# IR LED Circuit

Discussion in 'The Projects Forum' started by shawnster, Jul 13, 2009.

1. ### shawnster Thread Starter New Member

Jul 13, 2009
1
0
Hello,

I am interested in building an IR light out of a leftover AC/DC power supply from a cordless telephone and 5 IR lamps:

The power supply specifics:

9V @ 200mA

The IR lamp specifics:

1.2V @ 100mA (max)

To err on the side of caution I would like to load 5 IR lamps with a current of 90mA. I am thinking I can connect all 5 IR lamps in series and add a 140 mA resistor to the beginning of the series and everything will work out fine.

I really don't know much about designing complete circuits everything I have read so far doesn't address anything on the input side, it is always expressed as 6V or 12V or 24V. I am guessing that input needs to equal output? So on the input side 9V @ 200mA = 1.8 watts and on the output side (5) IR Lamps * 1.2V @ 90mA + 140mA resistor = 1.8 watts.

Not sure if that is how it works where input = output? I really don't know much other than some base Ohm's calculations. Other than that I am clueless. Any help is most appreciated. Thanks,

2. ### Audioguru New Member

Dec 20, 2007
9,411
896
If the "power supply' is just a cheap AC-DC converter then it probably is not regulated. Then with a load of 200ma its voltage should be 9v. But when its load is less its output voltage will be higher, maybe as high as 18V.

Don't add a resistor so that the load is 200mA. Just measure the power supply voltage and calculate a suitable current-limiting resistor value to limit the current to 90mA.

Most IR LEDs are pulsed so that they don't burn out at currents higher than about 30mA. The max allowed duration of the pulses is shown on the datasheet.

3. ### millwood Guest

most of those smps chargers ARE indeed regulated: tough to make them UNregulated these days.

4. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
If you bought the IR LEDs from Radio Shack, beware of that 100mA rating. That's the peak current at a very low duty cycle, but the duty cycle isn't specified. They'll quickly burn out if you exceed around 25mA.