Current limiting
Current limiting is the practice in electrical or electronic circuits of imposing an upper limit on the current that may be delivered to a load with the purpose of protecting the circuit generating or transmitting the current from harmful effects due to a short-circuit or similar problem in the load.
An inrush current limiter is a device or group of devices used to limit inrush current. Negative temperature coefficient (NTC) thermistors and resistors are two of the simplest options, with cool-down time and power dissipation being their main drawbacks, respectively. More complex solutions can be used when design constraints make simpler options infeasible.
Electronic circuits like regulated DC power supplies and power amplifiers employ, against 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 amplifiers.
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 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 and 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.
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