Editing Detecting short circuits (section)

==Short Circuit Protection==

Virtually all power supplies (voltage rail generators) use some form of short circuit protection in the way of an overcurrent protection. The following lines should give a short overview of the most commonly used protection circuits:

The simplest way for overcurrent protection is to use a resistor after the power supply. This is most often used in low power circuitry, where the current draw is very low (e.g. logic signals, low power battery gear like a TV remote). It is extremely reliable and cheap, but terribly inefficient and because of the voltage drop across the resistor unsuitable for rails that require precise voltages. Another problem with resistive overcurrent protection is that even in the event of a fault, the power is not turned off, it is just limited to a maximum value determined by the resistor (that limiting value should be at least four times (!) the nominal power draw for efficiency reasons).

The next method is fusing. Here, a device physically cuts the power if the current draw exceeds a certain value. The main advantage to using fuses is that the fuse physically cuts the power and therefore prevents any further damage to the shorted device, which is a main concern in high power circuitry (like house wiring or the electricity grid). It is used in high power circuits (more than 100{{nbsp}}W), where resistive protection is not economically viable or possible. The main disadvantage is that (most) fuses are single use, meaning after an incident the fuse has to be replaced. There are also resettable fuses (circuit breaker) or self-resetting ones (Poly-Fuse, only for low power!).

Another method of short circuit protection, which is commonly used in switching supplies, is active current limiting. Here, a controller measures the current draw on a supply (rail). If a set current is exceeded, the controller either reduces the voltage (lab power supply) or shuts the supply off (most switching supplies other than lab power supplies). If the supply is shut off, the controller decides if and when the supply should turn on again. '''Most supplies are configured in a way, where the supply shuts off when an overcurrent (short circuit) situation is detected, and turned on again after some time.''' If the short has not been removed, the supply turns off again. This can be heard as a "clicking" sound in higher power supplies (also called "power cycling"), and see on voltmeter as a non-stable voltage output.