# Is current source also a voltage source?

I'm confused between current and voltage sources; I get the text book definition but I am not able to understand real world difference. To me both current and voltage sources seem as the same. I understand that ideal sources doesn't exist. What is an example of practical current source? In order to produce current, we need voltage, so then isn't a current source also a voltage source? Since a battery is a voltage source and it produces current when connected to a circuit, isn't it also a current source?

Please help me understand real world example and usage of current source and how it is different from a voltage source.

• You're missing the word 'constant' in front of both. – user207421 Dec 31 '15 at 11:07
• @EJP : "ideal" may be a better term than "constant", considering AC sources. – Brian Drummond Dec 31 '15 at 11:30
• @BrianDrummond & EJP: I agree with you both, but @Brian: I'd say that saying "constant AC voltage source" is not wrong, as I assume it would be commonly understood as "AC voltage source with constant characteristics" (freq/P2P voltage/max power etc); "ideal" is a much more serious assumption here. Thus, I think that I could safely say e.g. that I've a constant, non-ideal AC voltage source - without any real risk of misunderstanding, as it's not the voltage that's constant here, but the source & its characteristics. – vaxquis Dec 31 '15 at 14:54
• As others have noted, the confusion comes mainly from inexact terminology. To produce current you MUST have voltage. To develop voltage across a load you must have current pass through it. Any practical power supply is both a current and voltage source. HOWEVER - for certain tasks we value having a (theoretically) constant voltage source or a (theoretically) constant current source. | With a CC source, as load varies voltage across the load MUST vary as V = IR = I x a constant. | With a CV source, as load varies current through the load must vary as I = V/R = V x a constant. – Russell McMahon Jan 6 '16 at 6:46

A voltage source provides, as closely as it can manage to the ideal, a constant (or only slightly varying) voltage at whatever current is needed (in real supplies, to the limit of the current it can supply)

A current source provides, as closely as it can manage to the ideal, a constant (or only slightly varying) current at whatever voltage is needed (in real supplies, to the limit of the voltage it can supply.)

If you short-circuit a voltage source, you get extremely large currents (and normally blow a fuse/trip a breaker, etc.)

If you short-circuit a current source, you get the rated current at extremely low voltage, and nothing exciting happens.

If you open circuit a voltage source, it sits there at its rated voltage and does nothing interesting.

If you open circuit a current source, it shoots to its maximum voltage. If it was an ideal current source, it would drive itself to enough kilovolts to form an arc and get the rated current flowing in plasma. We don't really want ideal current sources in most situations for that reason.

• Nice explanation of the open- and short-circuit states! – Ryan Griggs Dec 31 '15 at 4:52
• Good explanation but to avoid one possible confusion one might replace 'constant' with nominal or specified. The output of an audio amplifier, for example, could be described as a voltage source, but it's not constant. – nekomatic Dec 31 '15 at 11:15
• "(...)it would drive itself to enough kilovolts to form an arc and get the rated current flowing in plasma. We don't really want ideal current sources in most situations for that reason." Oh yes we do! (insert evil laughter and wild maniacal eyes) – Baard Kopperud Dec 31 '15 at 14:40
• @nekomatic I agree, but then I'd just add the clarification about the context of use of the word "constant" - learning the terminology is IMO the first step in understanding the concepts behind it; I suspect that if OP hadn't confused "current/voltage sources" with "constant current/voltage sources", he wouldn't have any problem with them at all... – vaxquis Dec 31 '15 at 16:01
• wow - 29 points, but was the original question answered? (Is a current source also a voltage source?) – LvW Jan 1 '16 at 9:54

An ideal voltage source would maintain a defined voltage regardless of the current drawn from it.

An ideal current source would maintain a defined current regardless of the voltage accross it.

Neither of these things actually exist. Both are simplifications we use when analysing circuits. Even if we could construct them we probablly wouldn't want to. A device with infinite open circuit voltage or infinite short circuit current would be extremely dangerous.

A real voltage source maintains a voltage close to it's defined value over some defined range of currents.

A real current source maintains a current close to it's defined valueover some defined range of voltages.

Some sources may exhibit both behaviours. A typical laboratory power supply is a good example, for low currents it will maintain a given voltage, but once the current reaches a given threshold the voltage will reduce to maintain a constant current.

An ideal current source in paralell with a resistor is equivilent to an ideal voltage source in series with a resistor. The resistor value is the same in both cases and is known as the "output impedance". The voltage vs current characteristic of such a circuit will be a straight line between the open circuit voltage and the short circuit current. More generally we can consider the output impedance to be dv/di .

