Posted on: July 1, 2014
Current limit techniques are used to control the amount of current flowing in a circuit. Limiting ensures that a safe amount of current flows throughout the circuit and a protection method is activated if this is exceeded. The current limiting devices and circuits may take various forms depending on the circuit sensitivity, amount of current, the response time and possible causes of the excess current.
An excess current may flow in a circuit due short-circuited components such as diodes, transistors, capacitors or transformers. Other causes include fault in the load (overload), an external object shorting the circuit parts, or an over voltage at the input power terminals.
Current limiting components
The choice of limiting device depends on several factors. Passive and active components may be used individually or as a combination. Below are the commonly used limiting components, usually connected in series with the load.
Fuse and Resistors
These are used for simple limiting of current. The fuse will normally blow if its rated current is exceeded.
Resistors are integrated in the design of the circuit and by using Ohm’s law I=V/R (where I is the current, V the voltage and R the resistance), the correct value of resistance can be obtained. Different physical sizes are available to suit any power dissipation requirements.
Circuit breakers are used to cut off power just like the fuse, but their response is slower and might not effective for sensitive circuits.
Negative Temperatures Coefficient (NTC) thermistors are used to limit the initial surge currents that flow when a device is connected to power. The thermistors have high resistance when cold and a low resistance at high temperatures. The NTC limits inrush current immediately the circuit is switched. When current starts to flow, its temperature rises and resistance decreases so that it is able to pass sufficient current.
Transistors and Diodes
Regulated power supply units use active current limiting circuits such as integrated circuits, transistors and diodes. The active circuits are suitable for sensitive circuits and responds by reducing the current or shutting down the supply to the affected short circuited section or the whole system.
Current limiting diodes
Current limiting diode (CLD), also known as constant current diode is used to limit or regulate the current over a wide range of voltages. The two-terminal current limiting device comprises of a JFET with a gate shorted to the source. It maintains a current constant irrespective of voltage variations, similar to how a zener diode maintains a constant voltage.