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MAINS POWER The simplest form of current limiting for mains is a Fuse . As the current exceeds the fuse's limits it blows thereby disconnecting the load from the source. This method is most commonly used for protecting the house-hold mains. A Circuit Breaker is another device for mains current limiting. Compared to circuit breakers, fuses attain faster current limitation by means of arc quenching. Since fuses are passive elements, they are inherently secure. Their drawback however is the single operation principle: once blown, they need to be replaced or retrofitted. IN ELECTRONIC CIRCUITS Electronic circuits like regulated DC power supplies and power amplifiers employ, in addition to fuses, active current limiting since a fuse alone may not be able to protect the internal devices of the circuit in an over-current or short-circuit situation. A fuse generally is too slow in operation and the time it takes to blow may well be enough to destroy the devices. A typical short-circuit/overload protection scheme is shown in the image. The schematic is representative of a simple protection mechanism employed in regulated DC supplies and class-AB power amplifiersǂ. Q1 is the pass or output transistor. Rsens is the load current sensing device. Q2 is the protection transistor which turns on as soon as the voltage across Rsens becomes about 0.65 V. This voltage is determined by the value of Rsens and the load current flowing through it (Iload). When Q2 turns on, it removes base current from Q1 thereby reducing the collector current of Q1. Neglecting the base currents of Q1 & Q2, the collector current of Q1 is also the load current. Thus, Rsens fixes the maximum current to a value given by 0.65/Rsens, for any given output voltage and load resistance. For example, if Rsens = 0.33 Ω, the current is limited to about 2 A even if Rload becomes a short (and Vo becomes zero). With the absence of Q2, Q1 would attempt to drive a very large current (limited only by Rsens, and dependent on the output voltage Vo if Rload is not zero) and the result would be greater power dissipation in Q1. If Rload is zero the dissipation will be much greater (enough to destroy Q1). With Q2 in place, the current is limited and the maximum power dissipation in Q1 is also limited to a safe value (though this is also dependent on Vcc, Rload and current-limited Vo). Further, this power dissipation will remain as long as the overload exists, which means that the devices must be capable of withstanding it for a substantial period. For example, the pass-transistor in a regulated DC power supply system (corresponding to Q1 in the schematic above) rated for 25 V at 1.5 A (with limiting at 2 A) will normally (i.e. with rated load of 1.5 A) dissipate about 7.5 W for a Vcc of 30 Vǂǂ (1). With current limiting, the dissipation will increase to about 60 W if the output is shortedǂǂ (2). Without current limiting the dissipation would be greater than 300 Wǂǂ (3) - so limiting does have a benefit, but it turns out that the pass-transistor must now be capable of dissipating at least 60 W. In short, an 80-100 W device will be needed (for an expected overload and limiting) where a 10-20 W device (with no chance of shorted load) would have been sufficient. In this technique, beyond the current limit the output voltage will decrease to a value depending on the current limit and load resistance. ǂ – ''For class-AB stages, the circuit will be mirrored vertically and complementary devices will be used for Q1 & Q2.'' ǂǂ – ''The following conditions are considered for determining the power dissipation in Q1, with Vo = 25 V, Iload = 1.5 A (limit at 2 A), Rsens = 0.33 Ω (for limiting at 2A) and Vcc = 30 V — ''
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