If a device requires 500 mA current (Specification) and let's say I have a power supply with max current of 100mA. What is likely to happen and why.?
closed as unclear what you're asking by Anindo Ghosh, PeterJ, Olin Lathrop, Dave Tweed♦, Nick Alexeev♦ Jul 10 '13 at 17:19
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It depends on the source, and to a lesser extent the target device.
If the source has circuitry to specifically limit the current, that circuitry will kick in if the load exceeds a certain limit, so the device will only receive a smaller portion of the current it "wants", usually causing source voltage to droop.
If the source has Over-Current-Protection, thermal overload protection, etc., it could temporarily or permanently shut off. Resetting the source could require replacing a fuse, waiting for a polyfuse to cool down, flipping a circuit-breaker, waiting for IC automatic timers to reset, etc.
If it has no specific protection circuitry what-so-ever, it would try to source whatever the device asks for. Since is was not specified to handles these loads, we enter the land of
Undefined Behavior(tm). It could work just happily, catch on fire due to thermal overload, have some quasi-output current limit, or some combination of the three (work one day, catch fire another day).
This is just a
Bad Idea(tm). At best your device and source might work, at worst you're going to end up with a damaged equipment, potential fire hazards, and a bad day.
This is by no means a conclusive list of what could happen, but I think covers most "common cases".