# Homework Help: Is a resistor a dependent source?

1. Apr 19, 2013

### ch3cooh

Is a resistor a voltage-controlled current source? WHY?
I think it is but I don't know whether I'm correct.

2. Apr 19, 2013

### Staff: Mentor

Hi ethanoic! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

Not many people would bestow that title on a humble resistor. A resistor can be used, in conjunction with a voltage source, to approximate a current source providing the voltage across that resistor is held approximately constant, and this condition is usually met by arranging that at all times source voltage ≫ load voltage.

By itself, a resistor is not a source of anything.

Last edited by a moderator: May 6, 2017
3. Apr 19, 2013

### cabraham

A resistor is a passive element, nothing more or less. It's I-V relation is Ohm's law. A voltage source across it will give a current. A current source across it will give a voltage. A resistor can translate V to I or I to V, either way.

Claude

4. Apr 19, 2013

### CWatters

Current sources have very high ouput resistance. One way to model a current source is to use a very large value resistor. The down side is you may need a very large voltage source as well.

For example suppose you needed a 1mA current source that operated over the range 0-1V.

If you used a 1000V supply and a 1MOhm resistor then the current would range from...

1000/1000,000 = 1000uA
to
(1000-1)/1000,000 = 999uA

which is reasonably constant over the required voltage range!

Even using a 10V source and a 10K Ohm...

10/10,000 = 1000uA
(10-1)/10,000 = 900uA

Call it 950uA +/- 5%

Within limits the current can be changed by varying the source voltage.