Hmmm... what's the white thing inside the battery case?

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It looks like...

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A red-black-black

resistor!!

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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:

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And, of course, the

current limiting produces heat in the

resistor. I can measure this with a very handy tool:

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Ambient temperature on my

desk is 24.5C:

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The

resistor, after running for only a minute or so, has already risen to 70C and is still rising as I type this:

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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. *