 An ideal current source, I, driving a resistor, R, and creating a voltage V A current source is an electrical or electronic device that delivers or absorbs electric current. Current sources can be theoretical or practical. A current source is the dual of a voltage source. Wikipedia does not have an article with this exact name. ...
A voltage source is any device or system that produces an electromotive force between its terminals OR derives a secondary voltage from a primary source of the electromotive force. ...
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It has been suggested that this article or section be merged into Current source. ...
Image File history File links Ohms_law_current_source. ...
Image File history File links Ohms_law_current_source. ...
In electronics, the word dual refers to two devices or circuits that have mathematical descriptions that are identical except that voltages in one correspond to currents in the other, and vice versa. ...
A voltage source is any device or system that produces an electromotive force between its terminals OR derives a secondary voltage from a primary source of the electromotive force. ...
Ideal current sources
In circuit theory, an ideal current source is a circuit element where the current through it is independent of the voltage across it. It only exists in mathematical models of circuits. If the current through an ideal current source can be specified independently of any other variable in a circuit, it is called an independent current source. Conversely, if the current through an ideal current source is determined by some other voltage or current in a circuit, it is called a dependent or controlled current source.
An independent current source with zero current is identical to an ideal open circuit. For this reason, the internal resistance of an ideal current source is infinite. The voltage across an ideal current source is completely determined by the external circuit. When connected to a short circuit, there is zero voltage and thus zero power. When connected to a load resistance, the voltage across the source approaches infinity as the load resistance approaches infinity (an open circuit). Thus, an ideal current source can supply unlimited power forever and so represents an unlimited source of energy. Connecting an ideal open circuit to an ideal non-zero current source is not valid in circuit analysis as the circuit equation is invalid, e.g., 5 = 0. Open circuit can mean:- In electronics, where there is nothing connected to a load and no current can flow. ...
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ...
For alternate meanings see Short circuit (disambiguation) A short circuit (sometimes known as simply a short) is a fault whereby electricity moves through a circuit in an unintended path, usually due to a connection forming where none was expected. ...
The input impedance or load impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to it. ...
No real current source is ideal (no unlimited energy sources exist) and all have a finite internal resistance (none can supply unlimited voltage). However, the internal resistance of a physical current source is effectively modeled in circuit analysis by combining a non-zero resistance in parallel with an ideal current source.
Physical current sources
Resistor current source The simplest current source consists of a voltage source in series with a resistor. The current available from such a source is given by the ratio of the voltage across the voltage source to the resistance of the resistor. For a nearly ideal current source, the value of this resistor should be very large but this implies that, for a specified current, the voltage source must be very large. Thus, it is impractical to construct a 'good' current source this way. Nonetheless, it is often the case that such a circuit will provide adequate performance when the specified current and load resistance are small. For example, a 5V voltage source in series with a 4.7k resistor will provide an approximately constant current of 1mA (+/- 5%) to a load resistance in the range of 50 to 450 ohms. A voltage source is any device or system that produces an electromotive force between its terminals OR derives a secondary voltage from a primary source of the electromotive force. ...
Active current sources Active current sources have many important applications in electronic circuits. Current sources are often used in place of resistors in analog integrated circuits to generate a current without causing attenuation at a point in the signal path to which the current source is attached. The collector of a bipolar transistor, the drain of a field effect transistor, or the plate of a vacuum tube naturally behave as current sources (or sinks) when properly connected to an external source of energy (such as a power supply) because the output impedance of these devices is naturally high when used in the current source configuration. An electronic circuit is an electrical circuit that also contains active electronic devices such as transistors or vacuum tubes. ...
Resistor symbols (non-European) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ...
Integrated circuit showing memory blocks, logic and input/output pads around the periphery Microchips with a transparent window, showing the integrated circuit inside. ...
The schematic symbols for pnp_ and npn_type BJTs. ...
Large power N-channel field effect transistor The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel in a semiconductor material. ...
In electronics, a vacuum tube or (outside North America) thermionic valve or just valve, is a device generally used to amplify, switch or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. ...
JFET and N-FET current source A JFET can be made to act as a current source by tying its gate to its source. The current then flowing is the IDSS of the FET. These can be purchased with this connection already made and in this case the devices are called current regulator diodes.. An enhancement mode N channel MOSFET can be used in the circuits listed below.
Simple transistor current source
 Typical constant current source (CCS) The image shows a typical constant current source (CCS). DZ1 is a zener diode which, when reverse biased (as shown in the circuit) has a constant voltage drop across it irrespective of the current flowing through it. Thus, as long as the zener current (IZ) is above a certain level (called holding current), the voltage across the zener diode (VZ) will be constant. Resistor R1 supplies the zener current and the base current (IB) of NPN transistor (Q1). The constant zener voltage is applied across the base of Q1 and emitter resistor R2. The operation of the circuit is as follows:
Image File history File links Typical constant current source (CCS) used in linear electronics applications. ...
