Noob question. 12 volts, 25 amps

[tieme]

Hey everyone, brand new to the forums and pretty inexperienced in electronics.

I am trying to wire up an LED array. They are 5mm and I'm trying to get 3 volts to run through them (a little under rated, but that's alright by me).

I have a computer power supply, the rail is 12 volts and 25 amps. I figured that I could put 4 leds in series and stick it on the rail with no problems. They light up for a split second and then burn out. Can someone explain what I'm doing wrong?

Thanks!

o/
Tieme

 

[saturn1bguy]

Can someone explain what I'm doing wrong?

You need a current limiting resistor in series with your string of LEDs.

Ideally, find out the precise forward voltage of the LEDs you're using via the datasheet, then you can figure out how many you can place in series for your 12V supply. Then add a series resistor to keep them from lighting mere milliseconds.

Say your LEDs have a Vf of 2.0V, place say five in series, and use a 100R resistor in series with them across your 12V supply (12V-(5x2V)/20mA).

 

[tieme]

You are prolly going to wince at the link... **broken link removed**

The spec sheet is on there. 3.6 max forward voltage. PSU tests at about 12.8-12.9 volts. If I have 4 in series that should be about 3.2 volts which is quite a bit UNDER the rated. I don't quite understand why it doesn't work. Can you maybe clear this up for me?

I guess I don't quite understand LEDs very well. Can you point me to any good resources for figuring this out? Thanks for the swift reply!

o/
Tieme

 

[saturn1bguy]

Let me see if I get this. I want my leds at 3.3 volts/20ma. my psu is 12.9 volts. I can put 3 leds (9.9 volts) leaving me with 3 volts to deal with with resistors. I want 20 ma so I need a 150 ohm resistor in series with 3 leds. Is that correct?

Close. They're spec'ed at 3.6V, not 3.3V. Three at 3.6V would leave 2.1V to drop across your resistor. That divided by 20mA = 105R, or 100 for a standard value.

However, a Vf of 3.6V looks suspect to me for red LEDs, which are often between 1.6-2.0V. Also, your 12.9V power supply willl likely drop a bit if loaded considerably, but this may not be an issue with what you're doing.

If yes, I have another question. Is there a better way to do it without wasting so much juice on resistors? That's almost 25% of my power being used in resistors.

With a 25A power supply I wouldn't worry about it. That would allow you 1250 such strings, theoretically. Plus, you're not burning 25% of your power on the dropping resistor: 12.9V*20mA = 258mW per string. The resistor dissipates 2.1V*20mA = 42mW, or 16%. Just for the record

 

[tieme]

Thanks. I thought 3.6 was max so I figured 3.3 would be a good place to be. I spose I'll try a few out at 3.6 and see how it goes. And thanks for the correction to my power calculation I was just doing a rough estimate. 16% isn't so bad then

I'll give it a try now and let you know how it goes. Thanks again for the help!!

o/
Tieme

 

[saturn1bguy]

I thought 3.6 was max so I figured 3.3 would be a good place to be.

The data given by your eBay seller looks generic, as if it could apply to the many types of LEDs they sell, so they don't have to keep changing their listings. Real LED datasheets don't specify a range that one can choose from, but given a tested and measured transfer function of forward current versus forward voltage, they specify a typical forward voltage for that particular product. A single value and not a range, just FYI.

You can measure your LEDs. If you have constant current source, set it to 20mA, drive one LED, and measure the voltage drop across it. That's the value you're looking for.

Typical values have always been 1.6V for red, 2.0V for yel, and 2.2V for grn, and typical currents of 10mA for T1 (3mm) and 20mA for T1 3/4 (5mm). But things have changed with new technology, especially for super bright units, blu, wht, and IR units. So, it's always best to consult the datasheet...

 

[tieme]

Alright, thanks. I've contacted the seller to see if they have actual data sheets for the LEDs.

If I come up with working circuit (say 5 LEDs then a 100R resistor) but want to save space on the number of resistors I use would it work to put the resistor to a rail and then run 5 LED chains from the rail? As long as I don't exceed the wattage on the resistor that should still work fine, right?

o/
Tieme

 

[saturn1bguy]

If I come up with working circuit (say 5 LEDs then a 100R resistor) but want to save space on the number of resistors I use would it work to put the resistor to a rail and then run 5 LED chains from the rail? As long as I don't exceed the wattage on the resistor that should still work fine, right?

You're saying you want one resistor to source five LED strings of five each. Yes, that works but it's not ideal. But if your LEDs are all identical it may work out okay. You'd have to make the resistor 1/5 as big, i.e. 20R, because it's now sourcing 100mA (5*20mA). And that's 210mW ea and you've maxed out a quarter-watt resistor.

 

[tieme]

Awesome. Thanks a lot for the advice. You've saved me a lot of time and a lot of dead LEDs. I'll mess around a bit and come post again if I have any more problems.

o/
Tieme

 

[tieme]

Just to let you know what I ended up doing:

4x red LEDs (assumed 2 volts) and 1x blue LED(rated 3.6 according to spec sheet). 11.6 volts. PSU is at 12.26 12.26 - 11.6 = .66v/27r = 24.44 ma I have 5 sets wired up and they seem to be running beautifully. I am waiting on more board so I will let these run a few days and see if I can notice any dimming or failing.

Thanks again for the help.

 

[k7elp60]

As a side note, the forward current flow also determines the intensity of the LED. I never run the LED's at the maximum current. Depending upon the specific application the current can be quite low. I have some blue and white T1 size LED's that have a MCD rating a little over 2000 MCD. They are both real bright when the forward current is about 3mA.
The attactment is the forward voltage for some common LED's

 

[tieme]

k7,

Thanks for the heads up. I am actually using them to try to grow with, hence the red/blue. So I really want the maximum light I can squeeze out of them without damaging them. Thanks for the attachment too!

o/
Tieme

 

[audioguru]

The LEDs are cheap no-name-brand Chinese ones. They might last a couple of days.
Please let us know.