Automotive LEDs...

[Sinner]

I bought some automotive LED bulbs, and tested them on a PC powersupply...

One that has 12 superbright LEDs in it...they're red, and I think I'll be using it to light my trackball...

The others are dual superbright clears...I bought sockets for them too, and plan to have them in behind the rejector on the coin door...

Neither of these had amperage specs on the packaging, so I'm just guessing as to how I can hook these up...the LED clust has a resistor inside the casing and a diode (to block current from going the wrong way)...this one I connected on it's own, without an resistors, and it seems to be OK...the other ones, I tried with a 200 ohm resitor in line...

anyone know if this is an OK setup, or is there something I should know, before doing this??

Thank you,

Matt

[Fat_Trucker]

Automotive LED's should be designed to work with 12v out of the box so should be ok to use directly with the 12v connection from a PC supply.

[Sinner]

Automotive LED's should be designed to work with 12v out of the box so should be ok to use directly with the 12v connection from a PC supply.

I'm not concerened about the voltage, but more so about the amperage...if a lamp is rated for 12V and 5A, and I dump 12V at 25A through it, the lamp will most likely not like it...

I guess I should have looked and found a package of bulbs with more specs on the package...

Thanks for the reply though...

[Teebor]

I wouldnt worry about the amp's coming from the pc power supply.  I have seen people directly hook up a single LED to the supply and the LEd has been fine.

Or the other side of the coin if you are worried about damaging the power supply just buy one with a bigger rating

[Sinner]

I wouldnt worry about the amp's coming from the pc power supply.  I have seen people directly hook up a single LED to the supply and the LEd has been fine.

Or the other side of the coin if you are worried about damaging the power supply just buy one with a bigger rating

Hmmm, OK...I have connected a 5V LED to the power supply (5V of the PS of course), with only a 100ohm resistor...it lasted about 2 weeks, and burned out...I have a 200ohm (might be 200K, I can't remember) and it runs 24/7 and has been fine for months...either I had a bad LED, or I was running too much current through it...

[bb040]

sounds like you had a bad led

[Sinner]

sounds like you had a bad led

Maybe, but I searched around this site, and seems like some people have had the same problem...most of the suggestions I've found say to put a resistor in there...could be a bad batch, or could be that the LEDs I bought were not rated for the power I was putting into them...
I'll give them a try with 100ohm and see if they last...I could kick myself for not checking the package for specs...I fugured it would just be a given that they would be there...

[Toonces]

The thing with the LED that burnt out is that the 100Ohm resistor may not be the correct one to use. Current is a draw, not a push. The LED will draw whatever current it needs. The Power Supply doesn't push current through. To properly size the resistor you need to know what current the LED is rated for and the voltage it needs. Depending on the color and intensity it was designed to run at, the LED could be rated for ~2.5-3.5V and ~20-30ma. So a 100Ohm resistor may not be the right choice for that particular LED. With a 5V supply, it's probably a good ballpark but is the LED rated for 20 or 30ma? if the resistor is too low you could be giving it 3.5V and 30ma when it's really rated for 2.5V@20ma and you'd be overdriving it. LEDs typically have different ratings per color, even at the same MCD rating. You best bet is to use an LED calulator to make sure you are using the right resistor. Google LED Calculator and choose your favorite. I usually use the one at http://led.linear1.org/1led.wiz as they have calculators for series/parallel and multiples and even a wizard type calculator.

Basically, you may be overdriving the LED because the resistor is not the right one for your LED.

Toonces

[Sinner]

The thing with the LED that burnt out is that the 100Ohm resistor may not be the correct one to use. Current is a draw, not a push. The LED will draw whatever current it needs. The Power Supply doesn't push current through. To properly size the resistor you need to know what current the LED is rated for and the voltage it needs. Depending on the color and intensity it was designed to run at, the LED could be rated for ~2.5-3.5V and ~20-30ma. So a 100Ohm resistor may not be the right choice for that particular LED. With a 5V supply, it's probably a good ballpark but is the LED rated for 20 or 30ma? if the resistor is too low you could be giving it 3.5V and 30ma when it's really rated for 2.5V@20ma and you'd be overdriving it. LEDs typically have different ratings per color, even at the same MCD rating. You best bet is to use an LED calulator to make sure you are using the right resistor. Google LED Calculator and choose your favorite. I usually use the one at http://led.linear1.org/1led.wiz as they have calculators for series/parallel and multiples and even a wizard type calculator.

Basically, you may be overdriving the LED because the resistor is not the right one for your LED.

Toonces

Unfortunately, all I know is that they are superbright, automotive (12V) clear LED lamps (not individual LEDs)...
The 100 ohm resitir I used before, was on a single superbright LED...
Without all the information, I guess it's just a matter of trial and error...
Thanks for the info...

[whammoed]

A single led is likely not meant for 5 volts, so you need a resistor for it.  Those 12volt automotive leds already have a resistor built into them, so you just plug them in to any 12 volt source.

[rwhip41]

The IPAC from Ultimarc recommends using a 220 ohm resistor ro 5V LEDs.

Here is a schematic involving a 5V input (scroll down 2/3 of the page)...
http://www.ultimarc.com/ipac2.html

[Sinner]

A single led is likely not meant for 5 volts, so you need a resistor for it.  Those 12volt automotive leds already have a resistor built into them, so you just plug them in to any 12 volt source.

