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|---+
| + ----- |
--- Adj [62R]
- 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|---+
| ----- |
-------- Adj [15R]
| + | | |
| 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
