# Current and voltage source

#### jasonhaykin

Joined Mar 8, 2007
23
Hi guys,
what is a current source ?
if someone says a supply of 5A , what do they mean ?
does current source have a voltage rating also ?
Thanks

#### beenthere

Joined Apr 20, 2004
15,819
The basic electronic formula, E = IR, says it must. E is voltage, I is current, R is resistance.

The 5 amp rating means thar the regulator can supply up to 5 amps current before the output voltage sags or the ripple increases beyond spec.

#### tiger_prodigy

Joined Mar 10, 2007
2 just a thought but is a current source the same as a voltage source except that it provides a constant current value and a adjustable voltage source as to allow for a higher amount of watts to be drawn on? form the formula W=EI and so when some one says 5A source does that not mean that you have a constant supply of 5A but a varying voltage value according to the amount of watts demanded by the circuit? Please correct me f I'm wrong thanks.

#### beenthere

Joined Apr 20, 2004
15,819
Most power supplies work in the volltage mode, so they maintain a constant voltage despite a varying load. Some have a current regulation mode where the voltage changes to maintain a constant current.

It's easier maintaining a constant voltage, unless the amperage limit is exceeded. Constant surrent is not possible when the load gets too resistive - not enough voltage to push the current. And there is also the limit to the amount of current that can be supplied.

• CiaranM

#### awright

Joined Jul 5, 2006
91
Current sources and voltage sources are exact analogues of each other and they each have their appropriate applications. We are most familiar with voltage sources as power supplies for most applications.

A VOLTAGE SOURCE such as a common bench power supply or a battery ideally provides a fixed voltage independent of the amount of current required to maintain the fixed voltage. It appears to the load to have a ZERO SOURCE RESISTANCE. That is, viewing the junction of the voltage source output and the load input as the center point of a two-resistor voltage divider fed from an infinitely stiff, fixed voltage source, if the top (source) resistor is zero, the voltage at the junction ramains fixed for any value of the lower (load) resistance other than zero. Of course, the whole analogy breaks down at the value of load resistance to which the source cannot supply enough current. That point would be the knee of the voltage/current curve of the supply. Any voltage source will have some rated maximum current it can supply before voltage deviates more than some specified amount from the set voltage.

The degree to which the voltage source deviates from zero source resistance is the degree to which it deviates from perfection, as all real devices must. On a voltage/current graph, the ideal voltage source would have a straight, horizontal line out to the point at wihich it cannot supply any more current and would then have a vertical line down to zero volts at its limiting current. In real life, the horizontal region slopes slightly or dramatically out to a sharp or very gradual knee (or burnout), followed by a steep or gradual slope down to zero volts.

A CURRENT SOURCE ideally supplies a fixed current independent of the voltage required to maintain that current. It appears to the load ideally to have an INFINITE SOURCE RESISTANCE supplied by an infinite voltage (neither being actually true). A current source will have a voltage compliance specification, that is, some voltage at which it can no longer maintain the desired current within some specified accuracy. Note the inverse analogy with a voltage source. Viewed as a two-resistor voltage divider supplied by a quasi-infinite voltage, if the upper resistor is extremely large (approaching infinite) then the value of the lower (load) resistor will have no influence on the total current flowing and the voltage at the junction will vary in exact proportion to the load resistance. This is a current source.

Both devices are imperfect, the degree of imperfection depending upon the intended application, the quality of the design, and where in the range of capabilities of the source you are operating, among many other factors.

A voltage (vertical axis) vs. current (horizontal axis) graph of the output of the two types of sources would be a straight horizontal line for a perfect voltage source and a straight, vertical line for a perfect current source.

Many modern bench power supplies can be set up by the user for either constant voltage or constant current function by proper use of the voltage limit and current limit knobs.

A common application of a current source that you use every day would be a battery charger that limits the initial charging current to some safe value specified by the battery manufacturer. Another is as the "long tail" of a differential amplifier stage. Schematic diagrams will show such "long tailed" configurations in one or more stages of an operational amplifier IC and in many audio power amplifiers. In real applications, of course, there is no infinite resistance or infinite voltage supply - it is all simulated with active electronics. Take a look at the schematic of an op amp and look for two mirror-imaged transistors and notice how the emitters are connected directly or via a low resistance. The circuit below those connected emitters is a constant current source.

Another common application of current sources is in circuits powering resistance strain gauges. If the simulated source resistance appears to be high enough, the output voltage from the strain gauge will be directly proportional to the resistance of the gauge and, therefore, the amount of strain.

Hope this is interesting to you.

awright

• CiaranM

#### jasonhaykin

Joined Mar 8, 2007
23
thanks , this is very interesting . #### solarnovice

Joined Apr 12, 2008
3
i wondered if I could deviate slightly with my question for a home-made solar charger... Say you have a solar panel putting out 15V 100MilliAmp giving 1.5Watt power rating. hope my numbers are right. Now most USB devices require a 5 to 5.3V supply (max 500ma) to charge up. So i looked around and found it was easy enough to limit the voltage to 5.3V. But the thing that is bothering me is that the extra volts were dissipated via resistor into heat. Could there be a way to use those extra volts to push up the amps put out by the circuit instead? I mean, the solar panel produces the extra volts but not the amps. Further, if I attached a device that required more amps than the solar panel can generate, what will be the effect ?

my name says it all - i am a novice to this subject. could be i am no novice in others! Thanks for any suggestions/clarifications.

Joined Apr 16, 2008
1

#### Caveman

Joined Apr 15, 2008
471
solarnovice,

Shoulda been a new post, not a thread hijack, but since you asked...

What you want is a switching power supply. It is able to convert high voltages at lower currents to lower voltages at higher currents (minus some efficiency loss, which becomes heat).

If you want to do it really simply, look into the TI PTH08080W. It costs a bit more (\$8.60), but it can be done. If you want to do it cheaper, you will have to learn a lot more.

Oh and read the entire datasheet. These kinds of things require attention to detail.

#### studiot

Joined Nov 9, 2007
4,998