Author Topic: Connecting LED help for an electronics noob  (Read 5941 times)

Offline Sadler

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on: January 23, 2008, 11:43:06 AM
I've bought a SY-IR53A - a quick google will give the specs. Here's where I bought it from:

http://www.maplin.co.uk/module.aspx?ModuleNo=2253&doy=23m1#overview

I've tried to work out what resistor I need but I'm getting confused. Can someone help?

I'm planning to use a laser pointer that holds 3x1.5V button batteries for a total of 4.5 volts or a single AA or AAA battery. I haven't got one yet but there's a few on eBay I've got my eye on.

Thanks in advance.
« Last Edit: January 24, 2008, 03:23:24 AM by Sadler »



Offline Salomon926

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Reply #1 on: January 29, 2008, 05:38:37 PM
So, I found a datasheet here:
http://www.100y.com.tw/pdf_file/SY-IR10,12,15,33,53,74.pdf

According to the datasheet, it says the typical voltage drop is 1.6v at 20mA. Thus, if you want to use a 4.5v battery, you have to dissipate 2.9v at 20mA. This will give you a resistor of 145 ohms (voltage across the resistor = current through the resistor x resistance. 1 V = 1 A * 1 ohm)

If you want to go brighter, you can, but you'll have to be careful. The limit you're likely to hit first is the 100mW power rating. LED Power = (voltage across LED x current through the LED. 1 W = 1 V * 1 A)

Say you try a 50 ohm resistor. (At this point I'm making up numbers) Then measure the voltage across the LED with a voltage meter to get 2v (the voltage drop slightly increases as current increases.) Then measure across the resistor to find 2.5v (Or measure the battery voltage at 4.5v, then 2v + 2.5v = 4.5v) Then the current is resistor voltage / resistance = 2.5 v / 50 ohm = 50 mA. Then the power dissipated in the LED is 50 mA * 2v = 100 mW, which is the maximum power allowed. So, you can decrease your resistor as long as you don't exceed 100mW (which you'll have to measure.) I recommend giving some safety room, and if I had to guess, I'd say 100 ohm would be decent.

Good luck.
« Last Edit: January 29, 2008, 05:40:43 PM by Salomon926 »



Offline imarzouka

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Reply #2 on: April 02, 2008, 03:36:13 AM
Use this calculator to do the calculations http://ledcalculator.net