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London UK
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What is a short circuit and what internally happens when such occurred?
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Try making google and wikipedia friends they help a lot , this is an easy to follow article on short circuit

http://en.wikipedia.org/wiki/Short_circuit

thanks
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Valencia, Spain
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What is a short circuit

Anything less than 5cm is considered short.
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For the Arduino, a short circuit is a wired connection from one circuit to another circuit in which that connection can carry much more current than the designer would approve of. A common short circuit is when a grounded wire touches the 5 volt power supply wire on the Arduino.

Many troubles can happen quickly during a short circuit on Arduino's internal wires. A current of 30 amps might flow through a wire that is too narrow to carry 5 amps. When that short circuit happens, the wires get hot in less than 0.4 seconds. The temperature in the Arduino wire can rise to over 1000 degrees Celsius so that copper wires melt and move the way a liquid moves : surface tension causes the liquid copper to bead up into a spherical shape. That movement of liquid copper causes the wire to disconnect from the nodes which the designer intended to be connected. That is called an open circuit. Current stops flowing through the liquid copper and it cools off, leaving cold balls of copper and a void where copper was previously residing.

Inside of an Arduino silicon integrated circuit, a short circuit can also cause melting or "latch-up", which also causes melting. Vaporization can occur when the temperature of aluminum wires in Arduino chips goes over 2520 degrees C. Vaporization can cause rapid expansion of solid ceramic IC packages and a loud bang may be heard and recorded using a microphone.
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Cumming, GA
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Quote
Anything less than 5cm is considered short.

I laughed so hard I almost spit my coffee...

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Lacey, Washington, USA
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A "short circuit" is any undesired connection, or connection that has a much lower resistance than is desired, or designed for, or is safe.

So an 8 ohm headlight for my car is not a short circuit for the car battery, but it would be if connected to a wall outlet or an Arduino output. Even 100 ohms could be a short circuit if it occurs unintentionally in a high impedance circuit. On the other hand, a short copper bar may not be an open circuit if it is a support on a 2.4GHz antenna, because it is in the right place with the right length.

Two pieces of metal touching, 100A of current flowing, and forming an arc could be a massive short circuit on my house wiring. Or maybe I'm welding, in which case it is not a short circuit any more.

A 3V 250mA flashlight bulb is a short circuit if connected directly to an output on an Arduino. Put a driver transistor with appropriate base resistor in between, and the bulb is no longer a short circuit.

So what constitutes a short circuit is entirely relative.
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Steve Greenfield AE7HD
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A short circuit is an unintentional low impedance current path.

What happens when one occours is at a lot more current flows than is expected normally and things get hot or set on fire.
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