Constant current source?

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  • #26
psparky
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I would be careful not to confuse *constant* current source with *controlled* current source. A transistor, bjt or fet, is not CCS, but controlled current source as its value of drain or collector current is contrplled by an input signal. The CCS for dring LED lamps is constant, not controlled by a signal. As far ad a CCS being fiction, I would agree that ideal ccs is fictions, but a real ccs is just as real as a cvs.
Claude
So to sum up, a CONSTANT current source is basically a controlled voltage source to make up for voltage drop or voltage increase due to distance in lines and loads starting or turning off?

Fixed?
 
  • #27
jim hardy
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I was aware streetlamps are sometimes series wired, but that requires provision for one in the string open circuiting.
I suppose those individual airport light transformers have some sort of magnetic shunt to limit the voltage when a lamp burns put?

Lamps are pretty robust devices. Series connection allows one conductor to serve a lot of loads, but opens the door to magnetic induction.
However - sensitive electronics , big loops and changing magnetic fields sounds to me like a risky mix(think power lines and geomagnetic storms) . The phone company learned of that decades ago when they shared poles with the electric companies..

i'll stick with parallel conductors in close proximity.
 
  • #28
sophiecentaur
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So to sum up, a CONSTANT current source is basically a controlled voltage source to make up for voltage drop or voltage increase due to distance in lines and loads starting or turning off?

Fixed?
That is not a definition, though. In a text book (whatever we conclude must agree with that, surely) a constant current source is defined as producing a certain value of current. It's an ideal component that is placed in equivalent circuits of many amplifying devices.

How it's achieved is not considered. Feedback is only a smart way of avoiding the need for 100000MV and a Gizillion Ohm series resistor. In the linear world, any source of current has an equivalent of an ideal constant current source and a resistor. A "Controlled Current Source' is a common (?) description of a real circuit / device that behaves a bit like a Constant Current Source.

Whatever the practicalities are behind this, I don't think you can get away with describing a Constant Current Source in any other way than its original (formal) form unless you want to re-write a lot of very basic stuff. Implementation is an entirely different matter.
 
  • #29
jim hardy
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Whatever the practicalities are behind this, I don't think you can get away with describing a Constant Current Source in any other way than its original (formal) form unless you want to re-write a lot of very basic stuff. Implementation is an entirely different matter.

quite agreed.

We think in terms of ideal circuit elements. Ideal current source is one of them, and it's no more achievable than an ideal voltage source.

OP, who presumably is early on in his studies, needs to have it in his repertoire.
 
  • #30
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So to sum up, a CONSTANT current source is basically a controlled voltage source to make up for voltage drop or voltage increase due to distance in lines and loads starting or turning off?

Fixed?
NO!!! I've already addressed that. What you just described is a constant voltage source! If the output being measured and controlled is voltage, then it's a voltage source. Say we generate a voltage at a power plant, using feedback to hold the terminal voltage constant. Five miles down at the load end, a small voltage drop takes place. So we modify the system by observing the voltage at the load end and adjusting fuel burn and field current to keep the load voltage constant to compensate for 5 miles of T-lines and the incurred drops.

This is still a constant voltage source. The constant voltage is at the load end of the T-line whereas in the first case it was at the source end.

A constant current source is produced by a system which measures output current, then adjusts fuel burn and field current so that as the load varies, the current is held fixed. Of course the voltage varies with loading. Any battery can be built for CCS mode. It's not done because lifetime is much better in CVS mode. Generators can go either way, but due to vast difference in loss between insulator and conductor, CVS is better.

A car alternator can be set up for CCS mode, but since the battery is CVS, it is built to regulate voltage. Current sources can be built just like voltage sources. An ideal CVS has 0 series resistance and 0 series reactance. With ac sources, the 0 reactance is impossible. With ac or dc, 0 resistance can be done with superconducting windings. But inductance is always present so 0 reactance is not going to happen. As soon as current is drawn, inductive reactance drops voltage. To keep terminal voltage fixed, field current needs adjustment.

An ideal CCS has a shunt impedance that is infinite, i.e. infinite resistance, infinite reactance. In ac domain, capacitance is non-zero, so infinite shunt reactance cannot happen. A real CCS has limited compliance.

