Laptop PSU To Power LEDs?

S

sitalchauhan

Limp Gawd
Joined
May 30, 2006
Messages
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I am planning a project to run some LEDs off my mains plug. To convert my mains AC to DC, I am thinking of using an old laptop PSU like the following:

http://www.rolta.co.uk/EbayPics/adapters/IBM20V325A.jpg

I was just wondering, can I just snip off the end round power connector to reveal the 2 positive and negative wires, or is it likely to to be much more complex with multiple wires in the cable?

Thanks
 
Yep, the only thing in the power cable is the positive and negative wires. Open it up, the Red cable should be positive and Black will be ground (negative).
 
You might be better off getting a jack from ratshack so you don't have to cut any wires on the psu. That will let you detach it later if you need to pack up and move your case.
 
sitalchauhan said:
I am planning a project to run some LEDs off my mains plug. To convert my mains AC to DC, I am thinking of using an old laptop PSU like the following:

http://www.rolta.co.uk/EbayPics/adapters/IBM20V325A.jpg

I was just wondering, can I just snip off the end round power connector to reveal the 2 positive and negative wires, or is it likely to to be much more complex with multiple wires in the cable?

Thanks
Click to expand...

It'll work fine, you just need current limiting resistors...
 
Unless you plan on running several leds that you can put in series or otherwise spread out the load, you'll be wasting a lot of power dropping that 20V down (and probably need a good sized resistor to handle the heat).... For just a couple of led's, I'd use a lower voltage like a common 12V wallwart.

But yes, there are only 2 wires in the cable, +/-. The negative could be cable shield rather than stranded wire, but it's the same.
 
Thanks for the replies.

The white LEDs I am getting are each forward voltage 4.5V and 20mA.
I am planning to set it up as:
g.php


My calculations show that if I get a 9V PSU, I will not need to use any resistors since they are 4.5V each, and 2 on each parallel line so they are getting the correct voltage.

However, I noticed that you mentioned "current limiting resistors". What are these? Are they just like normal carbon film resistors like this:
d3_1.JPG

Will I need to use any?

Thanks
 
Yes.

When a PSU starts up, or the load is changed on the fly, it often spikes or ripples the current... This can burn out LED's.

For all practical purposes though, I woulnd't worry about it.

Those LED's are PROBABLY rated for 4.5v MAX, and running 2 of them in series could seriously damage them (as a 9v supply can be within 5% of 9v, 5% over max is enough to kill a LED in a few minutes) not to mention their temperature will skyrocket.

Just get a PSU rated for 4v @ 200-300ma or so... Then run the LED's in parallel from them.
 
I thought that LEDs had to be at their exact rated voltage to light up :rolleyes:

1) In a 4.5V LED, if I ran them at approx 4.0V Im guessing they will still light up, but will they be dimmer than if at 4.5V?

2) If I got a PSU rated at 1000mA, would the LEDs run ok off it because they will only draw the 20mA that they need, or is having a high current PSU going to cause problems? Would I need thicker wires to run a 1000mA PSU?

Thanks

I have been studying physics for god knows how many years and now finishing A2 Level, yet they just dont teach you the practical stuff like LEDs in circuits, just the useless stuff about astronomy and how the earth was supposedly created in the big bang lol! :D
 
I would strongly encourage the use of a current-limiting resistor(s). It's a regular resistor, called "current-limiting" because of its function.

You see, diodes are a bit like your bathtub--no water will flow out until the water level reaches the edge of the tub. Once the water gets over that level, it'll pour out as fast as it gets poured in. Diodes work the same way--they won't conduct any current until the voltage gets to a certain level, which is the LED's forward voltage. Once the voltage gets high enough, the LED will try to conduct as much current as you supply it. In theory, if you supply 3.1V to a 3.0V LED, it will conduct infinite current and burn up. (in practice, it's a little squishier)

What kills LEDs is too much current, caused by applying too much voltage. A current-limiting resistor will take care of the excess voltage and regulate the current. If you have a 12V source and a 3V LED that you want to run at 20mA, you'll need a resistor that will drop the remaining 9V while conducting the same 20mA. So 9V = .02 * R (remember V=IR), and R turns out to be 450Ohms. You can also put multiple diodes in series and add their voltage as well, so your resistor isn't dropping so much voltage. So you could put three of these 3V LEDs in series, and have the resistor drop only the remaining 3V at the same current, and use a 150Ohm resistor.

agent420 linked an LED calculator for one LED. The same site also has a multiple LED calculator which will be very helpful.
 
thanks for that excellent explanation.

