A friend purchased a microcontroller-controlled lighting device and in the process of testing it, shorted the VCC and GND together for a second or two. Coincidental or not, the device stopped working thereafter, and he asked me if shorting the power could have been the cause.

It is my understanding that such a short could only really pose a threat to the battery or power supply (or the wire involved on the power supply side), given lack of current-limiting options or a fuse. The device should only experience power loss.

Can a power supply short cause a small circuit to permanently fail? If so, how?

I explained that the circuit might have failed because of the nature of the short: two wires making brief, intermittent contact. It might have created repeated instances of high inrush current which normally only happens once during power-on. Though, I doubt this theory because there shouldn't be much inrush current on subsequent power-on "events" as not enough time has elapsed for capacitors to discharge, etc.

  • \$\begingroup\$ Would the lighting device have some form of power / voltage regulator or some fuse? Would you be able to check if some such component has failed? \$\endgroup\$ – Anindo Ghosh Jun 16 '13 at 6:31
  • \$\begingroup\$ No, I checked the device out and it literally had no form of voltage regulation, polarity protection, etc. The microcontroller on it (an Attiny45) appeared to be dead. The seller of the device supplied a 3.6V battery pack, and included explicit instructions to never exceed that voltage. \$\endgroup\$ – JYelton Jun 16 '13 at 6:36
  • \$\begingroup\$ The ATtiny45 is rated for 1.8-5.5V (ATtiny45V) or 2.7-5.5V (ATtiny45), so it is probably not dead due to overvoltage - and it certainly does not die even if exposed to extreme undervoltage (stops operating till voltage comes back up). One possibility is that the ATtiny's load drew too much inrush too many times in a row. \$\endgroup\$ – Anindo Ghosh Jun 16 '13 at 6:49
  • \$\begingroup\$ So my theory might be correct? I began to wonder if inrush current was not the culprit if the events happened in such a short time frame. \$\endgroup\$ – JYelton Jun 16 '13 at 6:51
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    \$\begingroup\$ Batteries tend to have pretty high parasitic inductance. I'm betting that when the short was fixed, the battery supplied voltage way above the rated 5.5V. \$\endgroup\$ – avakar Jun 16 '13 at 6:58

A fast drop of DC input power can kill devices if they are not specifically designed to withstand this case. There are two main causes of damage:

  1. Current was actually running backwards thru a path it was only ever meant to run forwards thru. Note that just disconnecting the power doesn't cause backwards current flow, but actively shorting it can and probably does. Many linear regulators, including the otherwise rock solid 7805, are damaged when the output is held at a higher voltage than the input. Since the output of a linear regulator will have some capacitance on it, it will momentarily stay the same voltage when the input is suddenly driven to 0 by the short.

  2. Startup problems when some internal voltages exceed the power voltage as it comes back up. Think of any capacitance on the pin of a IC. If the power is suddenly brought to 0, that capacitance will discharge thru the IC pin, thru its positive protection diode, to the supply that is now at 0. If the supply comes back up while this current is still flowing, the whole chip can go into SCR latchup. That causes it to draw lots more current than designed for, and will often kill it or the linear regulator feeding it as a result.

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  • \$\begingroup\$ similar to 2, but the act of shorting can come with sparking, which can generate LARGE voltages well and truly beyond anything the device was designed to accept in the first place. This is especially true if it was a human using a probe to touch something, as the spark usually causes the person to jump a bit and cause even more sparking. \$\endgroup\$ – akohlsmith Jun 17 '13 at 4:22

The conversational scenario of "The VCC was shorted to GND for about a second" can be an unreliable report of the full extent of what actually happened. There are many ways this accidental short may have happened and as such there may have been other improper connections going on just before, during or after the communicated event.

Take for example - The friend was using a proto plug board with many open component leads. A simple wrong motion with a hand can result in things being shorted in unexpected ways.

enter image description here

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