# High current load from low current source [closed]

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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• What kind of power supply? What kind of device? In general, in the case you described 100mA will be drawn from a power supply, but your device won't work. However, depending on the parameters of the supply and the load, there may be additional constraints. – Vasiliy Jul 8 '13 at 18:44
• You will bring down the power supply voltage. The rest depends on the design of the 100mA power supply. If it doesn't have over-current protection, it could burn. If there is a fuse, it will open. If there is a crowbar circuit, it will turn off the supply. If there is a μC, it will reset. – Nick Alexeev Jul 8 '13 at 18:46
• You're asking a question about specification abuse. But, you're not providing justification as to why you might get away with this abuse. Broad question. – Nick Alexeev Jul 8 '13 at 18:47
• possible duplicate of Choosing power supply, how to get the voltage and current ratings? – Olin Lathrop Jul 10 '13 at 13:15
• I am sorry for posting the incomplete question.Power supply is regultaed one. – Abhishek Jul 10 '13 at 15:09

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".

• suppose the voltage supply is 5V, but the load requires 10V threshold voltage - below this voltage it's input impedance is infinite. Is your answer applicable to these conditions? – Vasiliy Jul 8 '13 at 19:10
• That is known as brown-out, and some devices do have brown-out detection/handling. Usually the device would enter a "shutdown-mode" and just not turn on until the voltage exceeds the brown-out voltage limit. If not, again we enter the land of Undefined Behavior(tm) and your guess is as good as mine (without further information of the actual source/device). Again, another Bad Idea(tm). – helloworld922 Jul 8 '13 at 19:16
• Indeed. I think there should be no answers unless additional information is provided in the question. – Vasiliy Jul 8 '13 at 19:20