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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
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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
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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
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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.
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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
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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.
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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
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are simple options while the positive one (PTC) is used to limit max current afterward as the circuit has been operating (with cool-down time drawback on both). More complex solutions using
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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."
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conditions. Foldback also significantly reduces the power dissipation in the load in fault conditions, which can reduce the risks of fire and heat damage.
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One style of current limiting circuit is shown in the image. The schematic represents a simple protection mechanism used in regulated DC supplies and
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111:). 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, R
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The inrush current of an incandescent lamp causes a bench power supply to limit its output current with a foldback current limiter.
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An issue with the previous circuit is that Q1 will not be saturated unless its base is biased about 0.5 volts above V
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is the load current sensing device. Q2 is the protection transistor which turns on as soon as the voltage across R
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current limit, the output voltage will decrease to a value depending on the current limit and load resistance.
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Power dissipation vs. load resistance for linear voltage regulators with different overload handling. Here
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Some electronic circuits employ active current limiting since a fuse may not protect solid-state devices.
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can be used for Q1 for lower dropout voltage. Due to its simplicity, this circuit is sometimes used as a
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is a device or devices combination used to limit inrush current. Passive resistive components such as
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to protect the circuit generating or transmitting the current from harmful effects due to a
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To reduce the heat that must be dissipated by the pass devices under a short-circuit,
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current limiting is used, which reduces current in the short-circuit case. Under a
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V-I curves for voltage regulators with different overload handling:
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becomes about 0.65 V. This voltage is determined by the value of R
123:= 0.33 Ω, the current is limited to about 2 A even if R
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can be used when more straightforward options are unsuitable.
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These circuits operate more efficiently from a single (V
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output is located at similar location as PNP example)
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fixes the maximum current to a value given by 0.65/R
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Current limiting resistor calculator for LED arrays
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Active current limiting or short-circuit protection
213:The prime purpose of foldback current limiting in
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456:Constant current & foldback current limiting
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57:(with power dissipation drawback), or negative
421:"The New Stuff!!! Constant Current Source #1"
399:. McGraw-Hill Professional. p. 1.113.
22:is the practice of imposing a limit on the
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205:design is three times lower than in the
201:=0.17 A. The maximum dissipation in the
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16:Safety feature in electrical engineering
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310:Current limiter with NPN transistors (
95:Q1 is the pass or output transistor. R
373:The Art of Electronics Second Edition
326:Current limiter with PNP transistors
446:Current Limiting for Stepper Motors
375:, Cambridge University Press, 1989
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107:and the load current through it (I
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396:Switchmode power supply handbook
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486:Over-current protection devices
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371:Paul Horowitz, Winfield Hill,
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271:Single power-supply circuits
73:In electronic power circuits
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26:that may be delivered to a
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393:Keith H. Billings (1999).
256:switched-mode power supply
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90:class-AB power amplifiers
481:Electrical power control
207:constant current limited
148:constant current limited
59:temperature coefficient
39:Inrush current limiting
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217:is to keep the output
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127:becomes a short (and V
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51:inrush current limiter
45:Inrush current limiter
353:Fault current limiter
298:for high-power LEDs.
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215:linear power supplies
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119:. For example, if R
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348:Electrical ballast
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223:power dissipation
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63:thermistors
465:Categories
359:References
219:transistor
152:unlimited
55:resistors
332:See also
236:overload
203:foldback
187:= 10 V,
180:= 12 V,
160:foldback
144:foldback
383:, p.316
343:Crowbar
209:design.
194:= 1 A,
24:current
427:4 July
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292:MOSFET
230:under
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61:(NTC)
248:ohmic
232:fault
429:2012
401:ISBN
377:ISBN
246:non-
234:and
125:load
121:sens
117:sens
113:sens
109:load
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101:sens
97:sens
28:load
192:max
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