How Do Constant Current Sources Differ From Other Power Sources?

[sophiecentaur]

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 realize 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?

 

[psparky]

Absolutely not, imo. A Constant Current Source produces whatever current it is specified to produce. How this is achieved is totally irrelevant. I realize 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.

 

[sophiecentaur]

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.

 

[zoki85]

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)

 

[cabraham]

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

 

[zoki85]

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...

 

[cabraham]

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

 

[analogdesign]

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.

 

[zoki85]

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?

 

[analogdesign]

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). ;)

 

[zoki85]

Yes, you got the point! ;)