I think I will just order the 4.5V LEDs first and see at what voltage they start to light up a decent amount (probably abut 4V).
Dont want to risk running them at their full 4.5V rated forward voltage, then one time the PSU spikes to just above its rated voltage and burns out all the LEDs :eek:

Then will buy suitable resistors and PSU for their required voltage to start lighting up.

Thanks again for the help:D
 
They should light up between around 2.7v (dim) and 4.5v (bright).

A good value to shoot for is around 3.8v IMO.
 
sitalchauhan said:
I think I will just order the 4.5V LEDs first and see at what voltage they start to light up a decent amount (probably abut 4V).
Dont want to risk running them at their full 4.5V rated forward voltage, then one time the PSU spikes to just above its rated voltage and burns out all the LEDs :eek:
Click to expand...
When you test it out, use a resistor in series. That will let you know a) what voltage the LED lights up at, and at what voltage it attains the brightness you desire, and b) how much current (by measuring the voltage across the resistor and using V=IR) it is drawing. And plus, it will help prevent you from blowing out the LED by accident.

There's this little thing called thermal runaway that can cause LEDs to burn out if they're run without a series resistance, even if you're using the proper voltage :)
 
You know, all this talk about the led voltages should make clear that is the voltage drop created by each led.... That really has nothing much to do with voltage used to power them, other than choosing the appropriate resistore to limit the current for the voltage source. You can power those led's just fine from 12V, heck you could right from 120v mains ac givien the correct circuit.

edit - actually, if anything, the Vf can be considered the minimum voltage required to illumate the led to it's rated output.

Arcygenical said:
They should light up between around 2.7v (dim) and 4.5v (bright).

A good value to shoot for is around 3.8v IMO.
Click to expand...
Imo it is better to think in terms of current than voltage for led circuits. It's not clear if the 20ma rating the original poster provided was typical or max, but I always shoot for the closest resistor that provides <= the typical current.
 
Just a friendly hint, looks like it's been covered, but I wanted it to be clear:

Do NOT hook up LEDs in series, even if their "voltage" is sufficiently high to a high-power source. They *will* die. Horribly.

You can hook up any LED to any voltage source as long as there is the proper current limiting resistor.

LED calculators are available at many places on the net.
 
mwarps said:
Just a friendly hint, looks like it's been covered, but I wanted it to be clear:

Do NOT hook up LEDs in series, even if their "voltage" is sufficiently high to a high-power source. They *will* die. Horribly.
Click to expand...
Um...actually, there is absolutely nothing wrong with hooking LEDs up in series, as long as you have the current-limiting resistor. You shouldn't put LEDs in parallel with each other, however, as manufacturing tolerances will likely lead to one or another LED drawing more than its share of current and becoming a "friode".
 
Mohonri said:
Um...actually, there is absolutely nothing wrong with hooking LEDs up in series, as long as you have the current-limiting resistor. You shouldn't put LEDs in parallel with each other, however, as manufacturing tolerances will likely lead to one or another LED drawing more than its share of current and becoming a "friode".
Click to expand...
Series is fine like you've said, but always use a resistor. I've seen people try to play the voltage matching game and not use limiting resistors, and that's when you'll get into trouble.

Just to clarify, parallel use is fine and probably even more commonly used than series, provided each branch has it's own limiting resistor. It's when you try to use one limiting resistor to power several parallel led's that you run into that kind of trouble.

If you think about it, most indicator panels (even your pc power / drive leds) are technically in parallel from a common 5v source, but each has it's own resistor... Likewise, every 7 segment display is also parallel...
 
agent420 said:
Series is fine like you've said, but always use a resistor. I've seen people try to play the voltage matching game and not use limiting resistors, and that's when you'll get into trouble.

Just to clarify, parallel use is fine and probably even more commonly used than series, provided each branch has it's own limiting resistor. It's when you try to use one limiting resistor to power several parallel led's that you run into that kind of trouble.

If you think about it, most indicator panels (even your pc power / drive leds) are technically in parallel from a common 5v source, but each has it's own resistor... Likewise, every 7 segment display is also parallel...
Click to expand...
^^^ Can we make this a sticky? Seriously, we need a "LED Sticky" thread in this forum, as it seems a large percentage of the posts here are about powering LEDs...

 
Just my 2c, but I think there are quite a few excellent tutorials out there, no need to reinvent the wheel. Personally, I think a good collection of links would suffice. A good general purpose site including led 411 would be The Electronics Club.

And on that note I share with you my personal favorite heavy duty electronic reference ;) Ctrl+C and impress your friends LOL
 
You know, the sticky idea is a good one. (Didn't we have one back in the day?)

If that whole esd mess deserves a sticky, imo a good links one does too.
 
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