Zener diode schematic symbol A Zener diode is a type of diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger (not equal to, but larger) than the rated breakdown voltage known as Zener knee...
International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ...
In electricity, current refers to electric current, which is the flow of electric charge. ...
Types of diodes closeup, showing germanium crystal In electronics, a diode is a component that restricts the direction of movement of charge carriers. ...
Resistor symbols (non-European) Resistor symbols (Europe, IEC) Axial-lead resistors on tape. ...
Assorted discrete transistors A transistor is a semiconductor device, commonly used as an amplifier. ...
Voltage across R2 (VR2) is given by VZ - VBE, where VBE is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by
Since VZ is constant and VBE is also constant for a given temperature, it follows that VR2 is constant and hence IE is also constant. Due to transistor action, IE is very nearly equal to the collector current IC of the transistor (which in turn, is the current through the load). Thus, the load current is constant and the circuit operates as a constant current source. As long as the temperature remains constant (or doesn't vary much), the load current will be independent of the supply voltage, R1 and the transistor's gain. R2 allows the load current to be set at any desirable value and is calculated by Assorted discrete transistors A transistor is a semiconductor device, commonly used as an amplifier. ...
 or  , since VBE is typically 0.65 V for a silicon device.
(IR2 is also the emitter current and is assumed to be the same as the collector or required load current, provided hFE is sufficiently large). Resistance R1 at resistor R1 is calculated as
 where, K = 1.2 to 2 (so that R1 is low enough to ensure adequate IB),  and hFE(min) is the lowest acceptable current gain for the particular transistor type being used.
Simple transistor current source with diode compensation
 Typical constant current source (CCS) with diode compensation Temperature changes will cause the above circuit to change the output current since VBE is sensitive to temperature. This can be compensated for by including a standard diode D (of the same semiconductor material as the transistor) in series with the zener diode as shown in the image on the left. The diode drop (VD) tracks the VBE changes due to temperature and thus suppresses temperature dependence of the CCS. Image File history File links Diode compensated constant current source. ...
Resistance R2 is now calculated as
Since VD = VBE = 0.65 V,
Therefore, 
(In practice VD is never exactly equal to VBE and hence it only suppresses the change in VBE rather than nulling it out.)
and R1 is calculated as
 (the compensating diode's forward voltage drop VD appears in the equation and is typically 0.65 V for silicon devices.)
This method is most effective for zener diodes rated at 5.6 V or more. For breakdown diodes of less than 5.6 V, the compensating diode is usually not required because the breakdown mechanism is not as temperature dependent as it is in breakdown diodes above this voltage. Zener diode schematic symbol A Zener diode is a type of diode that permits current to flow in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger (not equal to, but larger) than the rated breakdown voltage known as Zener knee...
Breakdown may refer to: Electrical breakdown Chemical breakdown Biological decomposition Nervous breakdown Societal breakdown A breakdown is a statement explaining the details of something such as a bill or cost of a plan A script breakdown for a play, film, or comic book Breakdown (album), by Melissa Etheridge. ...
Simple transistor current source with LED
 Typical constant current source (CCS) using LED instead of zener Another method is to replace the zener diode with a light-emitting diode LED1 as shown in the image on the left. The LED voltage drop (VD) is now used to derive the constant voltage and also has the additional advantage of tracking (compensating) VBE changes due to temperature. R2 is calculated as Image File history File links Constant current source using LED. File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...
Led is also the past tense of the verb to lead Blue, green and red LEDs. ...
and R1 as
 , where ID is the LED current.
Feedback
 Typical op-amp current source. The transistor is not needed if the required current doesn't exceed the sourcing ability of the op-amp. The current will be the zener voltage divided by the sense resistor. Another common method is to use feedback to set the current which removes the dependence on the Vbe of the transistor. Very common is to use an op amp with the non-inverting input connected to a voltage source (such as the zener in an above example) and the inverting input connected to the same node as the resistor and emitter of the transistor. This way the generated voltage is across the resistor, rather than both the resistor and transistor. Image File history File links No higher resolution available. ...
Current mirror Another form of current source can be realized with a current mirror mirroring the constant current through a resistor. Variations to the basic current mirror are the Widlar current source and the Wilson current source. A current mirror is a circuit designed to copy a current flowing through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. ...
Widlar current source. ...
A Wilson current source is a circuit configuration designed to provide a constant current source or sink. ...
Other practical sources In the case of opamp circuits sometimes it is desired to inject a precisely known current to the inverting input (as an offset of signal input for instance)and a resistor connected between the source voltage and the inverting input will approximate an ideal current source with value V/R.
Inductor type current source
 Constant Current Source with LM317 Amongst other applications, these are used to present a source of constant current in Class E (switching) electronic amplifiers Image File history File links Size of this preview: 800 × 299 pixelsFull resolution (1070 × 400 pixel, file size: 38 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Current source ...
Image File history File links Size of this preview: 800 × 299 pixelsFull resolution (1070 × 400 pixel, file size: 38 KB, MIME type: image/jpeg) File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Current source ...
