Current limiting is the practice of imposing a limit on the
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
that may be delivered to a
load to protect the circuit generating or transmitting the current from harmful effects due to a
short-circuit
A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the 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
An
inrush current limiter
An inrush current limiter is a component used to limit inrush current to avoid gradual damage to components and avoid blowing fuses or tripping circuit breakers. Negative temperature coefficient (NTC) thermistors and fixed resistors are often us ...
is a device or group of devices used to limit inrush current. Passive resistive components such as
resistors
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...
or negative
temperature coefficient
A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature. For a property ''R'' that changes when the temperature changes by ''dT'', the temperature coefficient α is def ...
(NTC)
thermistors
A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''.
Thermistors are divided based on their conduction ...
are simple options, with power dissipation and cool-down time being their main drawbacks, respectively. More complex solutions using
active component
Passivity is a property of engineering systems, most commonly encountered in analog electronics and control systems. Typically, analog designers use ''passivity'' to refer to incrementally passive components and systems, which are incapable of po ...
s can be used when more straightforward options are unsuitable.
In electronic power circuits
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
class-AB power 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 soon as the voltage across R
sens becomes about 0.65 V. This voltage is determined by the value of R
sens and the load current through it (I
load). When Q2 turns on, it removes base current from Q1, thereby reducing the collector current of Q1, which is nearly the load current. Thus, R
sens fixes the maximum current to a value given by 0.65/R
sens. For example, 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 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.
![Voltage regulator foldback](https://upload.wikimedia.org/wikipedia/commons/6/62/Voltage_regulator_foldback.svg)
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
A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit ...
, where the output voltage has reduced to zero, the current is typically limited to a small fraction of the maximum current.
![Voltage regulator linear foldback](https://upload.wikimedia.org/wikipedia/commons/4/45/Voltage_regulator_linear_foldback.svg)
The prime purpose of foldback current limiting in
linear power supplies is to keep the output
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
within its safe
power dissipation
In thermodynamics, dissipation is the result of an irreversible process that takes place in homogeneous thermodynamic systems. In a dissipative process, energy (internal, bulk flow kinetic, or system potential) transforms from an initial form to ...
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
For power semiconductor devices (such as BJT, MOSFET, thyristor or IGBT), the safe operating area (SOA) is defined as the voltage and current conditions over which the device can be expected to operate without self-damage.
SOA is usually presented ...
under
fault and
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.
Many power supplies employ
constant current A constant current (steady current, time-independent current, stationary current) is a type of direct current (DC) that does not change its intensity with time.
Sources
If the load is constant, a steady current can be obtained via a constant vo ...
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-
ohmic devices that draw a constant current independent of 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
A switched-mode power supply (switching-mode power supply, switch-mode power supply, switched power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently.
Lik ...
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 switchmode power supply when it is a component in a product that is independently certified to meet regional safety standards.
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.
These circuits operate more efficiently from a single (V) 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 is at least a few volts, a
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
can be used for Q1 for lower dropout voltage. Due to its simplicity, this circuit is sometimes used as a
current source
A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
A current source is the dual of a voltage source. The term ''current sink'' is sometimes used for sources f ...
for high-power LEDs.
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
See also
*
Current source
A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.
A current source is the dual of a voltage source. The term ''current sink'' is sometimes used for sources f ...
*
Crowbar
*
Electrical ballast
An electrical ballast is a device placed in series with a load to limit the amount of electric current, current in an electrical network, electrical circuit.
A familiar and widely used example is the inductive ballast used in fluorescent lamp ...
*
Fault current limiter A fault current limiter (FCL), also known as fault current controller (FCC), is a device which limits the prospective fault current when a fault occurs (e.g. in a power transmission network) ''without'' complete disconnection. The term includes sup ...
References
External links
Current Limiting for Stepper MotorsCurrent limiting resistor calculator for LED arraysConstant current & foldback current limiting
{{DEFAULTSORT:Current Limiting
Analog circuits
Limiting
In electronics, a limiter is a circuit that allows signals below a specified input power or level to pass unaffected while attenuating (lowering) the peaks of stronger signals that exceed this threshold. Limiting is a type of dynamic range comp ...
Electrical power control
Over-current protection devices