Which Starting System Component Uses A Small Amount Of Current To Control A Large Amount Of Current?

Current limiting is the practice of imposing a limit on the current that may be delivered to a load to protect the circuit generating or transmitting the current from harmful effects due to a short-circuit or overload. The term "electric current limiting" is likewise used to define a type of overcurrent protective device. According to the 2020 NEC/NFPA 70, electric current limiting overcurrent protective device is divers every bit, "A device that, when interrupting currents in its electric current-limiting range, reduces the electric current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid usher having compatible impedance."

Inrush current limiting [edit]

An inrush current limiter is a device or group of devices used to limit inrush current. Passive resistive components such as resistors or negative temperature coefficient (NTC) thermistors are simple options, with ability dissipation and cool-down time being their main drawbacks, respectively. More complex solutions using agile components tin can exist used when simpler options are unsuitable.

In electronic power circuits [edit]

Agile current limiting or curt-circuit protection

Some electronic circuits employ active current limiting, since a fuse may not protect solid-state devices.

One way of current limiting excursion is shown in the epitome. The schematic is representative of a uncomplicated protection mechanism used in regulated DC supplies and class-AB ability amplifiers.

Q1 is the pass or output transistor. R_sens is the load current sensing device. Q2 is the protection transistor which turns on as before long as the voltage beyond R_sens becomes nearly 0.65 Five. This voltage is adamant past the value of R_sens and the load current through information technology (I_load). When Q2 turns on, it removes base current from Q1 thereby reducing the collector electric current of Q1. which is very about the load current. Thus, R_sens fixes the maximum current to a value given by 0.65/R_sens. For instance, if R_sens = 0.33 Ω, the current is limited to about 2 A even if R_load becomes a short (and V_o becomes zip).

Further, this ability dissipation will remain every bit long equally the overload exists, which ways that the devices must exist capable of withstanding it for a substantial menstruation. This ability dissipation will exist substantially less than if no current limiting excursion had been provided. In this technique, across the current limit the output voltage will subtract to a value depending on the current limit and load resistance.

V-I curves for voltage regulators with different overload handling: foldback, constant current limited, and unlimited.

To reduce the heat that must exist dissipated by the pass devices under a short-circuit, foldback electric current limiting is used, which reduces electric current in the short-circuit case. Under a short circuit, where the output voltage has reduced to zero, the current is typically limited to a small fraction of the maximum current.

Power dissipation vs. load resistance for linear voltage regulators with different overload treatment. Here V _in = 12 V, Five _OC = 10 V, I _max = ane A, I _SC=0.17 A. The maximum dissipation in the foldback pattern is 3 times lower than in the constant current limited design.

The prime purpose of foldback electric current limiting in linear power supplies is to keep the output transistor within its safety power dissipation limit. A linear regulator dissipates the departure between input and output voltages equally rut. Nether overload conditions the output voltage falls and so the deviation becomes larger, thus increasing dissipation. Foldback helps to keep the output transistor within its safe operating area under fault and overload weather condition. Foldback also significantly reduces the power dissipation in the load in fault conditions, which tin reduce the risks of fire and rut damage.^[1]

Many ability supplies employ constant current limiting protection; foldback goes ane step farther by reducing the output current limit linearly as output voltage decreases. However it adds complexity to the ability supply and can trigger "lockout" conditions with non-ohmic devices that depict a abiding current independent of supply voltage (such as op-amps). A foldback electric current limiter may besides use a transient filibuster to both avoid lockout and limit localized heating at the brusk circuit.

A switched-style ability supply operating at the current limit with the output curt circuited does non have increased power dissipation in the ability transistor(s), and so foldback current limiting is an application feature simply rather than one that also prevents a load fault from also destroying the power supply. The safe benefit of reducing the power delivered to a brusque circuit in the load is proportional to the operating current limit. Foldback current limiting is most likely to exist plant in a switchmode power supply when it is a component in a product that is independently certified to see regional safety standards.^[2]

The inrush electric current of an incandescent lamp causes a bench power supply to limit its output current with a foldback current limiter.

Unmarried power-supply circuits [edit]

An issue with the previous circuit is that Q1 will not be saturated unless its base is biased about 0.5 volts to a higher place V_cc.

These circuits operate more efficiently from a single (Five_cc) supply. In both circuits, R1 allows Q1 to plough on and pass voltage and electric current to the load. When the current through R_sense exceeds the pattern limit, Q2 begins to plough on, which in plough begins to turn off Q1, thus limiting the load electric current. The optional component R2 protects Q2 in the event of a short-circuited load. When V_cc is at least a few volts, a MOSFET can exist used for Q1 for lower dropout voltage. Due to its simplicity, this circuit is sometimes used as a current source for loftier-power LEDs.^[3]

Electric current limiter with NPN transistors (Vo output is located at like location as PNP example)
Electric current limiter with PNP transistors

References [edit]

^ Paul Horowitz, Winfield Loma, The Art of Electronics Second Edition, Cambridge University Press, 1989 ISBN 0-521-37095-7, p.316
^ Keith H. Billings (1999). Switchmode power supply handbook. McGraw-Hill Professional. p. 1.113. ISBN978-0-07-006719-half dozen.
^ "The New Stuff!!! Constant Current Source #1". Instructables. Retrieved 4 July 2012.