13

enter image description here

For example this circuit, A current source is something with would alter the voltage across it to meet a certain current.

But where is there a voltage source to do this or even have a voltage at Vx. How is this circuit functioning without even having a voltage supply?

I have looked here : Is current source also a voltage source?

To try to understand the difference between what a current source is and a voltage source but nothing helped answer this question.

Could in this case the current source act as a voltage supply?

Thanks

fred
  • 451
  • 1
  • 5
  • 16
  • Think of a solar cell, current is generated from incoming light, there is no voltage source. – sstobbe Oct 18 '18 at 00:30
  • 14
    You said it yourself: "A current source is something with would alter the voltage across it to meet a certain current". That's it -- the current is constant, forced by the current source, and the voltage just has to rise enough to satisfy that. – TimWescott Oct 18 '18 at 00:53
  • 11
    Also, how can a Voltage Source possibly create any currents? After all, it has to produce voltage alone and nothing else! (Wrong, "voltage source" actually means constant voltage with variable current.) – wbeaty Oct 18 '18 at 01:03
  • 3
    "current source" and "voltage source" do not mean what they say on the tin, they are shortened forms (for convenience) from the more accurate descriptions: "source with regulation to (try to) maintain constant current" and "source with regulation to (try to) maintain constant voltage". – Ben Voigt Oct 18 '18 at 03:28
  • 2
    If a circuit only has a voltage source and no current source where does the current come from to supply the circuit? – uglyoldbob Oct 18 '18 at 16:41
  • A "current source" is the correct technical term to what we would sometimes call "current regulator" - a device designed to increase or decrease voltage as necessary until the desired current is generated. Similarly a "voltage source" is a voltage regulator that will increase or decrease voltage as necessary until the desired voltage is generated. Where the "electricity" (voltage + current) comes from depends on the physical device - batteries generate them using chemical reactions, dynamos generate them using a rotating magnetic field – slebetman Oct 19 '18 at 01:07
  • If a circuit only has a voltage source and no current source where does the current come from to supply the circuit? – user253751 Oct 19 '18 at 03:11
  • @uglyoldbob (and immibis - are you both the same person?) The *power* to supply both current and voltage comes from somewhere outside of the circuit - the chemical reactions in a battery, light falling on a solar panel, the steam turbine or nuclear reactor that supplies electricity to your house, or whatever. – alephzero Oct 20 '18 at 09:27
  • A positive displacement water pump moves a fixed amount of water per cycle. It doesn't "create" water pressure, but any resistance to the flow, such as your thumb pressed over the end of the hose, does. Consider the current source to be something that moves a fixed number of electrons per second. That's what current is. Voltage is a result of external resistance to that flow. – Dave Oct 23 '18 at 01:49

9 Answers9

72

An Ideal current source will produce whatever voltage is necessary to permit it to deliver its specified current.

An ideal voltage source will deliver whatever current the rest of the circuit requires when it is delivering its specified voltage.

Real current and voltage sources will have limits on the voltage (for a current source) or current (for a voltage source) that they can deliver.

Peter Bennett
  • 57,014
  • 1
  • 48
  • 127
  • 18
    I wish I had such a simple and precise explanation when I was a student. I spent so much time trying to figure out what a current source was, and the teacher never managed to explain it in such a good way. Great answer! – Tim Oct 18 '18 at 08:18
  • 1
    ...and I hope it is clear to you now that - physically spoken - there is no such thing like a current source (which would be able to "deliver" a voltage). All such devices contain a voltage source - and it depends only on the source resistance (static or externally controlled) if we call it "voltage source" or "current source". – LvW Oct 19 '18 at 07:45
  • 1
    @LvW: I would say that a Van de Graaf generator is much closer to being a current source than to being a voltage source, physically speaking. – hmakholm left over Monica Oct 20 '18 at 00:08
  • Henning - I am not quite sure about this - but this subject deserves a more detailed discussion (not possible here at this location). – LvW Oct 20 '18 at 08:35
14

These are idealized elements. Neither voltage sources nor current sources exist in reality. Reality has generators and galvanic cells etc.

