Want to make crummy flashlight brighter using e.g. QX5252

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Joined Sep 17, 2016
I want to make a not very good flashlight a lot brighter. It's got two LEDs and I don't know what the LEDs are rated for but I'm guessing it's 20 mA each (maybe it's not; all I know for sure is that they're cheap, white and 5 mm). At 3 V the current is 7 mA and at 2.7 V (batteries only a bit discharged) it's 2 mA!! I want to use a little wotzit to increase the current to 20 mA and I was previously recommended to use the QX5252 so I bought some.

The datasheet is rubbish but using the "LED power setting" equation I get the following:

The LEDs need 4.1 V to get 40 mA of current to them. That's 164 mW. I'm providing 2.4 V (dropping to ~1.6 V) .

Inductor needed at start = 2 x 2.4 V x 10E-6 / 0.164 W = 29 uH. Is that calculation correct?

When I try to breadboard I get about 8 mA with 470 uH, 12 mA with 33 uH and 17 mA with 1 uH. Should I go lower? How much uH does a piece of wire have (i.e. what would happen if I short out the inductor)? In the breadboard I used a 108 ohm resistor to simulate the LEDs passing 40 mA at 4.1 V.
Last edited:


Joined May 20, 2015
Turn on the light-emitting diodes in series. You are using a homemade choke? I recommend using a winding inductance cylindrical ferrite core and a sufficiently thick wire.