EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: paulca on November 14, 2017, 08:38:51 pm
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This is probably something a beginner could build themselves, though I'm not quite sure how to yet.
I imagine a device which has various input connectors (USB, Round, Headers) and various output connectors (USB, Headers, Breadboard pins).
It would have a jumper or switch which can be either 0.5A or 1A.
Should the current through the device exceed that current a buzzer sounds and an LED lights.
It sounds completely mickey mouse, but it would instantly point out to the a newbie that:
a) they have a short circuit
b) they are pulling too much power from the wall wart PSU.
If you could package this (if it doesn't exist) into a simple off the shelf component I think it would be really useful.
Anything pulling enough amps to heat stuff up will sound the buzzer the instant they connect it or turn the power on.
EDIT: I'm pretty sure it's just an op amp with a current shunt or sense resistor type thing.
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Yes, you need shunt and comparator, perhaps amplifier after the shunt, possible inspiration is here http://www.electronicdesign.com/sites/electronicdesign.com/files/uploads/2014/05/0714_POV_harmon_F2.gif (http://www.electronicdesign.com/sites/electronicdesign.com/files/uploads/2014/05/0714_POV_harmon_F2.gif)
Very very crude way of indicating overload may be based on something like this https://snag.gy/miAchB.jpg (https://snag.gy/miAchB.jpg)
R1 determines the current "limit", it happens when the voltage drop is roughly equal to 0,7V.
This circuit is temperature dependant (cca 2mV/K of BE voltage, at 1R resistor it means 2mA/K), has significant voltage drop (0,7V at current of power limit) and the current "limit" indication isn't straight bi-stable, but it's brain-dead simple and sometimes does the job. Variants of this circuit were common decades ago, often with thyristor instead of transistor - then the overcurrent indicator doesn't go off until power supply is interrupted, what may be desirable sometimes.
The sense resistor and comparator is technically better way of doing this task, basically eliminating all of the problems of this simple circuit, at marginally higher cost and complexity.
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Polyfuse and a P channel MOSFET. Source to PSU side of MOSFET, gate to load side and buzzer and indicator lamp between drain and Gnd. Jumper in a second polyfuse to double the current limit.