LED resitor calculation

[biffen]

I've got a 100W PSU from an old shiatty HP. The 5V line is 10A.

What kind of resistance am I going to need for a 5V, 30mA LED? What about 2 LEDs?

I used the famed LED calculator over at bit-tech, but it tells me to round up to 1 Ohm from 0 Ohms after calculation. I don't know much about this kind of thing, and that seemed odd. Can anyone clarify?

 

[xonik]

If you truly think your LEDs are rated for a 5 volt forward voltage drop, you don't need any resistors. Connect the LEDs in parallel (cathode to cathode to +5 volts; anode to anode to ground). You don't need a resistor if the forward current rating is listed. Your +5 volt rail won't give the LEDs any more current than they draw; the 10 amp figure is a reference to the maximum amount of current draw on the power supply under ATX-specified load and environmental conditions.

 

[eternalspire]

The calculator was right....kinda. Since your LED can run on 5 volts you do not need a resistor to run it off your 5 volt line(current will only pass thru if it can). To run two led's just attach them in parallel.
EX:


_____ ______
|____U_____ |
|.......U.........|
5v...............ground

note: periods are just for spaceing(my ascii art is slacking)

 

[Teancum]

Having even a 1 ohm resistor is a much better idea than no resistor at all. Diodes basically drop a fixed amount of voltage, no matter what current is going through them (assuming it's going in the right direction). So without a resistor to control the current, you could (potentially) put a whole lot more current through than you're supposed to.

So use a resistor.

Also, You'll want to connect the cathode to ground and the anode to the +5 volts--not the other way around.

 

[camay123]

For the record, the calculation is this :

Led specs
Voltage = 1.5V, Amp = 0.015 A. Therefore, to calculate resistance needed for a 5V line-in;

Example
Ohm = (Mains Voltage - LED Voltage) / LED Amperes, so;
Ohm = (5 Volts - 1.5 Volts) / 0.015 A
= 233.33 Ohms or 240 Ohms

 

[xonik]

Also, You'll want to connect the cathode to ground and the anode to the +5 volts--not the other way around.

That's correct, thanks for the correction. For the thread starter:

http://www.electronics2000.co.uk/data/images/LED.gif

Originally posted by camay123
For the record, the calculation is this :

Led specs
Voltage = 1.5V, Amp = 0.015 A. Therefore, to calculate resistance needed for a 5V line-in;

Example
Ohm = (Mains Voltage - LED Voltage) / LED Amperes, so;
Ohm = (5 Volts - 1.5 Volts) / 0.015 A
= 233.33 Ohms or 240 Ohms

For the record, LEDs have different specs, so that would only work with an LED specified to have a 1.5 volt forward drop.

 

[biffen]

Cool. Thanks guys. Everything works and I have a 1 Ohm resistor in there just in case. I got the 5 volt LEDs so I wouldn't have to do much else but plug them in.

Rock n Roll

 

[camay123]

For the record, LEDs have different specs, so that would only work with an LED specified to have a 1.5 volt forward drop.

I tought my "Example" was clear enough

 

[specialk]

Originally posted by camay123
I tought my "Example" was clear enough

Honestly, I think that you should have put "example" before the whole thing. When you define "Led specs" first as "Voltage = 1.5V, Amp = 0.015 A", then it defines an LED as having a forward voltage drop of 1.5V and a current draw of 0.015A (15mA), which is not universal. And even though it is specific for 1 LED, you should not assume the reader would know that.

You should have done this:

LED Resistor Calculation
Ohms = (Supply Voltage - LED Voltage Drop) / LED Current Draw

Example
LED Voltage Drop = 1.5V, LED Current Draw = 0.015 A
Ohms = (5 Volts - 1.5 Volts) / 0.015 A
= 233.33 Ohms or 240 Ohms

-special [k]