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### Topic: Power Supply and limit resistor for Infrared LED (Read 1 time)previous topic - next topic

#### Andy R

##### Jun 04, 2010, 04:22 am
What is a good power supply for an Infrared LED, TSAL 7600?
http://www.vishay.com/docs/81015/tsal7600.pdf

I'm using 4 AA batteries that are about 5.5 volts. The LED is in series through a 47 Ohm resistor. It is powered through a transistor to overcome the Arduino's current limit.

How do you tell from the spec sheet what the maximum voltage and limit resistor to use?

#### Andy R

#1
##### Jun 04, 2010, 01:50 pm
Instructables has a circuit http://www.instructables.com/id/Power-LED-s---simplest-light-with-constant-current/, but this seems to be for a much more powerful LED, like something you would wear.

This only needs 200 mA peak.

How much power can 4 AA's make? Has any one played around with different power circuits?

#### Grumpy_Mike

#2
##### Jun 04, 2010, 01:55 pm
200mA is quite a lot of current for an LED. Is the pulsed or continuous?

Quote
How much power can 4 AA's make?

Depends on the batteries you have in the AA format. It can delver the power sure but for how long depends on the mA hours rating of the battery.
A typical rechargeable AA can has a 2000mAH rating so simply you would not expect to exceed 10 Hours. Probably closer to 5 hours.

#### Andy R

#3
##### Jun 04, 2010, 03:44 pm
The batteries are regular, non-rechargeable batteries. They are fairly fresh. I don't have a battery checker, but if the power dropped, the arduino would probably reset.

Is going over 5 volts ok?

If 5.5 volts is ok, this doesn't like a battery problem.

The TSAL 7600 can take 200 mA peak. I'm using NEC format. That uses a 38 MHz carrier. When the signal is high, the wave would be high 1/2 the time. So overall when transmitting it is on around 1/4 the time.

The current through the LED would be: 5.5 volts/50 ohms = 110 mA

That sounds good too. I tried a 25 ohm resistor, but that just made it much worse. (20% success at 10' instead of 80% success at 10', head-on.)

So I don't understand why dropping the resistor caused performance to deteriorate. And it doesn't sound like there is a problem with the amount of power. But maybe the constant current supply is worth doing? Do people get good results without doing this?

Infrared is kind of frustrating. It is hard to tell what is going on.

#### Grumpy_Mike

#4
##### Jun 04, 2010, 04:02 pm
Quote
And it doesn't sound like there is a problem with the amount of power

Well it sounds to me like it is a question of power. If you drop the resistor and it gets worse then it is likely that the extra current through the LED is causing the voltage to droop and that is affecting performance.

Quote
Infrared is kind of frustrating. It is hard to tell what is going on.

If you have a digital camera or a web cam then use it to look at the LED you can see it light up and judge the relative brightness.

#### Groove

#5
##### Jun 04, 2010, 04:02 pm
Quote
That uses a 38 MHz carrier

I really, really do not think so.

#### Grumpy_Mike

#6
##### Jun 04, 2010, 04:04 pm
Haven't we had this conversation once today?

#### Groove

#7
##### Jun 04, 2010, 04:53 pm
Hmmm, now that you mention it...

#### Andy R

#8
##### Jun 04, 2010, 10:20 pm
s/MHz/KHz/p
It uses a 38 KHz carrier.

The 20 Ohm resistor does seem like a lack of power, but it's not clear that the 47 Ohm resistor causes a power problem.

I'll try the webcam.

#### AWOL

#9
##### Jun 04, 2010, 10:49 pm
Quote
It uses a 38 KHz carrier.

sp. "kHz".
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

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