To summarize, ideal CVS and/or CCS cannot be made. But real CVS & CCS can offer very good performance. By the way, to make a CCS, we do not need a CVS plus high series resistance. That is a poor way to construct a CCS, very lossy.

Google constant current LED drives and you will find that switching converters and inductors are used to keep LED current constant. The voltage generated is only the LED forward drop plus a little to cover drops incurred in wires, traces, sensing resistor, etc. If I wish to drive an LED with 100 mA, and it's forward drop is 3.0 volts, a CCS is the way to do it. A 100 volt supply, with a large series resistor of 970 ohms is a terrible way to do it. The drop across the resistor is 97.0 volts, and at 100 mA, that is 9.7 watts of heat loss. The LED power is 3.0V*0.100A = 0.300W. That is an efficiency of 3%, terrible.

Designing a CCS is not that hard, but please avoid large CVS with large resistors, esp. if large power is involved.

Claude
 
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  • #31
sophiecentaur
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So to sum up, a CONSTANT current source is basically a controlled voltage source to make up for voltage drop or voltage increase due to distance in lines and loads starting or turning off?

Fixed?
What I would like to know is why you imply that it's ok to talk in terms of a controlled voltage source, as if it's a more basic component than a controlled current source. This controlled voltage source is just as difficult to achieve as a controlled current source would be.

Would there be any fundamental difference between using the collector of a PNP transistor or the emitter of an NPN transistor as the output stage, giving you a constant ("controlled") current generator (if you use appropriate feedback)? Those two terminals can be treated, in an elementary way, as constant current and constant voltage sources, respectively. Imo, you are giving too much emphasis on the way the source is actually achieved and seem to be assuming that you'd be starting with a low impedance amplifier output stage. If you had spent your life with thermionic valves, I suggest you might be viewing it the other way round. :smile:
 
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  • #32
jim hardy
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If you had spent your life with thermionic valves, I suggest you might be viewing it the other way round.
thermionic valves ? Beam Power Pentode, add to that the inductance of impedance matching transformer, ? Small wonder you mentioned audio way back there.....

6AQ5's forever !
 
  • #33
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I was aware streetlamps are sometimes series wired, but that requires provision for one in the string open circuiting.
I suppose those individual airport light transformers have some sort of magnetic shunt to limit the voltage when a lamp burns put?

Lamps are pretty robust devices. Series connection allows one conductor to serve a lot of loads, but opens the door to magnetic induction.
However - sensitive electronics , big loops and changing magnetic fields sounds to me like a risky mix(think power lines and geomagnetic storms) . The phone company learned of that decades ago when they shared poles with the electric companies..

i'll stick with parallel conductors in close proximity.
No they don't have shunts. The primary has a constant current so the secondary has a constant EMF at all times.

It's funny that you should mention sensitive electronics. Some of the newer traffic signs had sophisticated electronics for brightness calibration and power factor correction. An airport ordered one for a circuit powered by an ancient CCS. The sign didn't work. The computer in the sign kept shutting itself down. I called the manufacturer and they took me through their troubleshooting steps. At the end they told me that I must have a poor meter and I need to buy a really expensive one then try again. That sounded like a last resort to me. Even though the CCS had the proper power rating to handle the addition of this sign I had the thought that maybe it wasn't operating well enough to power it. Perhaps it was unstable. I rewired the airport at a junction box to cut out half of the sign circuit and the sign starting working. I also rewired the airport to power the whole sign circuit with one of their newer sources and the whole system worked. I had a phone conversation with my boss to try and convince him to sell the airport a new circuit. That wasn't going to happen. It finally occurred to me that the airport bought the absolute cheapest isolation transformer there is. The one they ordered was square but better models are toroidal. I drove to our office and got a toroidal transformer with identical ratings and wallah! the sign worked.
 
  • #34
jim hardy
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The primary has a constant current so the secondary has a constant EMF at all times.
Sounds like an interesting transformer to have in one's "bag of tricks".

old jim
 
  • #35
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The current equivalent source –and the voltage equivalent source-it is an ideal source – as per Norton [or Thevenin] theorem. See:
http://en.wikipedia.org/wiki/Norton's_theorem
IEC 60909 Short-circuit currents in three-phase a.c. systems – employs an equivalent voltage source- c*Un-in the short-circuit location in order to simplify the short-circuit current calculation.
 
