Running a torch from USB??

Hi guys,

I've got a 9 led torch - it takes 3x aaa batteries. So 4.5 volt?

I want to get it running from a USB cable.

I tried today - I used a USB to mini usb cable - the black and red wires.

The torch lit up -- but the LEDs were CRAZY bright - started to flicker, and the touch was getting hotter and hotter. When I tried it again with batteries. Only 2 LEDs work now.

So... I've got a new torch

I don't understand how to get my project to work. USB is 5 volts - torch is 4.5 volts. Why did the torch overheat?? I'm using an iphone USB plug for power.

How can I make it work? Any help would be great

Thankyou.

Well, maybe those batteries are actually wired in parallel, and you're actually driving your LEDs with 1.5V, Now when you connected it to 5V...

itsallgood said:

I don't understand how to get my project to work. USB is 5 volts - torch is 4.5 volts. Why did the torch overheat??

Click to expand...

You learned that a cheap Chinese flash... er torch uses the internal resistance of the little battery cells to limit the current. Powered with USB the voltage was too high and therefore since the LEDs are diodes, not incandescent light bulbs, then the current was much too high.

Guessing that the internal resistance of cheap Chinese carbon-zinc AAA cells drops the loaded voltage to 4.0V and guessing that the LEDs work well at 20mA then you add a series resistor that is 1V/20mA= 50 ohms (use 47 ohms or 51 ohms) or try many different values until the brightness is normal.

My cheap Chinese flash... er torch also uses 3 cheap Chinese AAA carbon zinc cells but it has 24 LEDs that are extremely bright. I betcha it blows up when I replace the battery cells with modern alkaline AAA cells that have a very low internal resistance.

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barry said:

Well, maybe those batteries are actually wired in parallel, and you're actually driving your LEDs with 1.5V, Now when you connected it to 5V...

Click to expand...

Some red LEDs are only 1.5V. Have you ever seen a red torch? Of course a burning torch is red. I mean have you ever seen red LEDs in a white flashlight?

I took apart my flashlight and see that the 3 AAA cells are in series and it measures 4.47V turned off. With 24 LEDs lit the battery voltage drops to only 3.7V but it is dropping more very quickly.