Installing LED mod. How to power and such...

[djheater]

{moved from general hardware}

I've just bought five of these bad boys (10,000 mcd!) and want to install them as into the backlight of an LCD to make it supah-bright.
I have an old power-supply sitting around to use to power it, could someone point me in the right direction of what I'd need to make it work. do i need a PCB? The space within the backlight tray is pretty tight. do I need a resistor?

I guess what I need is a primer course in electronics How do these things work???

The two leads from the back of the LED hook directly up to the +5v and Ground on the molex connector. If you need some voltage limiter, you are on your own- I just hooked mine right up to the molex.

If I'm reading the specs right for the LED's I'm getting that I'd need a resistor, but where do I find that and how do I know which one?

 

[djheater]

something like this is what I'm thinking. I could use the wires from an old case but I'll still need a resistor. where would I get one and which one would I get??

 

[djheater]

bump for the morning crowd.

 

[OulOat]

Radioshack sells resistors, for a decent price. Using the equation Power = Current * Voltage, you find out that the voltage needed for the LED is 4 Volts. You don't really want to go over that voltage, since it will just cause the LED to burn out and explode. So basically you need to reduce the voltage from the 5V or 12V line to 4V, this is where resistors come in. To find the voltage dissipated by a resistor use this equation, Voltage = Current * Resistance. Since your current is fixed at 70mA (from the LED specs), and you want the voltage dissipated to be 1 V (assuming you are using the 5V line), 14 Ohm resistor. If you can't find a 14 Ohm resistor, just connect some resistors in series (tail to head) and the final resistance would be the sum of the individual resistance. Make sure to add them in series, and not parallel.

Edited---
Oh yeah, that is just for 1 LED. If you want multiple LEDs to be hooked up to the same resistor, it is suggested that you hook up the LEDs in parallel (head to head, tail to tail) and you increase the current required respectively. i.e. if you want to hook up all 5 LEDs to the resistor, you would multiply the current in only the V=IR equation by 5. This will lower the resistance required.

 

[Mark R]

. If you want multiple LEDs to be hooked up to the same resistor, it is suggested that you hook up the LEDs in parallel (head to head, tail to tail) and you increase the current required respectively. i.e. if you want to hook up all 5 LEDs to the resistor, you would multiply the current in only the V=IR equation by 5. This will lower the resistance required.

That is not recommended - LEDs should not be connected in parallel on the same resistor. You run the risk of burning them out because the current will NOT be evenly divided between the LEDs.

The recommended way is to connect them in series (therefore they all take the same current) - if they are blue or white, you can connect 3 in series from a 12 V supply. A typical blue LED is 3.7 V (but this varies) - so if you have three in series connected to a 12 V supply - the calculation becomes:

Vf = 3.7 x 3 = 11.1 V
Vres = 12 - 11.1 = 0.9 V
I = 30 mA (the max current for your LED)
R = 0.9/0.03 = 30 Ohms

30 Ohm resistors are not easily available - therefore a 33 Ohm resistor should be used.

 

[djheater]

OK so If I want them in series a 33ohm resistor and only 3 per 12v line. If I want them individual then 14ohm on 5v line? Is this correct? To be honest I'd prolly go with them each on thier own line. At least initially.

 

[OulOat]

Originally posted by: Mark R

. If you want multiple LEDs to be hooked up to the same resistor, it is suggested that you hook up the LEDs in parallel (head to head, tail to tail) and you increase the current required respectively. i.e. if you want to hook up all 5 LEDs to the resistor, you would multiply the current in only the V=IR equation by 5. This will lower the resistance required.

That is not recommended - LEDs should not be connected in parallel on the same resistor. You run the risk of burning them out because the current will NOT be evenly divided between the LEDs.

I dunno, that is what my ECE professor recommended when I asked him. Why wouldn't the current be evenly divided, since the diodes all dissipate the same voltage?

Editted--
Yes, I remember when we were working on BJTs (forgot which type) when we have multiple diodes (same voltage) in parallel, current is just divided evenly between them.

 

[OulOat]

Originally posted by: djheater
OK so If I want them in series a 33ohm resistor and only 3 per 12v line. If I want them individual then 14ohm on 5v line? Is this correct? To be honest I'd prolly go with them each on thier own line. At least initially.

No, that is my bad, I misread the chart when I did V=IR. I read I=70 instead of it's true value of 30 mA since they were right next to each other. You will need a 33 Ohm resistor.

 

[Mark R]

Why wouldn't the current be evenly divided, since the diodes all dissipate the same voltage?

Because the diodes don't have the same forward voltage drop. LEDs are not like silicon diodes, which have only a small range in forward voltage; LEDs can have a very wide range of voltage drop, even within the same batch.