You can model a galvanic cell as either

  • a voltage source with an internal resistance in series

  • a current source with an internal resistance in parallel

Of course you can always add more things to your model, but these two options are the minimum. If you drop the internal resistance connected the right way, you aren't talking about real world things any more but you are talking about your modeling.

And that's what these circuit lessons and practice is about. Learn to understand the modeling. So you can build and understand models for real world elements.

I think you can now easily understand what to do with R2, and what the current source and R1 can be transformed into afterwards.

Janka
  • 13,636
  • 1
  • 19
  • 33
  • So - a galvanic cell with a parallel resistor behaves like a current source? Is that what you mean? I have some severe doubts (in spite of the 14 points). – LvW Oct 20 '18 at 15:02
  • Because you are thinking wrong. A galvanic cell behaves like a current source with a parallel resistor. Not what you said. – Janka Oct 20 '18 at 16:32
  • Janka - without the resistor, is the galvanic cell a voltage source? I think you will agree - YES!. Now, with a parallel resistor there will be a current through this resistor - however, the voltage across this parallel combination remains constant, does it not? So - where is a current source? Can you answer this? I am afraid you were thinking of a theoretical model for an IDEAL current source which - with a parallel resistor - turns into a real current source. – LvW Oct 21 '18 at 08:39
  • A galvanic cell has an internal resistance. The model is incomplete without it. If you assume the internal resistance to be zero you fool yourself. An ideal current source with a parallel resistance is equivalent to an ideal voltage source with a series resistance. – Janka Oct 21 '18 at 12:11
  • 1
    Janka, do you believe Wikipedia? Excerpt: "A galvanic cell, or voltaic cell..... is an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell. Volta was the inventor of the voltaic pile, the first electrical battery. In common usage, the word "battery" has come to include a single galvanic cell, but a battery properly consists of multiple cells. " (End of quote).The device we may call "current source" is a voltage source with a large SERIES resistance! That`s for sure! – LvW Oct 21 '18 at 13:03
  • *do you believe Wikipedia?* No. – Janka Oct 21 '18 at 13:06
  • So, what is wrong in my quotation? Perhaps you should consult one of the available textbooks. Do you need some recommendations? – LvW Oct 21 '18 at 13:12
  • It's missing the point. Please read again what I wrote. Then you understand why you are far out with your false assumptions of what I wrote. – Janka Oct 21 '18 at 13:16
  • Univ. of Illinois: "An electrochemical reaction in an electrochemical cell requires both a reduction half reaction and an oxidation half reaction. A voltmeter in the circuit shows the total cell voltage (Etot = Ered + Eox). " Any further question? – LvW Oct 21 '18 at 13:25
  • What you wrote is completely misleading. A galvanic cell is a non-ideal source of voltage. You can use it as a non-ideal current current source with a large internal SERIES resistor. But with a parallel resistor it remains a (loaded) non-ideal voltage source. Is this correct or wrong? What is your answer? – LvW Oct 21 '18 at 13:30
  • If you want someone to answer your questions, post them as such, please. – Janka Oct 21 '18 at 13:36
  • OK - I see. No answer. – LvW Oct 21 '18 at 13:46
  • The OP shows a 5A current source. It is quite unhelpful to claim "neither voltage sources nor current sources exist in reality". Of course no source can produce infinite voltage or infinite current, but 5A and, say, 24 V are well within the reach of reality. Since by inspection the circuit has less than 3 ohms, 5A would generate less than 15V. Modeling galvanic cells is totally NOT what the question is about. Learning how to divide the *exactly* 5A current between two parallel resistances is. – Dave Oct 23 '18 at 00:49
6

A current source does have to have a voltage source. But, because a current source is designed to supply a fixed amount of current, we can ignore its inner workings and just focus on its purpose: to be a current source.

If you think about it, we treat sources as ideal a lot, and it works out just fine. If we couldn't do that, a voltage source in a circuit would have to include all the components that allows it to output that particular voltage. And if it were plugged into the wall, it would have to include a circuit that went all the way back through transformers and many miles of wire to the electricity source...

Annie
  • 875
  • 5
  • 13
3

A constant current (CC) source has its own DC source or one needs to be supplied.

You can buy a DC-DC CC regulator or an AC powered CC power supply.

An AC powered CC power supply will have an AC-DC power supply input and DC-DC CC regulator output.