It has been suggested that Rf power amplifier be merged into this article or section. ...
High voltage current sources A Van de Graaff generator behaves as a current source because of its very high output voltage coupled with its very high output resistance and so it supplies the same few microamperes at any output voltage up to hundreds of thousands of volts (or even tens of megavolts) for large laboratory versions. Van de Graaff generator. ...
In electronics or physics, a megavolt is 1 million volts. ...
Comparison between a current source and a voltage source Most sources of electrical energy (the mains, a battery, ...) are best modeled as voltage sources. Such sources provide constant voltage, which means that as long as the amount of current drawn from the source is within the source's capabilities, its output voltage stays constant. An ideal voltage source provides no energy when it is loaded by an open circuit (i.e. an infinite impedance), but approaches infinite energy and current when the load resistance approaches zero (a short circuit). Such a theoretical device would have a zero ohm output impedance in series with the source. A real-world voltage source has a very low, but non-zero output impedance: often much less than 1 ohm. Conversely, a current source provides a constant current, as long as the load connected to the source terminals has sufficiently low impedance. An ideal current source would provide no energy to a short circuit and approach infinite energy and voltage as the load resistance approaches infinity (an open circuit). An ideal current source has an infinite output impedance in parallel with the source. A real-world current source has a very high, but finite output impedance. In the case of transistor current sources, impedances of a few megohms (at DC) are typical. An ideal current source cannot be connected to an ideal open circuit. Nor an ideal voltage source to an ideal short circuit, since this would be equivalent to declaring that "5 is equal to 0". Since no ideal sources of either variety exist (all real-world examples have finite and non-zero source impedance), any current source can be considered as a voltage source with the same source impedance and vice versa. These concepts are dealt with by Norton and Thevenin's theorems. Mains may mean or refer to, or be a subject of: Mains electricity Electricity transmission Public utility, about mains services, including electricity, natural gas, water, and sewage disposal Mains (Scotland), about the central steading of a townland BMX racing See also: Main Mane Category: ...
Four double-A (AA) rechargeable cells A Duracell AA alkaline cell In science and technology, a galvanic cell is an electrochemical cell that stores chemical energy and makes it available in an electrical form, and a battery is a string of two or more cells in series. ...
A voltage source is any device or system that produces an electromotive force between its terminals OR derives a secondary voltage from a primary source of the electromotive force. ...
International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ...
Open circuit can mean:- In electronics, where there is nothing connected to a load and no current can flow. ...
Electrical impedance, or simply impedance, is a measure of opposition to a sinusoidal alternating electric current. ...
The input impedance or load impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to it. ...
For alternate meanings see Short circuit (disambiguation) A short circuit (sometimes known as simply a short) is a fault whereby electricity moves through a circuit in an unintended path, usually due to a connection forming where none was expected. ...
The ohm (symbol: Ω) is the SI unit of electric resistance. ...
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ...
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ...
The input impedance or load impedance of a circuit or electronic device is the impedance actually experienced by a signal which is connected to it. ...
Open circuit can mean:- In electronics, where there is nothing connected to a load and no current can flow. ...
Infinity is a word carrying a number of different meanings in mathematics, philosophy, theology and everyday life. ...
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ...
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ...
The ohm is the SI derived unit of electrical resistance (derived from the ampere and the watt). ...
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to the flow of an alternating current (AC) of a particular frequency as a result of resistance, induction and capacitance. ...
Nortons theorem for electrical networks states that any collection of voltage sources and resistors with two terminals is electrically equivalent to an ideal current source, I, in parallel with a single resistor, R. For single-frequency AC systems the theorem can also be applied to general impedances, not just...
Thevenins theorem for electrical networks states that any combination of voltage sources and resistors with two terminals is electrically equivalent to a single voltage source V and a single series resistor R. For single frequency AC systems the theorem can also be applied to general impedances, not just resistors. ...
Image File history File links File links The following pages link to this file: Current source Voltage source ...
Image File history File links File links The following pages link to this file: Current source Voltage source ...
Image File history File links A circuit symbol. ...
Image File history File links A circuit symbol. ...
Image File history File links A circuit symbol. ...
A voltage source is any device or system that produces an electromotive force between its terminals OR derives a secondary voltage from a primary source of the electromotive force. ...
Four double-A (AA) rechargeable cells A Duracell AA alkaline cell In science and technology, a galvanic cell is an electrochemical cell that stores chemical energy and makes it available in an electrical form, and a battery is a string of two or more cells in series. ...
See also Each entity in this world may be presented as a system of connected components. ...
A current mirror is a circuit designed to copy a current flowing through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. ...
Widlar current source. ...
A Wilson current source is a circuit configuration designed to provide a constant current source or sink. ...
A current loop describes two different electrical signalling schemes. ...
A high output constant current welding power supply for use with GTAW and SMAW A welding power supply is an electrical device that provides an electrical current to perform welding procedures which require electricity. ...
Manual Metal Arc welding, also known as stick or MMA welding is one of the most common forms of welding. ...
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