I've got a datasheet for some blue LEDs: Vf @ 20 mA is listed as betwwen 3.3 and 4.5 V. Combined with the highly non-linear nature, it is easy to conceive a situation where a pair of LEDs on one resistor share current in a 90:10 ratio.

I've got a number of LEDs which are closely matched for forward voltage, and these do work acceptably when connected in parallel, the current split is only 60:40 for a typical pair. When I've combined other LEDs which weren't closely matched, splits of 70:30 or worse or common.

 

[djheater]

Ok so a 33 ohm resistor either way i go. Going to radio shack tonight.

 

[Fallen Kell]

V = IR

That is all I have to say

Well, that and the fact that the current will run through the path of least resistance. Each LED acts as a resistor to the electricity. It is HIGHLY unlikly that the 3 LEDs you have have the exact same resistance. Even resistors vary in the resistance they actually produce, that is why they have so many different markings on them (the strip paterns and colors represent the base resistance and the variance that the resistor was tested to be within). The less variance allowed in the resistor, the higher quality it is and the pricier it is as well. The same will hold through to an LED, but I don't know how careful LEDs are manufactured as well as the tests they are run through. You will need to build a parrallel circuit to run 3 LEDs properly, but to do that you will need to test the resistance of each LED. Once you know that value, you will then need to place a resistor on each LED so that the resistor+LED have the same resistance for each of the three LEDs. You can then use one power connection to power those three LEDs in that configuration.

Or you could go the cheap way and use a more powerful resistor then necessary, thus restricting the amount of amps that can reach the LEDs so that even if one LED gets more power then the other 2, it will still be within spec for that LED to handle. You might lose some overall brightness doing it this way, but its certainly easier.

 

[djheater]

OK so I just hooked up the 33ohm resistor to the 5v line and turned it on... At first is was dim and blue (it's a white LED) then it burnt out....
I hooked another one up for a few seconds and it was blue too. So I powered off and as the voltage fell it brightened and became white?

The resistor package says 1/2 watt? is that meaningful?

 

[HiTek21]

Use This To calculated what resistors you need to power up your LEDs.

 

[OulOat]

You sure you hooked it up to the 5V line and ground? And the LED are hooked up in the right direction?

 

[djheater]

Originally posted by: OulOat
You sure you hooked it up to the 5V line and ground? And the LED are hooked up in the right direction?

Beats me Could you clarify a few things for me. Just so I'm absolutely sure.

1)Which side gets the resistor and therefore power, the cathode or anode?

2)What color is the 5v line on a molex?

 

[djheater]

::bump::

 

[djheater]

Originally posted by: HiTek21
Use This To calculated what resistors you need to power up your LEDs.

The calculator gives the following results:

Supply voltage 5 V
Diode forward voltage 4V
Diode rated current 30mA

Calculated resistance value 33.3Ohms
Next standard 5% resistor 39 Ohms
Resistor color markings orange white black
Power dissipated in LED 103mW
Power dissipated in resistor 26mW

So should I buy 39ohm resistors? Adn what's the meaning of the 1/2 or 1/4 watt ratings?

 

[OulOat]

Originally posted by: djheater

Originally posted by: OulOat
You sure you hooked it up to the 5V line and ground? And the LED are hooked up in the right direction?

Beats me Could you clarify a few things for me. Just so I'm absolutely sure.

1)Which side gets the resistor and therefore power, the cathode or anode?

2)What color is the 5v line on a molex?

1) It doesn't matter which side gets the resistor, since they are in series.

2) Red is your 5V line. The longer lead on the LED is the positive side and the shorter lead is the negative side. So you need to make sure the longer lead is connected to the 5V line (doesn't matter if you put the resistor before) and the shorter lead is connected to the ground wire (any of the two black wires.

 

[djheater]

Originally posted by: OulOat

Originally posted by: djheater

Originally posted by: OulOat
You sure you hooked it up to the 5V line and ground? And the LED are hooked up in the right direction?

Beats me Could you clarify a few things for me. Just so I'm absolutely sure.

1)Which side gets the resistor and therefore power, the cathode or anode?

2)What color is the 5v line on a molex?

1) It doesn't matter which side gets the resistor, since they are in series.

2) Red is your 5V line. The longer lead on the LED is the positive side and the shorter lead is the negative side. So you need to make sure the longer lead is connected to the 5V line (doesn't matter if you put the resistor before) and the shorter lead is connected to the ground wire (any of the two black wires.

Well crap, that explains it. I used the yellow. I searched and found a webpage that referenced it as the 5v line. Should have verified my data.
or maybe I should buy a voltmeter.

 

[OulOat]

How did it turn out?