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I am thinking of buying this UV flashlight.

It runs on either 1 18650 battery (4.2v) or 3 AAA batteries (4.5v). Since the flashlight can already take 4.5v, will it be safe to run on 5v USB?

I found this image of the circuit board inside the flashlight in the reviews if that helps. Flashlight circuit board

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  • \$\begingroup\$ Think about dropping the voltage within the specify battery V range \$\endgroup\$
    – D.A.S.
    Commented Sep 23, 2019 at 18:35
  • \$\begingroup\$ Probably, but it will run hot, and most likely reduce it's already short Chinese life. \$\endgroup\$
    – MadHatter
    Commented Sep 24, 2019 at 2:35

1 Answer 1

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From your picture it looks like the LED driver has some active electronics, most likely a primitive PWM regulator. Which means that it is not critically sensitive to input voltage if it is above the LED voltage threshold.

The specs also say that it will run for 14 hours, which means that it has a 200 mW LED (assuming 3xAAA = 3 W*hr), and it will take something 1000/14 = 70 (x inefficiency) = maybe 100 mA from the source. So it looks like it is pretty safe to run this flashlight from any USB port.

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