  • #36
sophiecentaur
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So to sum up, a CONSTANT current source is basically a controlled voltage source to make up for voltage drop or voltage increase due to distance in lines and loads starting or turning off?

Fixed?
Absolutely not, imo. A Constant Current Source produces whatever current it is specified to produce. How this is achieved is totally irrelevant. I realise that a CCS is often achieved with a voltage amplifier somewhere plus feedback but a could also point out that an electron beam in a high voltage tube is as good a source of a precisely defined current as you would want. There is a bit of chicken and egg here but we should not confuse the result with how its achieved. Who is to know that the 'controlled voltage device' is not constructed (internally) with current amplifiers which give the impression of voltage amplification? Voltages may be a bit more familiar to us than currents but what is the relevance of that?
 
  • #37
psparky
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Absolutely not, imo. A Constant Current Source produces whatever current it is specified to produce. How this is achieved is totally irrelevant. I realise that a CCS is often achieved with a voltage amplifier somewhere plus feedback but a could also point out that an electron beam in a high voltage tube is as good a source of a precisely defined current as you would want. There is a bit of chicken and egg here but we should not confuse the result with how its achieved. Who is to know that the 'controlled voltage device' is not constructed (internally) with current amplifiers which give the impression of voltage amplification? Voltages may be a bit more familiar to us than currents but what is the relevance of that?
You still picking on me about that comment? lol. Its just a reasonable guess to what might be going on. Sometimes I work in "close enoughs"....just part of my care free personality.

But I understood what you are saying and thank you for the correction as usual.
 
  • #38
sophiecentaur
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You still picking on me about that comment? lol. Its just a reasonable guess to what might be going on. Sometimes I work in "close enoughs"....just part of my care free personality.

But I understood what you are saying and thank you for the correction as usual.
I'm sorry about that. For some reason I was answering an older post that you had already been bashed about. Your example about controlling current into a remote device was valid but not a general definition.
 
  • #39
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When we speak about current sources, solar cells come first to my mind. V-I characteristic look almost like a perfect current source (of course, the thing should not be overloaded-nothing has infinite power):
http://dc220.4shared.com/doc/3fWupKdS/preview_html_7e5664ff.jpg

All other current sources I know of are realized "artificially" (by use of combinations of voltage sources, electronic components and feedback)
 
  • #40
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Zoki85 - Voltage sources are also "artificial". Feedback is used by the power company to keep the grid constant voltage. A car alternator has a voltage regulator with feedback to adjust field current and maintain constant bus voltage. All current sources and voltage sources are realized by using some means of regulation, with the only exception I can think of being a battery. A battery could be built for constant current, but constant voltage works much better. Shelf life for CC is awful vs. CV.

Claude
 
  • #41
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Claude, we can say every man made electrical power source is artificial. But constant voltage control in general is much easier to achieve than current control. Voltage regulation of sync generator is not a good example becouse it is not trivial (besides it is AC voltage). You mentioned battery... And how about self-excited compaund wound DC generator? That's a pretty straightforward construction. No electronics, no chemistry...
 
  • #42
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Claude, we can say every man made electrical power source is artificial. But constant voltage control in general is much easier to achieve than current control. Voltage regulation of sync generator is not a good example becouse it is not trivial (besides it is AC voltage). You mentioned battery... And how about self-excited compaund wound DC generator? That's a pretty straightforward construction. No electronics, no chemistry...
I don't know why you would suggest that CV is easier to achieve than CC. Take a simple bicycle pedaled generator. If the handle bars are equipped with a field current control adjustment, an ammeter, and voltmeter, regulating either the current or the voltage is equally easy. If CC is desired, then as the load resistance varies, we can adjust field current and adjust pedaling effort to achieve steady current for any load resistance until it is too large and the person pedaling cannot output enough power.

Likewise, we can do the same for voltage. Say a person can output 50 watts of pedal power. We wish to output a fixed 10.0 volts. With a load of 100 ohms, we only need 0.10 amps at 10V, and the power is a mere 1.0W. Child's play for the biker. Adjust field current for 10V open circuit, then pedal until the voltage is 10 volts. Once the 100 ohm load is switched in, the biker will have to exert 1.0 more watt of effort, and the output voltage will drop very slightly due to stator inductance. Increasing field current slightly restores 10.0V.

