though

[l1ttle_d3vil]

sweet, works fine running 12v + and earth straight to it.

another stupid question - does this have to be setup using a relay to provide the 12v + signal, and the ECU earthing the relay at given RPM?
or can it be wired without a relay by using 12v+ ign straight to LED and ECU directly earthing the LED?

cheers

[touge_ae101]

from my understanding of a LED is that it acts as a small resistor due to the voltage drop across the diode and that why you can run them straight off a 12V or similar source. its more about the characterstic of the diode rather than having a built in 'resistor'. this is probably a theoretical representation of the resistance the diode itself causes by dropping the voltage 0.7V.

haven't brushed up on my digital electronics in a while so correct me if i'm wrong, i just thought someone would have mentioned this by now.

[Mr.Phreak]

You're wrong

[touge_ae101]

[sergei]

from my understanding of a LED is that it acts as a small resistor due to the voltage drop across the diode and that why you can run them straight off a 12V or similar source. its more about the characterstic of the diode rather than having a built in 'resistor'. this is probably a theoretical representation of the resistance the diode itself causes by dropping the voltage 0.7V.

haven't brushed up on my digital electronics in a while so correct me if i'm wrong, i just thought someone would have mentioned this by now.

You could not be more wrong.

Even if LED was linear device voltage drop across it would be exactly what the supply voltage is. So say 50 Ohm resistor plugged into 12V source will have 12V across it. same 100kOhm.

Closest thing what you describe is a light bulb - the hotter it is, more resistance it has. So light bulb is quiet tolerant of what voltage it runs off to a degree.

For LED it behaves same same way as standard diode more or less (except initial voltage is a bit higher), so more voltage you apply LESS resistance it has = more current goes through the LED. Imagine if you would connect a standard diode to 12V in conductive configuration. It would go "poof" within milliseconds. Semiconductors are not linear devices and exhibit a higher conductivity with higher voltage/temperature (temperature aids in electron-hole flow).

Unlike common misconception LED have operating current (not voltage), they only require a certain voltage to turn on, but when you calculate limiting resistor you calculate the current that will go through LED, because LED have certain current that they have peak efficiency at.

[QikStarlie]

sweet, works fine running 12v + and earth straight to it.

another stupid question - does this have to be setup using a relay to provide the 12v + signal, and the ECU earthing the relay at given RPM?
or can it be wired without a relay by using 12v+ ign straight to LED and ECU directly earthing the LED?

cheers

can use led without relay due to small current draw. long as what your connecting to the link doesnt draw too much current can be directly connected. otherwise needs relay. it says in the link manual somewhere how much you can directly switch though the aux outputs.

[l1ttle_d3vil]

can use led without relay due to small current draw. long as what your connecting to the link doesnt draw too much current can be directly connected. otherwise needs relay. it says in the link manual somewhere how much you can directly switch though the aux outputs.

Sweet, yeah the manual does have the limit but I can't remember the exact method for measuring it so thought it'd be safer to ask than get my reading wrong and damage something

The load must not draw more than 5 Amps of current. This means that a directly connected load should have a resistance exceeding 2-3 Ω. If the resistance is lower than this a relay should be used.

[sergei]

LED would draw about 20-50mA (or 0.02-0.05A).