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.
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:
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
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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.
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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 -
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
Would it be possible to have the batteries under the actor's shirt or belt, and run wires up to the hat?
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
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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.
Hook up your current meter in series as follows: ________ | +|---[meter]---[resistors & LED's]--, |battery | | |pack | | |_______-|----------------------------------'
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