Now the load is changed to 2.0 ohms. The biker will immediately feel greater opposition. The voltage will plummet. Biker sees the ammeter and voltmeter readings drop, so he/she pedals harder. The field current is increased and when the biker is outputting 50 watts, the field current is set for 10.0V under load. The biker is getting a good work out. Then the load is removed. Immediately the biker feels little pedaling resistance, and the voltage skyrockets. The field current is set for maximum load, so it must be adjusted down. Biker goes back to light pedaling, and 10.0V is restored.

With constant current, a larger load resistance means more biker effort. If we wish to have a constant 1.0 amp, with 1.0 ohm loading, it is easy, just 1.0 watt. Field current is adjusted so that reasonable pedal speed is achieved. Now the load is stepped to 50 ohm, a fill 50 watts with 1.0 amp.

With 1.0 amp and 1.0 ohm, the output voltage was 1.0 volt. But with 50 ohms, 50 volts is needed to maintain 1.0 amp. The biker must increase the pedal effort enough to get 50V and 50W. The field current gets adjusted so that it becomes a combination of both.

With either scenario, the output quantity being regulated is done artificially via feedback. Nature does not provide sources that maintain constant current or voltage. We do it with feedback. Re batteries, a CC battery would output minimum power when shorted. Thus the shelf life depends on how low the resistance of the conductors and electrolyte can go. With CV, the no power condition is open. It is much easier to achieve ultra-high resistance compared to ultra-low. Insulators lose less power than conductors.

Nuclear battery cells are constant current. They work better than CV. But I doubt that the general public will be seeing cells with fissionable materials sold over the counter.

So we agree that power sources are artificially regulated. I don't believe that CV is easier to achieve than CV. But losses incurred with CV are much lower than with CC. Power lines are a prime example. Transmitting at high voltage increases insulation loss. But that loss is so miniscule it isn't a problem. Lowering current decreases conduction loss. That loss is enormous compared to insulation, by many orders of magnitude. So not only do we transmit at full voltage in CV mode, variable current, we also step up V and step down I by a factor of a thousand or so to keep losses low.

Losses incurred in conducting eclipse those incurred in insulating, so we build our power sources, from tiny batteries, to gigawatt turbines, for CV mode operation. Anyway, the OP asked about current sources, and I just wished to answer by saying that the alternator in a car, the power plant turbine, etc., could just as well be configured for CC mode. The losses are too great, so we stay with CV.

There are other issues as well, like shock hazard due to varying voltages incurred with CC mode. Also, CC mode makes fixed frequency harder to achieve, and synchronous motors rely on fixed freq. Also, fixed freq facilitates multiple generators connectied together for load sharing. CV is without a doubt more convenient and less lossy.

Since all power sources are CV by design, it is easy for a person to acquire the mind set that voltage sources exist but current sources do not. Many people regard a current source as a theoretical abstraction, while viewing voltage sources as "real". Both are real, but one is more optimum. Sorry to be so long winded. Thanks for your input, enjoy the football games this weekend.

Claude
 
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  • #43
analogdesign
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Constant current sources are everywhere. Think of a common-emitter or common-source amplifier with an active load. Over the operating range of the amplifier, its load approximates a constant current source. If it didn't the usable swing of the amplifier (for a given distortion specification) would be ludicrously small.

The simplest "constant current source" I can imagine is the humble current mirror.
 
  • #44
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I don't know why you would suggest that CV is easier to achieve than CC.
Becouse in the majority of cases it is.
Constant current sources are everywhere. Think of a common-emitter or common-source amplifier with an active load. Over the operating range of the amplifier, its load approximates a constant current source. If it didn't the usable swing of the amplifier (for a given distortion specification) would be ludicrously small.
The simplest "constant current source" I can imagine is the humble current mirror.
And these sources, in order to operate as constant current sources, are powered by what kind of electrical source?
 
  • #45
analogdesign
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And these sources, in order to operate as constant current sources, are powered by what kind of electrical source?
Why a constant voltage source, of course, realized "artificially" (by use of combinations of voltage sources, electronic components and feedback). ;)
 
  • #46
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Yes, you got the point! ;)
 

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