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A Quick LED Question...

HobbyElec

Jul 14, 2015
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Hello, newbie here,

I've got a small project involving LEDs, and need help finding the right resistor/battery combo. If this has been asked before or there is a good thread already explaining this concept, please let me know.

I need to wire 2 LEDs in parallel, each with their own switch so they can be turned on independently from each other. The LEDs I will be using are here.

The problem is that I don't know which resistor to use with it.

I would like the circuit to be powered by a AA, AAA, 9V or similar household battery.

If you could explain what level resistor to use with what battery (and maybe post a link of the resistor from this electronics store) I would be very grateful. Also if you could explain in your response how you got to the answer (V=IR and all that) that would be great.

Thanks in advance
 

Alec_t

Jul 7, 2015
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Welcome to EP!
The LED in the link has a typical (it always varies from one example to another) forward voltage (Vf) of 2.3V and a current rating of 20mA.
Because of Vf variations it is not good to wire LEDs directly in parallel (one will tend to hog the current and so be brighter or have a shorter life). So each LED needs its own current-limiting resistor, unless 2 or more LEDs are connected in series, in which case the series string needs a single resistor. Such resistor-LED strings can be connected in parallel if required.
In your case there will be 2 resistors (1 per LED).
Since the Vf is about 2.3V you will need at least 3V to drive it (this allows a bit of 'headroom' for when the battery runs down). So, 2 AA alkaline cells do the job. It's not advisable to run LEDs at their maximum rated current (unless you don't mind them having a short life), so let's aim for a current of, say, 15mA. 3V-2.3V = 0.7V needs to be dropped across the series resistor, so its value is 0.7V / 0.015 Amps = 467Ω. The nearest standard (E12) value is 470Ω.
The resistor will dissipate about 0.1Watts, so a 1/4 Watt rated one is what you want. Jaycar don't list one.
 
Last edited:

HobbyElec

Jul 14, 2015
2
Joined
Jul 14, 2015
Messages
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Thanks for your help Alec,

Since the local electronics store doesn't supply the resistor we need, I figure I'll get it all on Amazon. I'll try and follow your working with these LEDs:

LEDs have a forward voltage of 1.9-2/1V and 20mA current.
We'll have 3V powering it, and 15 mA again. 3V-2.0V=1V to be dropped by the resistor.
Value is 1V/0.015mA = 66.6Ω. We can use a 68Ω resistor in that case, 1/4W again. This resistor appears to be suitable.

So will this combination of LED and resistor work?

Just to check that I followed your description of the circuit layout, is this how the circuit could be laid out?

___________ 3V Battery _______________
|PLEASE I GNORE THIS TEXT PLEASE|
|PLEASE IGNORE THIS TEX T PLEASE|
|PLEASE IGNORE THIS TEX T PLEASE|
|__ Switch __ 68Ω Resistor __ LED ___|
|PLEASE I GNORE THIS TEXT PLEASE|
|PLEASE I GNORE THIS TEXT PLEASE|
|PLEASE I GNORE THIS TEXT PLEASE|
|__ Switch __ 68Ω Resistor __ LED ___|
 

Alec_t

Jul 7, 2015
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Oops! My bad. In post #2, 467 and 470 should read 46.7 and 47 respectively.
is this how the circuit could be laid out?
Spot on!
 
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