The singles I bought were actually 5V LEDs, so I don't think the actual voltage was an issue...
I got them from Electrosonic and the specs were on the bag...

The automotive LEDs have resistors in them, but a car's electrical system outputs a certain amount of current...I assume those resistors are calculated for that amperage...
A PC powersupply puts out a certain amount of current and voltage...I guess what I'm trying to figure out, is the difference between a car's output (which I can't find) and a PCs (on the power supply)...That way I can determine if a resistor is needed, and then what size it should be...

[Sinner]

The IPAC from Ultimarc recommends using a 220 ohm resistor ro 5V LEDs.

Here is a schematic involving a 5V input (scroll down 2/3 of the page)...
http://www.ultimarc.com/ipac2.html

Thank you.

[whammoed]

You don't have to worry about what your power source is rated for unless you will be exceeding its rating with whatever device you are using.  (a few leds certainly won't exceed any power supplies rating.)

Look at ohms law:

I = V/R

where
I = current
V = volts
R = resistance

a 12 volt car bulb will draw the same amps it does from a 2 amp power supply as it does from a 200 amp supply.

[Sinner]

You don't have to worry about what your power source is rated for unless you will be exceeding its rating with whatever device you are using.  (a few leds certainly won't exceed any power supplies rating.)

Look at ohms law:

I = V/R

where
I = current
V = volts
R = resistance

a 12 volt car bulb will draw the same amps it does from a 2 amp power supply as it does from a 200 amp supply.

Then if some LEDs are connected to a powersupply, why do they need a resistor?  They have to be drawing more, or are they being driven?  Is the power supply putting out 12V+ at 26A, or will the bulb only draw 12V+ at 3A?  If it's the second, then there would be no need for a resistor, but if it's the first, then I would need to regulate how much current is going through the LED...
This is where I'm confused...

[whammoed]

led's need  a resistor when they are rated for a voltage lower than that wich you are supplying.
Bottome Line.
a 12 volt bulb rated at 0.5 amps will draw 0.5 amps from a 12 volt supply regardless of the power supply rating.

[Sinner]

led's need  a resistor when they are rated for a voltage lower than that wich you are supplying.
Bottome Line.
a 12 volt bulb rated at 0.5 amps will draw 0.5 amps from a 12 volt supply regardless of the power supply rating.

I understand what you're saying, but I don't necessarily agree...resistors would limit the current, not the voltage...

[whammoed]

Never said it limited the voltage.
Did you even look at ohms law?  It is pretty widely accepted. 

[Sinner]

led's need  a resistor when they are rated for a voltage lower than that wich you are supplying.

I guess I read this as you were saying you needed a resistor to change the voltage...

As far as Ohm's law goes, wouldn't I need to know the amperage rating of the LED to figure out the resistance needed?

[whammoed]

to figure out the resistor you need (if you need one) you need to know the amps and volts for the led and the volts you will be supplying.
this site will do the calculation for you:
http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

[whammoed]

I will break this down for you using ohms law:
Lets say you have an led and the ratings are:
Volts: 2
amps: 0.030  (30 milliamps)

so, lets find the resistance of the led:
R=V/I
R = 2/0.30
R = 66.7

now lets say you want to use this led on a 12 volt source, lets find the resistance needed to only pull 30 milliamps:
R-V/I
R=12/.030
R= 400

so, you need and extra 333 ohms (400-66.7) of resistance to use the led on a 12 volt supply.  You can likely find a 330 ohm resistor to do the job.  (pick a common resistor value close to the calculation)

[whammoed]

I will confirm also, once more that a 12v automotive led bulb is ready to go on any 12 volt source.

Even if you don't believe me, a cars alternator can typically deliver over 100 amps.  How many pc power supplies are rated for over 100 amps on the 12 volt rail?  Even if I am full of it, sounds like you're safe to me.

[Sinner]

to figure out the resistor you need (if you need one) you need to know the amps and volts for the led and the volts you will be supplying.
this site will do the calculation for you:
http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

What I've been saying from the begining is that I don't know the amerage of the bulbs...I'll have to take your word on it and try...what's the worst that can happen???  It blows and starts a fire in my cab, when I'm not home...

Thanks for the explanation...

[TOK]

I will confirm also, once more that a 12v automotive led bulb is ready to go on any 12 volt source.

Even if you don't believe me, a cars alternator can typically deliver over 100 amps.  How many pc power supplies are rated for over 100 amps on the 12 volt rail?  Even if I am full of it, sounds like you're safe to me.

I will re-confirm it. I've had 3 of these lighting my trackball for over a year plugged right into the 12v lead of a PC power supply. You don't need to dick around with resistors, just hook them up.

[Toonces]

to figure out the resistor you need (if you need one) you need to know the amps and volts for the led and the volts you will be supplying.
this site will do the calculation for you:
http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

What I've been saying from the begining is that I don't know the amerage of the bulbs...I'll have to take your word on it and try...what's the worst that can happen???  It blows and starts a fire in my cab, when I'm not home...

Thanks for the explanation...

If you look at the ratings of some of the brightest LEDs they run around 30milliamps. each.  So if you have a cluster, if you add them all up, you are going to need a hell of a lot of them to even get 1 amp. 

[Teebor]

Because you are going to be plugging in more than one LED (ie the cluster in the auto lamp) then you will not / should not need any resistors.