RS-485: Imported from Wikipedia (overwrite)

2026-05-02T18:00:43Z

Imported from Wikipedia (overwrite)

New page

{{Short description|Safety feature in electrical engineering}}

'''Current limiting''' is the practice of imposing a limit on the [[electric current|current]] that may be delivered to a [[Electrical load|load]] to protect the circuit generating or transmitting the current from harmful effects due to a [[short-circuit]] or overload. The term ''current limiting'' is also used to define a type of overcurrent protective device. According to the 2020 NEC/NFPA 70, a current-limiting overcurrent protective device is defined as, "A device that, when interrupting currents in its current-limiting range, reduces the 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 conductor having compatible impedance."

==Inrush current limiting==
{{Main|Inrush current limiter}}

An [[inrush current limiter]] is a device or combination of devices used to limit inrush current. Passive resistive components such as [[resistors]] (with power dissipation drawback), or negative [[temperature coefficient]] [[thermistors]] are simple options, while a positive temperature coefficient thermistor is used to limit current afterward, as the circuit has been operating (with cool-down time drawback on both). More complex solutions using [[Active device|active component]]s can be used when more straightforward options are unsuitable.

==In electronic power circuits==
[[File:Locale_RS6_Использование блока ограничителя тока.gif|thumb|right|200px|Active current limiting or short-circuit protection]]
Some electronic circuits employ active current limiting since a fuse may not protect solid-state devices.

One style of current-limiting circuit is shown in the image. The schematic represents a simple protection mechanism used in regulated DC supplies and [[Linear amplifier|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 the base current from Q1, thereby reducing the collector current of Q1, which is nearly the load current. Thus, Rsens fixes the maximum current to a value given by 0.65/Rsens. 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).

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. This power dissipation will be substantially less than if no current-limiting circuit had been provided. In this technique, beyond the current limit, the output voltage will decrease to a value depending on the current limit and load resistance.

[[File:Voltage regulator foldback.svg|thumb|V-I curves for voltage regulators with different overload handling: foldback, constant current limited, and unlimited.]]

To reduce the heat that must be dissipated by the pass devices under a short-circuit, ''foldback'' current limiting is used, which reduces 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.

[[File:Voltage regulator linear foldback.svg|thumb|Power dissipation vs. load resistance for linear voltage regulators with different overload handling. Here ''V''in = 12 V, ''V''OC = 10 V, ''I''max = 1 A, ''I''SC=0.17 A. The maximum dissipation in the foldback design is three times lower than in the constant current limited design.]]

The prime purpose of foldback current limiting in [[Linear power supply|linear power supplies]] is to keep the output [[transistor]] within its safe [[power dissipation]] limit. A linear regulator dissipates the difference between input and output voltages as heat. Under overload conditions, the output voltage falls, so the difference becomes larger, thus increasing dissipation. Foldback helps to keep the output transistor within its [[safe operating area]] under [[fault (power engineering)|fault]] and [[electrical overload|overload]] conditions. Foldback also significantly reduces the power dissipation in the load in fault conditions, which can reduce the risks of fire and heat damage.<ref>Paul Horowitz, Winfield Hill, ''The Art of Electronics Second Edition'', Cambridge University Press, 1989 {{ISBN|0-521-37095-7}}, p.316</ref>

Many power supplies employ [[constant current]] limiting protection; foldback goes one step further by linearly reducing the output current limit as output voltage decreases. However, it adds complexity to the power supply. It can trigger ''lockout'' conditions with non-[[Ohm's law|ohmic]] devices that draw a constant current independent of the supply voltage (such as op-amps). A foldback current limiter may also employ a transient delay to avoid lockout and limit localized heating at the short circuit.

A [[switched-mode power supply]] operating at the current limit with the output short-circuited does not have increased power dissipation in the power transistor(s), so foldback current limiting is an application feature only, rather than one that also prevents a load fault from also destroying the power supply. The safety benefit of reducing the power delivered to a short circuit in the load is proportional to the operating current limit. Foldback current limiting is most likely to be found in a switch-mode power supply when it is a component in a product that is independently certified to meet regional safety standards.<ref>
{{cite book
| title = Switchmode power supply handbook
| author = Keith H. Billings
| publisher = McGraw-Hill Professional
| year = 1999
| isbn = 978-0-07-006719-6
| page = 1.113
| url = https://books.google.com/books?id=RwAmBvRzA3EC&dq=foldback+power-supply&pg=PT115
}}</ref>
[[File:Locale_RS6_Lamp inrush current.png|thumb|The inrush current of an incandescent lamp causes a bench power supply to limit its output current with a foldback current limiter.]]

==Single power-supply circuits==
An issue with the previous circuit is that Q1 will not be saturated unless its base is biased about 0.5 volts above V{{sub|cc}}.

These circuits operate more efficiently from a single (V{{sub|cc}}) supply. In both circuits, R1 allows Q1 to turn on and pass voltage and current to the load. When the current through R_sense exceeds the design limit, Q2 turns on, which in turn begins to turn off Q1, thus limiting the load current. The optional component R2 protects Q2 in the event of a short-circuited load. When V{{sub|cc}} is at least a few volts, a [[MOSFET]] can be used for Q1 for lower dropout voltage. Due to its simplicity, this circuit is sometimes used as a [[current source]] for high-power LEDs.<ref>{{cite web|title=The New Stuff!!! Constant Current Source #1|url=http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/step6/The-new-stuff-Constant-Current-Source-1/|publisher=Instructables|accessdate=4 July 2012}}</ref>

<gallery>
File:Ограничение извести с помощью NPN-транзисторов.png|Current limiter with NPN transistors (''Vo'' output is located at similar location as PNP example) 
File:Ограничение извести с помощью PNP-транзисторов.png|Current limiter with PNP transistors
</gallery>

==See also==
* [[Current source]]
* [[Crowbar (circuit)]]
* [[Electrical ballast]]
* [[Fault current limiter]]

==References==
{{Reflist}}

== External links ==
* [http://www.cs.uiowa.edu/~jones/step/current.html Current Limiting for Stepper Motors]
* [http://english.cxem.net/calc/ledcalc.php Current limiting resistor calculator for LED arrays]
* [https://www.electronics-notes.com/articles/analogue_circuits/power-supply-electronics/current-limiter-circuit.php Constant current & foldback current limiting]

{{Authority control}}

{{DEFAULTSORT:Current Limiting}}
[[Category:Analog circuits]]
[[Category:Electric current|Limiting]]
[[Category:Electrical power control]]
[[Category:Over-current protection devices]]

Current limiting - Revision history

RS-485: Imported from Wikipedia (overwrite)