# LED+Resistor power consumption

So - I have a requirement for 6 LED IR Emitter/Receiver pairs (for a model rail layout).

The max current for the IR LED is 300ma, so I think "Ah for 6 LEDs I will need 1.8 watts of power".

But then I thinks - what do the resistors consume - and I came across this web site LED Series Resistor Calculator, which tells me that for a 9 volt supply with a 27R resistor, @ 300ma with a forward voltage drop of 1.7 I can expect an overall power consumption of 2.7w - times 6 this is a little over 16 watts! If I reduce my current to the LED to 150ma I still get 6 x 1.35 = a bit more than 8watts.

Just a tad more than the 1.8 watts I had expected.

Am I getting the right end of the stick? - 'cos if so, then the cost of a power supply (if I can find one!) could be a tad OTT for my purposes!

Unless anyone can suggest an alternative to using resistors to limit the current?

Cheers - Richard

P=IV, so with 0.3A and 5V, that is 1.5W.
x 6 = 9W
Resistor will dissipate (Vs - Vf) * I = watts of power.

Got a spec for the device you are using?
300mA might be absolute max, and then you wouldn't to be near that.

IR leds are often built for pulsing…
I found this sentence for one hi current led

(1) Pulse conditions of 1/10 duty and 0.1msec width

Many LEDs have limits like that, not just IR.
300mA seems awfully high tho.

RMurphy195:
So - I have a requirement for 6 LED IR Emitter/Receiver pairs (for a model rail layout).

The max current for the IR LED is 300ma, so I think "Ah for 6 LEDs I will need 1.8 watts of power".

Nope. It's actually much more than that. Watts is (amps x volts).

With a 5V supply you need 5 x 0.3 x 6 = 9 Watts.

More embarrassment - I have confused my watts and amps!

My original sentence should have read "The max current for the IR LED is 300ma, so I think "Ah for 6 LEDs I will need 1.8 amps"."

But the calculation of 2.7W @ 9 volts gives me a 3 amp current - i.e. the cap of a 9v transformer would need to be at least 3 amps. Panic sort of over - in my defence I've working (and trying to concentrate) all day with my neighbour's dog whining and howling 'cos they are out, and that does spoil my concentration somewhat!

Have looked again at putting LED's in series (I'd prefer not to for practical reasons), and the practicalities of doing so for my environment, plus running the LED's at 150ma (need more testing).

Thanks for all your replies - Richard

Well, 6 * 1.7V = 10.2V, so that would be feasible with a 12V supply assuming you needed all of them on all the time, and the receive side is where the action is.
http://www.dipmicro.com/store/DCA-1210
Connector options if you don't want to hack the end off:
http://www.dipmicro.com/store/KLDX-0202-A

You have failed the first requirement for posting here - you have not cited the datasheet for the IR LEDs.

The fact of the matter is that you would not be wanting to operate the IR LEDs at 300 mA, 30 mA would be as much as you need. As others have noted, the 300mA is most likely the maximum pulsed current limit, not the normal operating current by any means.

If you can cite the link to the datasheet, we might be able to make some sense of it all.

Datasheet attached - please explain the difference “Forward Current” v “Pulsed Forward Current”, and how I would recognise this from the datasheet.

Many thanks -Richard

UPDATE - Just came across this link http://www.ddpleds.com/engineered/glossary.php#12 which answers my “Pulsed” question.
Thank you for the tip Paul B - My “Electronics for Dummies” book doesn’t go quite this far!

IR-TSUS5400 IR Emitter.pdf (64.9 KB)

OK, so in fact the maximum forward current is specified as 150 mA and all other specifications are based on 100 mA. It would be reasonable if in fact you need to, to operate them at 100 mA (though 50 mA would likely suffice - just how do you propose to locate these sensors?), in which case you can make use of the maximum specified forward voltage of 1.7V and wire them in series pairs to have a voltage drop of 2.5V or so. You could try them out in pairs, with a 27 ohm resistor to a known 5V supply and see what the voltage across the pair comes to, if 2.5V or less then you could use actually series chains of three. This is practical as infra-red LEDs have the lowest voltage drop of any "colour".

In any case, you should have no trouble finding a few hundred milliamps to supply them - again if you really need the full 100 mA each - but you do not want to draw this current from the Arduino 5V regulator. If you have another source, you just calculate the appropriate series resistor. A 12V supply could supply all six IR LEDs in series.

Problem solved now Paul, thankyou

RMurphy195:
The max current for the IR LED is 300ma, so I think "Ah for 6 LEDs I will need 1.8 watts of power".

I think that all problems comes from here.

1 LED = 300mA, so for all 6 you need 1800mA total, not 1.8watts.

P = I*V, if the LED forward voltage = 2V, you need P = 2.0V * 1.8A = 3.6W.