So you could decide what an acceptable source impedance is for the variation in current to be sufficiently small over the output voltage range then transform the circuit from a current source with paralell resitor to a voltage source with series resistor.

In practice that doesn't work so well. To get a high output impedance by that method requires a high voltage source which is inefficient and can create safety hazards. So a typical current source will involve some form of feedback to adjust the voltage depending on the load. For such a source the voltage vs current graph will not generally be a straight line and hence the output impedance will vary depending on the voltage across the source.

Typically some form of transistor or op-amp circuit is used to do this. There are many variations depending on the characteristics the source needs to have.

What is an example of practical current source?

In arc welding, you must use either a constant current(CC) or constant voltage(CV) power source depending on which process is being used. Several of the most common welding processes use constant current power supplies(e.g. SMAW, GTAW).

When a SMAW("stick" welding) operator is welding, the constant current power source will show a relatively small change in amperage compared to a large change in voltage.

Using some example operating parameters for a CC power source, we have the machine set to 300A, and we check the voltage and amperage on the power source while the operator changes the arc length by holding the electrode closer or further away from the work:

• Short arc: 30V - 308A
• Ideal arc: 32V - 300A
• Long arc: 34V - 290A

Here we can see there is a relatively small change in amperage of 18A with a comparatively large change in voltage of 4V.

In order to produce current, we need voltage, so then isn't a current source also a voltage source?

No. Current source and voltage source are theoretical definitions that exist in order to analyze electrical circuits. If you look at the definitions, they could not both be true.

The essence is that a current source provides a reasonably stable(i.e. constant) current and a voltage source provides a predictable voltage (e.g. 12V batteries, 120V wall outlets).

• Agree somewhat, but (~= +/- 3%) ~= (~= +/- 6%) in this case. A wider range of one variable wrt to the other would be better for an example. – Russell McMahon Jan 6 '16 at 2:41

For ideal current and voltage sources, it is like this.

The current passing through a current source is fixed at a constant value by the current source. The voltage across a current source may take on any value.

The voltage measured from one terminal to the other of a voltage source is fixed at a constant value by the voltage source. The current through the voltage source may take on any value.

Does that make sense?

• A current source could be varying as a function of time (or be a function of some other quantity in a circuit, like in a transistor model). – Peter Mortensen Jan 1 '16 at 21:34
• @PeterMortensen, that is true, if it is a dependent current source. But I feel that the topic of dependent current sources might be too advanced for the OP. – mkeith Jan 2 '16 at 0:36

My understanding is that a real-life current source adjusts the output voltage to ensure the specified current flows through the circuit, while a voltage source produces a specific voltage at up to a rated current. But I think both are technically voltage (potential) sources, one being a variable voltage and the other fixed voltage.

Regarding current sourcing, years ago I had a mental block until an instructor made the simple statement that "the ability to source current is assumed to be infinite in equations, but in real life it is always limited by the capabilities of the source".

• For my opinion and as far as I can see, this is the only contribution that gives an answer to the question "Is the current source also a voltage source?". – LvW Jan 1 '16 at 9:56
• I'm curious: do high-quality current source instruments detect an open circuit condition, or do they just push the voltage up to the max available voltage? If they detect open-circuit condition, how is it done? – Ryan Griggs Jan 1 '16 at 21:43
• Yes it's obvious: any current source must be a voltage source because there must be a voltage potential in order for current to be "pushed" through the circuit. No voltage (i.e. pressure) equals no current (i.e. flow). – Ryan Griggs Jan 1 '16 at 21:44
• Thank you all for answers. Unfortunately i am still confused. I get it that ideal voltage or current source does not exist. my question is - we need voltage to produce current then are not current and voltage source same? – user3551094 Jan 12 '16 at 6:29
• Yes, my answer explains that a "current source" is simply a variable voltage source. The source monitors the current flow and changes the voltage to ensure a consistent current flow. So yes, both are "voltage potentials". – Ryan Griggs Jan 13 '16 at 16:57

You are right in thinking that there is no such thing as an ideal voltage source or ideal current source in the real world.

Instead there are just sources, which provide both voltage and current. The difference between them is which of the parameters is under the control of the source and which is under the control of the load.

For simple resistive loads you have Ohm's Law which illustrates it nicely.

You have three parameters - voltage, current, and resistance. Ohm's law relates the three together into a very simple formula - $I=\frac{V}{R}$

When you have two of those values you can calculate the third.

With a (constant) voltage source you have a fixed value of $V$ and a known value of $R$ (the load resistance) so the current $I$ is variable and can be calculated.