Here is a very basic DC-DC step down (buck) CC regulator driving some LEDs.

enter image description here

The current flows through the Rset resistor. The chip measures the voltage across Rset to monitor the current flow.

enter image description here

If the current is insufficient, the internal PWM signal will increase the duty cycle of the SW (switching) input, increasing the current flow in to SW. And vice versa.

When the duty cycle is at its maximum (maximum rated current) the CC source essentially becomes a voltage source.

It's a load related thing. When the CC reaches maximum voltage or current, it is then a voltage source.

Example if the forward voltage of these LEDs being driven is greater than the input voltage the maximum voltage is reached.

If the load is a resistive load that could draw more current than the CC source can provide, the maximum current is reached.

Misunderstood
  • 7,287
  • 1
  • 11
  • 24
2

"Current source" is just a (slightly misleading) name. In truth, the usual Ohm's Law works as usual, that is, there is some voltage, some resistance, and hence some current. It is not like there is some magic that somehow creates a current out of thin air.

You can imagine that a real current source (as opposed to the ideal version, where we do not care about its internals) has a regular voltage source (110V/220V AC), converts that to some internal DC voltage, and then does some nifty regulation which varies the DC voltage it serves to its client circuit such that it increases/decreases if the actual current is too low or too high. By changing the voltage, the current changes as well, depending on the load. As such, the real current source is still just a voltage source, albeit with a rapidly changing/adapting voltage, and the implementation is obviously very much more complicated than a standard voltage source.

The Wikipedia page on current sources lists quite a number of different implementations, using transistors, opamps or other active components.

AnoE
  • 490
  • 5
  • 15
1

The case you expose is an idealized element. So it is a model that you use to do calculations with other models.

Could in this case the current source act as a voltage supply?

If you look at a regulator based on a current mode DC-DC converter, you verify that it actually has two control loops:

  1. The "inner" loop regulates the output current as requested by the outer loop.
  2. The outer loop, in turn, "asks" the current needed so the output matches the desired voltage.

So effectively the whole regulator can be thought as a controlled current source, supplying the current needed such that the output voltage is the desired one.

Undertalk
  • 303
  • 1
  • 7
1

Think of a current source as an energy source that somehow outputs an almost constant current. In practice it would typically be constructed with electronic components that measure the current and try to stabilise it, but crudely you could make one with a million volt battery in series with a one megohm resistor. That will deliver pretty close to one amp no matter what you connect to it!

To measure a current source you would short circuit the terminals (almost) with a current meter. The internal voltage is unspecified and trying to measure it with an open circuit would possibly be dangerous.

Conversely, to measure a voltage source you would use a high-resistance meter that draws (almost) no current. The current it can deliver is unspecified and trying to measure that with a short circuit would possibly be dangerous.

Phil McKerracher
  • 111
  • 2
0

Current is a flow of charge. In a metal conductor we know electrons are moving randomly in all directions. You need a complete circuit and potential difference in order to get them to move (on average) towards the positive terminal of the cell.

No potential difference = no current

David Wicks
  • 17
  • 1
  • 1
    Current source is idealized component, which makes charges move somehow - be it some clever circuitry, creating potential difference or some jinn, urging electrons to flow in specific direction, doesn't matter at all. – Arvo Oct 19 '18 at 11:01
0

Not an expert like the others, but just wanted to add when you see a current source in a circuit picture, you can't assume that voltage across it is zero. It has some voltage which probably unknown to you at the start of the problem, but which you can find with the help of Kirchhoff's laws.

Another thought is that a current source is not a simple device like a battery, so behind that picture, there's a pretty complicated element which produces current. And as Peter said in his answer, it would adjust the voltage across it based on the resistance of the rest of circuit, so as to produce the current it's "advertised" to produce. Thus a current source of 1 amp will have higher voltage across it with a resistance of 200 ohm in the rest of the circuit, than with resistance of 100 ohm. (And as for how it gets this voltage, you can just imagine that a current source is a complicated device, not a simple one like a battery, that can adjust its own voltage as necessary to get the right current given the circuit it faces.)

Transistor
  • 168,990
  • 12
  • 186
  • 385
Daniel
  • 206
  • 1
  • 9