# voltage and current source

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#### Gaston

##### Member
what is the diference betwen a voltage and a current source. does that just mean that you can adjust the voltage or current?

#### checkmate

##### New Member
Ideal voltage source - Provide constant voltage regardless of load.
Ideal current source - Provide constant current regardless of load.

#### ag

##### New Member
The voltage source output (series) resistance should be as low as possible so that the input impedance of the next stage soaks up as much of the voltage as possible (as in a voltage divider), whereas a current source output (shunt) resistance should be as high as possible so that the maximum current flows through the input impedance of the next stage. Some amps have current inputs but voltage ouputs and vice versa. Er, does that help :?

#### lord loh.

##### Member
A voltage source provides a constant voltage. Irrespective of the load resistance. If the resistance drops, lot of current shall flow through and the voltage across the resistance shall drop. So, a voltage source shall supply extra current so that a potential difference across the resistor is maintained...

Think of it as a pipe. A water pressure of 1N is to be maintained across it. Suddenly the pipe widens and water is able to flow easily(voulume per unit time). So the pressure drops. But 1N must be maintained. So increase the volume of the water so that 1N pressure is maintained. (Pressure is analogus to voltage and the volume per unit time to current).

The current source shall give a constant current irrespective of the load resistance. That means the voltage shall change.

If the volume flowing per unit time through the pipe is to be maintained, and the pipe widens, decrease the pressure.

Current sources and Voltage sources are text book concept. There exist no such thing. It is only meant to simplify circuit analysis.

However it may be implimented...
A voltage source may be implimented by placing your load resitance parallel to the zener diode. Here a series equvalent restance comes into picture. There must be a resistor placed before the zener diode to prevent excessive current from flowing into the zener diode and damaging it. But this resistor shall be in series with your load resistor and shall form a potential divider network. Thus the voltage across the load resistor shll not be the drop provided by the zener diode but slightly less.

V across load resistor = Load resistance * zener diode voltage/(load resistance + series resistance)

A current source is a saturated transistor...with no resitance.

Hope this helps....

#### eblc1388

##### Active Member
lord loh. said:
A current source is a saturated transistor...with no resitance.
Can you tell me a bit more on how to build one using a satuated transistor with no resistance?

#### lord loh.

##### Member
Well, I never built one...But the theory is...

The transistor's operating point must be set into it's saturation region. You can know the saturation region from the datasheet of the transistor.

A BC547 transistor needs to have about 400+ microampere to drive it into the saturation region. When saturated, there is a maximum limit on the current that can flow through it, for a fixed given voltage. (Voltage across the collector and emitter) However if you do apply a greater load resistance, it shall decrese the current flowing through it and you will need to increase the voltage.

I read of a circuit in my study of the oscilloscopes. It had a saturated transistor that supplied a capacitor a constant current to generate the desired timebase.

I know that this explanation is full of holes but I know nothing better at the timebeing. If you find a good explanation or a circuit, do make it a point to let me know. (You may PM me.)

Thanks.

#### Styx

##### Active Member
current-sources do not usually use saurated trasistors (since they apeare as just a resistor -hence why not use a resistor).

a much better current-source is the use of a current-mirror that hold two transistors in their active region under control

#### lord loh.

##### Member
Hi Styx,

Why dont you please explain more about about the current-mirror or give a few lionks that might help...

Current mirror sounds great...

#### audioguru

##### Well-Known Member
Most Helpful Member
lord loh. said:
A current source is a saturated transistor...with no resitance.
Hi Bharath,
Nope, not saturated nor no resistance.
An active transistor's collector or the drain of an active FET is a current source (or sink) because they are high-impedance and their load current doesn't change much when the supply voltage of the load changes.

#### ElectricWraith

##### New Member
lord loh. said:
Hi Styx,

Why dont you please explain more about about the current-mirror or give a few lionks that might help...

Current mirror sounds great...
Here's an image of a BJT current mirror.

The transistors need to be matched, meaning betas and junction areas as close to identical as possible. This makes it hard to do with discrete transistors without some careful measurements.
Q1 is connected to make it function just like a diode, providing a stable reference current. I won't go into all the equations at the moment, but the collector current of Q2 will be close to that of Q1. The current transfer equation reduces down to 1/(1+(2/beta)), going to unity at very high beta values. Connecting the bases of additional transistors to the base of Q2 and tieing their collectors together will effectively multiply the current by multiplying the emitter-base junction area.

#### audioguru

##### Well-Known Member
Most Helpful Member
TI used to make some current-mirror ICs in a TO-92 transistor package. They had a wide voltage range, a very constant current and a very high impedance. They were also temperature stable.
I guess nobody except me bought them, so they stopped making them.

#### chemelec

##### Well-Known Member
I guess nobody except me bought them, so they stopped making them.
I Did, Now I make my own.

#### chemelec

##### Well-Known Member
I guess nobody except me bought them, so they stopped making them.[
I Did, Now I make my own.

#### altaf

##### New Member
Gaston said:
what is the diference betwen a voltage and a current source. does that just mean that you can adjust the voltage or current?
TRUE

#### audioguru

##### Well-Known Member
Most Helpful Member
Normally, a constant voltage source automatically adjusts the current to maintain a constant voltage output.
Normally, a constant current source automatically adjusts the voltage to maintain a constant current output.

You could adjust voltage or current yourself to be constant if you want but the automatic circuits are a lot faster than you could be.
You can also have the constant voltage and current adjustable, like in a power supply where you can set the output voltage to whatever voltage that you need and the circuit maintains that set voltage when the load current changes, until the load resistance is so low that not enough current can be supplied at that voltage.
Or you can turn the voltage to maximum and set the current to whatever you need and the circuit maintains that current when the load resistance changes, until the load resistance is so high that not enough voltage can be supplied at that current.

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