The attached schematic is the power supply portion of a larger project. The active component is a DC-DC converter that puts out +5 and -5v DC and a ground reference (COM). These voltages are tapped at the indicated +5V and -5V points to run the rest of the project. The +5v and COM outputs of the converter are also connected to a USB cable that can plug into and power an external device drawing 100mA through the V+ and V- termination points on the far right of the schematic. When the external device is not connected, there is no current flowing between these two termination points.

I would like to figure out a way to turn on an LED when current is flowing to the external device (i.e., between the V+ and V- termination points) but I cannot put the LED in series with the load because I need the full 5 volts (or close to it) to power the external device, and, of course, an LED can't take 100mA through it. Is there some kind of active device that could be put in series with the load and not drop the voltage (much) but would produce a signal when current is flowing through it? Or some other way to accomplish the goal?

Thanks,
Mac

 

A tiny resistor in series to V+ will produce a tolerable tiny voltage drop across it that can be amplified by a transistor or opamp to turn on a LED.

Image "2 of 3" has a schematic here :
----> http://www.electroschematics.com/9335/current-sensor-led-indicator/

 

That's the ticket! Thanks.

 

There are lots of players in this game, TI, Linear, Maxim, ST, Zetex and more, all make dedicated current sense amps. Pretty cool things but since they are new they only come in SMD packages. (Since it's 2017 you should work out ways to use them in any build.)

TI has one, INA301, with a built comparator. Only spec'd for 3mA drive which is a little lite to drive as LED but OK to drive a transistor to buffer a LED.

 

...current sense amps.

Cool. I'd never heard of such a thing. It would certainly reduce the component count.

Thanks,
Mac

 

For the thread... TI has a very extensive training module on current sense amplifiers.

 

For others who may read this thread in the future ...
Two other options. Specifics of both designs will depend on your circuit but design needs are specified.

 

You can connect two rectifier diodes in series (such as 1N4001) anode to cathode, and then a red high-intensity LED from end to end of the series-connected diodes. This would divert the 100 mA mostly through the rectifier diodes, but you lose a drop of voltage of about 1.5V by inserting this sensor assembly in the circuit.

This technique comes from Dave Johnson (discovercircuits.com) applied to a sensor for current from 120 VAC. If interested, the thread starter might search "Lamp current Indicator" at his site.

Regards,
Pete