- Jan 1, 1970
Today, I took some of your suggestions, and decided on trying out a
(8) AA version for a hat requiring only 24 LEDs.
I wired 8 series of 3 LEDs with the same 82 ohm resistors I already
had, which theoretically allowed 25 mA through the LEDs (I compromised
between the 30 mA I was using, and the 20 mA that was suggested). I
used 8 AA batteries because I was able to buy a prebuilt 8 AA holder
from Radio Shack. At 25 mA, in total it used only 200 mA.
Overall, I was pleased with the results. I posted a picture here:http://www.alokw.com/hats/hats.jpg
The only thing that might become a problem is the weight of 8 AA
batteries. It's a bit heavy and may hinder the actors movement with
the hats. I'm going to run this by the costume folk and see what they
Now, assuming the costume department will be okay with the weight,
would I be safe to use 8 AA batteries to run as many as 66 LEDs? That
*should* draw 550 mA at 25 mA per LED. Is this okay? Also, should I
keep using resistors to keep the current at 25 mA? or should I stick
with 20? or 30?
Thanks for all of your help again guys. I wouldn't be able to do it
without you all.
I have another unrelated question. I hooked up my micrometer to the
leads on the 8 AA battery holder to check how much current it was
pushing. I set it to 10A and the display read somewhere around 1.200.
Does that mean it pushes 1200 mA? The snaps on the battery holder also
started smoking. Does that mean I was drawing too much current for the
battery holder to handle? or the snaps? or something else?
Also, I hooked up each of the micrometer leads to an end on one of the
LEDs while it was turned on to try to check how much current it was
actually getting. I assume I did something wrong here, because the
display said 0.05. How would I go about checking this correctly? Or
did I do something else wrong?
Thanks so much everyone!
An ideal ammeter will look like a short, so if you put it across the
batteries, the batteries will be shorted out. While thing can be
entertaining, especially if things smoke, the practice should be
avoided. ;-) You probably popped a fuse in your DVM (which you are
calling the micrometer.)
I don't think 0.58A is a problem for those AA cells. However, this is
a case where alkaline AA cells will probably be at 1/3rd capacity.
This is because alkaline capacity is measured over a range of 1.5V
(i.e. initial) to 0.5V, i.e. where the battery is dead. Your design
probably won't work well once the cell voltage reaches 1V, which is a
third of the capacity. So 2.5AH AA alkaline will have capacity of
about 0.8AH. This is why you should use nicads or NiMH. Their capacity
is rated at the point where the cell voltage is at 1V. So even a
1.5AHr nicad would last twice as long as a 2.5Ahr alkaline.