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# Wiring LEDs together

M

#### [email protected]

Jan 1, 1970
0
Hi everyone,

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
think.

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.

E

#### ehsjr

Jan 1, 1970
0
alokw said:
Hi everyone, Thanks again for all of your very helpful replies!

I looked over everything you all said and did some thinking. I like
Ed's (10) AA battery solution and I'm pretty sure we have the space
allowance in the hats for them. However, before I commit to purchasing
the battery holders, resistors, batteries, and such, I want to be sure
that the AA batteries will be able to push 500 mA of consistent
current - I can't have these guys going dim on me. I wasn't able to
find the maximum peak current on any battery manufacturer's web sites.
Do you all know for a fact that 10 AAs can push at least 500 mA?

Groan - the two new pieces of information mean the 10 alkaline
cell solution is not good. The max power rateing for your LEDS
will be exceeded if you run at 30 mA (see below), and the
requirement for no dimming necessitates a constant current supply
and higher voltage, to keep it simple and reliable. It means you
have to add 1 small part to each hat for the constant current
supply - that's the good news. The bad news is that you
need to go to 14 NiCd cells.

You will definitely get dimming, even at 20 mA per LED with
the 10 alkaline cells. To guarantee *completely* avoiding
dimming, the design has to be changed.
Go to 14 AA NiCds, and do this:

-----
+--------Vin|LM317|---+
| + ----- |
- 16.8V | | + + + +
--- +------+--[LED]--[LED]--[LED]--[LED]--+
- |
| |
+----------------------------------------------------+

The LM317 and 62 ohm resistor form a constant current
supply of 20 mA (R would be 42.5 for 29.4 mA)
You need to make the circuit above 14 times per hat, but
need only 1 set of cells per hat. You'll need 14 LM317's
(Mouser # 511-LM317LZ $.28) in the TO-92 package and 14 62 ohm resistors (Mouser # 271-62-RC$.09). So your
real expense will be in the construction time and the NiCd
cost. To save on the NiCds get CAT# NCB-3AA from Allelectronics
http://www.allelectronics.com/
It is 3 NiCd cells (850 mAh each) in series for $2.00. You need 5 of them for 1 hat, which brings you to 15 cells (18 volts) per hat. The same circuit works with either 14 or 15 cells. The cost is now approaching$20 per hat,
but you save on battery holders. You still need a charger.
To make that, you need another x LM317's (where x = the
number of hats) so you can charge the batteries for all the
hats at once, and x 15 ohm resistors, plus a suitable supply
such as stock # 15576 PS from MPJA for \$10.95.
http://www.mpja.com/
It produces 24 volts at 1.6 amps, and you will be charging at
85 mA per hat, so you could charge up to 18 hats at once. A
single charge (14 hours of charge time) will charge the hats
for all 20 performances, but there is no reason not to
recharge after say 5 or 10 performances, and it is a good idea.

The charger circuit (1 LM317 & resistor per hat):
-----
+-------------Vin|LM317|---+
| ----- |
| + | | |
| 24 V | +------+----> To hat battery +
| supply |
| 15576 |
| - |
--------
|
+-------------------------------> To hat battery -
In response to Werty's post, I checked out Harbor Freights web site,
but the only 18vdc batteries I found were drill batteries. If these
are what you were referring to, they definately won't work, just due
to their size and weight. I've listed the exact specs of the LEDs we
have below.

Emitted Colour : WHITE
Size (mm) : 5mm T1 3/4
Lens Colour : Water Clear
Peak Wave Length (nm) : N/A
Forward Voltage (V) : 3.2 ~ 3.8
Typical Voltage (V) : 3.4
Reverse Current (uA) : <=30
Luminous Intensity Typ Iv (mcd) : Average in 13000
Life Rating : 100,000 Hours
Viewing Angle : ±10°
Absolute Maximum Ratings (Ta=25°C)
Max Power Dissipation : 80mw
Max Continuous Forward Current : 30mA
Max Peak Forward Current : 75mA
Reverse Voltage : 5~6V
Lead Soldering Temperature : 240°C (<5Sec)
Operating Temperature Range : -25°C ~ +85°C
Preservative Temperature Range : -30°C ~ +100°C

Should I really try to run these at 20 mA as opposed to the 30 mA that
I've been calculating for?

Yes. The max power dissipation is 80 mw. At 30 ma,
they will dissipate .03*3.2 = 96 mW *minimum* and
up to .03*3.8 = 114 mW. At 20 mA they will dissipate
a maximum of 76 mW at 25 C.

If you decide to accept some dimming and stick with
the 10 Alkaline cell design, you need to change the
resistor to 70 ohms (70.6 ohms is the closest standard value)
You could experiment with one hat to see if the dimming
is acceptable - if not, you'll need to go with more
cells and change to NiCd as above. You could go with
AAA NiCds to save a bit on size/weight at a higher
cost than the AA NiCds from Allelectronics

Ed

R

#### redbelly

Jan 1, 1970
0
Hi everyone,

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
think.

Would it be possible to have the batteries under the actor's shirt or
belt, and run wires up to the hat?
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?

I think you're already at or over your useful limit.
According to Energizer***, the cells will drop to 1.4 V in about 1/2
hour at 250 mA. At that voltage, each LED in your circuit would be
drawing just 12 mA. Since you're now running slightly less current,
175-200 mA, you will get over 1/2 hour with the 24-LED circuit.
Consider going to more (10?) battery cells (and the appropriate
resistor) in order to get less current drop.

At 550 mA, I'd say forget about it. Consider going to C batteries.

*** See graph titled "Constant Current ... 250 mA" on page 2 at
http://data.energizer.com/PDFs/E91.pdf
Also, should I
keep using resistors to keep the current at 25 mA? or should I stick
with 20? or 30?

It seems you need to be in "battery conservation mode" to make this
work, so lower current is better. Because of the way our vision
system perceives brightness, a 50% change does not appear as drastic
as you might think.
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!

Hook up your current meter in series as follows:
________
| +|---[meter]---[resistors & LED's]--,
|battery | |
|pack | |
|_______-|----------------------------------'

Regards,

Mark

R

#### Rich Grise

Jan 1, 1970
0
Hey all!

Thanks for the reply. These LEDs are for a theatre production. They
have to stay on each night for about 1 minute and theres about 20
performances. If I can power them for 45 minutes on one battery, I'll
be happy.

The LEDs are also installed inside hats, so there's not a lot of room
to place a big lead acid battery. I'd like to stay smaller than around
8 cubic inches or so (they're top hats).

Thanks! Let me know if there's any other info that might be helpful. I
really appreciate your help!

How many LEDs per hat? Or are all these hats wired together? It'd seem
simpler if you could have a battery per hat, or are all 54 on one hat
and you need to make multiples?

Thanks,
Rich

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