Light up LED only when a (large) current flows

Other Hardware General Electronics
1

Hi, I've got a (probably) easy question, but as a newbie I can't fogure out the solution.

I'm building a bench power supply from a PC ATX power supply, and I also added a USB port for powering devices at 5V (e.g. a cellphone). Now I'd like a small LED to light up when something is connected to the USB and is drawing power (e.g. a cellphone while charging). How can I achieve this?

I cannot connect the LED in series (since from this USB I'm going to draw different amount of Ampères, and it could deliver up to 2.5A (not for a cellphone of course)), so it would fry (or draw too less to light up).

I also cannot work with transistors, since the voltages before and after the USB terminals are always the same (+5V and 0V), regardless of a device attached or not...

Any suggestion is welcome, thanks! :wink:

2

One generally measures current flow with a small value (1 Ohm or less) series resistor. The voltage drop across the resistor is proportional to the current flow. An op-amp can be used to boost this tiny voltage to a reasonable level to be read by an analog input.

For example if you use 0.1 Ohms between the ground (-) pin and the load you would see 0.1V across the resistor under a 1 Amp load. If you use an op-amp to boost that by a factor of 20 you will get voltages from 0 to 5V for currents from 0 to 2.5 Amps.

3

Thank you very much for the clear answer :wink: