parallel resistance Rs of a current source

[PG1995]

Hi

I have always thought of a current source as a very, very high voltage supply with a very, very large resistor, Rs, in series (I mean this large resistor would make series connection with the load resistor, RL). The larger the series resistor, Rs, the less effect the change in load resistor has on the load current. The Rs should be a variable resistor which is adjusted to get the wanted current through the RL. In other words, a current source is also a special type of voltage source.

In a regular voltage source the Rs is very small which can be adjusted to vary the voltage across the RL. If you want more volts to appear across the RL then you need to decrease the resistance of Rs.

Please correct the stuff above if you deem something wrong. Thanks

Now I'm coming to the main question(s).

Please have a look on the linked diagram (or attached one):
https://img843.imageshack.us/img843/3518/imgan.jpg

A current source is represented as shown in Fig. 3 in the linked and attached diagram with the Rs in parallel configuration. I don't get it. Why? As I say above, in my view, a current source is a special kind of voltage source. They say, Rs, is an internal resistance and makes a parallel connection (as shown in Fig. 3). I don't get it. Do you get where I'm having problem? Please help me. Thanks

Regards
PG

 

[carbonzit]

I have always thought of a current source as a very, very high voltage supply with a very, very large resistor, Rs, in series (I mean this large resistor would make series connection with the load resistor, RL). The larger the series resistor, Rs, the less effect the change in load resistor has on the load current. The Rs should be a variable resistor which is adjusted to get the wanted current through the RL. In other words, a current source is also a special type of voltage source.

While this may work conceptually, with a large R in series, it certainly doesn't work for any real-world current source, unless you're supplying infinitesimally small amounts of current. Think about it: large R allows only a very small I.

 

[ronv]

Constant current sources usually employee an active element to vary the voltage to keep the current constant for a varying load.

Check out the LM317 datasheet for a simple one.

 

[Ratchit]

PG1995,

I have always thought of a current source as a very, very high voltage supply with a very, very large resistor, Rs, in series (I mean this large resistor would make series connection with the load resistor, RL). The larger the series resistor, Rs, the less effect the change in load resistor has on the load current. The Rs should be a variable resistor which is adjusted to get the wanted current through the RL. In other words, a current source is also a special type of voltage source.

You thought wrong. You are confusing an ideal current source, which is a black box that automatically adjusts its voltage to keep up a set current, even if it has to arc across its terminals, with a practical current source that has a finite voltage, and an adjustable finite resistance. Using a voltage source to make a practical current source does not make it a special voltage source.

A current source is represented as shown in Fig. 3 in the linked and attached diagram with the Rs in parallel configuration. I don't get it. Why? As I say above, in my view, a current source is a special kind of voltage source. They say, Rs, is an internal resistance and makes a parallel connection (as shown in Fig. 3). I don't get it. Do you get where I'm having problem? Please help me. Thanks

Since the current source in figure 3 is not perfect, it has a finite output impedance which is represented by a parallel resistance of high value. If it were a ideal current source, its parallel resistance would be ∞. Now, what is your problem?

Ratch