Conversely for a (constant) current source you have a fixed value of $I$ and a known value of $R$ so the voltage $V$ is variable and can be calculated.

So in summary:

• In a voltage source the voltage is fixed and the current changes depending on the load
• In a current source the current is fixed and the voltage changes depending on the load

Just to add some maths V= RI (ohm's law) Now what voltage source does is mathematically saying V is constant do therefore is make (RI) constant this would imply

1. For increase in resistance(LOAD) less current is drawn.
2. However power dissipation is same Implying a possibility of current in circuit to be lower if power required is same.

The reverse happens for current source where even low voltage would fulfill required power barrier. Mathematically this is the fundamental difference between both sources.

You asked for some practical applications of current loops. Here are a few. Some are historic, and some are still being used today.

Early Teletype machines, like the Model 15, used 60 mA current loops between machines. Later models, like the Model 33, used 20 mA loops. The advantage in both cases is that you could run lines for several miles between machines without the need for any repeaters, since the constant current overcame any losses due to the resistance of the lines. Of course the voltage drop across these distances increased as the distance increased, and some lines were operated at supply voltages up to 125V.

Another advantage is that you could add additional machines in series with the others anywhere in the loop, and the power supply will automatically compensate by raising the voltage driving the loop.

These Teletype loops used an absence of current for a "space" condition, and the presence of current in the line for a "mark". Since a spacing condition (no data) was the default condition, this reduced power consumption in the power supply circuits most of the time.

Model 33 Teletype machines were widely used as computer terminals for minicomputers in the 1970s-1980s, and thus most of them came with a 20 mA interface. Even the original serial card for the IBM PC had provisions for a current loop interface.

MIDI is another example of a current loop interface. It uses 5 mA.

Another type of current loop was and still is being used in some places for instrumentation. It is called 4-20 mA current loop (10-50 mA has also been used). Unlike the constant current in the loops discussed above for sending digital data, the 4-20 mA loops are used to convey instrument readings such as pressure, temperature, level, flow, pH or other process variables. Usually 4 mA represents a reading of 0, and 20 mA represent a full scale reading. So if the full scale of an instrument was 160, each 100 µA increase in current would represent an increase of one in the reading.

A device known as a Transmitter is used to convert the reading into a varying current. Modern ones are rather complex.

Like the 20 mA and 60 mA digital loops, an advantage of the 4-20 mA current loops is that they could be run over a telephone pair, for example, for long distances.

The reason they started with 4 mA instead of 0 mA, is the latter was used to indicate a fault (open loop).

Try pondering on this notion - slowly and calmly. Current is real. It is a physical reality [electrons moving in some way]. It is measurable. It is variable [more or less moving electrons]. It can be seen with a range of instruments [electron microscope]. So step 1 is to come to terms with the existence of the mechanical form of electrical current - it exists. Voltage is not real. It has no mechanical constituents whatsoever. So for all of you who mistakenly believe that BOTH current and voltage are real and exist and depend on each other to have some further meaning - you are wrong. The term voltage needed to be described back in the day to EXPLAIN electricity in a simple way rather than leave the subject confused and un-explained. The key point to grasp here is the meaning of EXIST!. Current exists. It is a mechanical component [it has mass] comprising several building blocks [electrons; particles; atomic structure, plus interaction between the constituents pursuant to the laws of physics]. Voltage DOES NOT exist because it has no mass. We create the value of Voltage ourselves by interposing a purpose designed and labeled measuring instrument into a closed circuit that allows the continuation of or commencement of a current to circulate. Depending on the physical parameters [at the electron level] of the circuit, will depend on what we see on our humble Voltage Measuring device. Interestingly we don't really EVER need to define Voltage as a separate parameter if we are willing to stick just to the reality of the two circuit constituents that actually EXIST and define electron flow precisely [circuit resistance and current]. Trys it sometime - it soon become quite easy to do the math.

• Thank you all for answers. Unfortunately i am still confused. I get it that ideal voltage or current source does not exist. my question is - we need voltage to produce current then are not current and voltage source same? – user3551094 Jan 12 '16 at 6:29
• "Voltage is not real". This is utterly WRONG! Voltage is energy per unit charge and energy is as real as matter (It's Einstein's legacy, E=mc^2!!!). It is measurable even in non-nuclear reactions, although with very sensitive instruments: in a chemical reaction where energy is produced, the total mass of the products is less than the total mass of the reagents. The difference is the equivalent mass of the produced energy! – Lorenzo Donati Apr 21 '18 at 14:54