Hmmm... what's the white thing inside the battery case?
View attachment 18496
It looks like...
View attachment 18497
View attachment 18498
A red-black-black
resistor!!
View attachment 18499
It's 20 ohms. That's the
current limiting for the LEDs.
The batteries are delivering 4.5V, the
resistor provides a 1.5V
drop, resulting in 3V on the string of LEDs:
View attachment 18502
View attachment 18503
View attachment 18504
And, of course, the
current limiting produces heat in the
resistor. I can measure this with a very handy tool:
View attachment 18505
Ambient temperature on my
desk is 24.5C:
View attachment 18506
The
resistor, after running for only a minute or so, has already risen to 70C and is still rising as I type this:
View attachment 18507
Reminder: the boiling
point of water is 100C.
Power resistors can handle considerably higher temperatures than that. This is why the
resistor is packaged inside a thermally protective white
braid sheath (see first pic above).
Now, the math!!! Woo hoo!!
Ohms law to find the
current draw:
I = V/R
I = 1.5V / 20 ohms = 0.075A = 75mA
And we also want to know how much
power, in Watts, is being dissipated in the
resistor:
W = VA
W = 1.5V x 0.075A = 0.1125W
That's just over 1/10th of a
Watt in heat. Judging by the size of the
resistor, it's most likely rated for 1/2W (0.500W), so there's plenty of
headroom there. It's hot, but it's inside a
sheath inside a container. It's relatively safe.
Want to run this string at 12V instead? The
LED voltage will always be 3V, that's the Vf, or Forward
Voltage specification of the LEDs. The V we use in our
ohm's law calculation is the
drop from supply down to 3V. In this case, we solve for R:
R = V/I
R = (12V - 3V) / 0.075A
R = 9V / 0.075A
R = 120 ohms
W = VA
W = 9V x 0.075 = 0.675W
Using a 12V
power supply or battery, you need a 120
ohm 1W
resistor, protected to avoid fire and/or injury. You'll be dumping more than 2/3 of a
Watt in heat.
I hope this helps.
Jim
RC4
* I have written all of this very quickly, and I cannot be responsible for errors or omissions. Every attempt has been made to provide accurate information. Math should be double-checked and real-world tests should be run with caution to ensure all is well. *