[ot] [sdiy] Battery questions

[Tim Parkhurst]

Hi Robot,

 

Some real basic LED info (my apologies if you already know this). The
battery life will depend on how much current you’re drawing from it. To
figure out how much current you’ll need to light your LED, you need to find
out a couple of things: 1) the forward voltage drop across the LED and 2)
the maximum current the LED will take. Both of these can usually be found in
a spec sheet for the LED. There are a couple of ‘general’ assumptions you
can make to get you in the ballpark: The forward voltage drop is usually
around 1.7V. Some LEDs drop as much as 2.2V, so again, check the spec sheet
if at all possible. Second assumption – most LEDs will take a MAXimum of
20mA (0.020A) current. With that in mind, you can figure out what resistor
you’ll need to run the LED.

 

R = Vbatt-Vled / Iled (this is just Ohm’s law R = V / I, but remember we
have to subtract the voltage drop across the LED)

 

For a 3V source (two batteries in series), and running 10mA (usually gives
good brightness), we get

 

R = 3 – 1.7 / 0.010

R = 1.3 / 0.010

R = 130

 

So a 130 Ohm resistor and a three volt supply will give you 10mA through the
LED. You could probably get away with even less current. It depends on the
LED. Some high efficiency LEDs will have a lower voltage drop and will give
very good light levels with at very low currents. 

 

After you know how much current you need to run through the LED, THEN you
can check the mAH rating of the batteries. This is where I’m a little
unclear – so perhaps a battery guru can step in. Basically, mAH is a rating
of how much current the battery can supply over time. I don’t know how to
apply this at this point, but this can probably be found out with a little
Googling (funny how that word has become a verb).

 

 

Tim (needs more verbs in his life) Servo

"Imagination is more important than knowledge." - Albert Einstein

-----Original Message-----
From: Robotboy8 at aol.com [mailto:Robotboy8 at aol.com] 
Sent: Monday, January 17, 2005 9:06 AM
To: synth-diy at dropmix.xs4all.nl
Subject: [ot] [sdiy] Battery questions

 

Two questions.  Firstly, how long can I expect two AA batteries to last when
connected to a single LED (no other circuitry)?  And will the color of the
LED matter?  Secondly, what the heck is a UM-2 battery?  I have an old tape
recorder that calls for four um-2's, they seem to be the same size as "C"
cells, are they the same thing?

       -eric

 

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