Capacitance between parallel conductors in a cable

[plugwash]

The main reason is the near impossibility of synchronising the phases of the two countries.

IIRC they run a synchrnoised grid all the way from poland to portugal.

Afaict there are three main reasons for using HVDC in international links.

1: capacitive losses, underground/undersea cables have much higher capacitance to ground than overhead line. In an AC system those capacitances cause increased losses (both from any non-ideal behaviour in the capacitors themselves and from the current flow caused by constantly charging and discharging them).
2: political considerations, runing a synchronised grid requires you to cooperate to run the whole thing as one big grid. That may be politically difficult in some cases (I wouldn't think this would be too big a deal between britan and france though
3: incompatible systems (this one clearly isn't an issue in the britan to france case)

I strongly suspect that if we had a land border with france we would have an AC link between our grids.

 

[plugwash]

An ideal current source can be a voltage source of value = infinite, in series with a resistor of infinite value.

Infinite resistance (which an open circuit is) means no current can flow so not a current source.

A perfect voltage source driving a short circuit would result in infinite current, a perfect current source driving an open circuit would result in infinite voltage. What this clearly implies is that neither a perfect voltage source or a perfect current source can actually exist.

It's a basic principle of circuit theory that a perfect voltage source with a resistor in series is equivilent to a perfect current source with a resistor in paralell. We can consider a source to be "acting as a voltage source" in regions of it's graph where changing the current only has a small impact on voltage and we can consider it to be acting as a current source in regions of it's graph where changing the voltage has only a small impact on current.

So getting back to the LED driver case if the difference between the forward voltage of the LED and the supply voltage is much greater than the variation in LED forward voltage then the resistor can be regarded as a current source. For indicator LEDs on a 5V system this is easy, the 3V or so drop in the resistor is far greater than the variation in forward voltage of a typical LED. For 3.3V systems it gets more dubious but you usually get away with it because honestly the exact current through an indicator LED doesn't matter that much. The LED will provide acceptable indication anywhere from 5ma to 20ma so if you aim for 10ma you have a LOT of margin for error.

OTOH for larger LED systems where consistancy and posiblly efficiency are more important than in a basic indicator application building an electronic current source/sink can make sense. You can build a current sink that works nicely for strings of LEDs with a transistor and three resistors. This means you can also use an unregulated supply to power the LEDs.

 

[Porque223]

Yeah, with this LED thing I got wrapped around the axle.

 

[JohnW2]

OTOH for larger LED systems where consistancy and posiblly efficiency are more important than in a basic indicator application building an electronic current source/sink can make sense.

Indeed but, as I said, if the light output (or consistency of light output) were, for whatever reason, a crucial factor, then having a constant current source may not be enough - one might then need to tweak each LED individually.

You can build a current sink that works nicely for strings of LEDs with a transistor and three resistors.

Indeed and, probably a bit better (particularly if supply voltage is not well regulated), a transistor, two resistors and a zener diode.

Kind Regards, John