Voltage sources and Current sources are both sources of Energy. Both have a positive/negative terminal, supply voltage and there is current coming out from both of them. But what is the difference?

By nature we are more familiar with voltage sources. Example: batteries. There are various types, think for the 9V battery. It means it provides 9 volts! But here is the trick. 9 volts are NOT always 9 volts. It is when we have an open circuit yes, but once you add a load it is a whole different story.

In short:

  • Voltage Source
    • maintains constant voltage irrespective of the load
    • Internal resistance << load for stable voltage
  • Current Source
    • maintains constant current irrespective of the load
    • Internal resistance >> load for stable current

Essentially, the resistance of the load determines the accuracy of the source. A voltage source will try to maintain a constant voltage but vary the current, whereas the current source will provide a constant current but will vary the voltage it can supply.

So, what is often confusing: They both do the same thing: supply voltage & current but

  • the voltage source is good at supplying constant voltage
  • the current source is good at supplying constant current

Symbols

Voltage and Current Source Symbols

Voltage Source

  • Good (stable) voltage source irrespective of the load
  • Current varies
  • The internal resistance r is in series with the EMF E
    • If the circuit below is a battery, A and B are the batterie’s terminals
    • r is inside the battery
  • If a load is attached, there will be a current flowing in the circuit.
Voltage source: current drawn vs voltage supplied graph

  • From Ohm’s Law V=IR or I=V/R it is clear that the current is inversely proportional to the resistance: the smaller RLoad is, the large the current is.
  • The load will not receive the entire voltage E can supply. Why? Because of the voltage drop across the internal resistance. The load will only receive the voltage at AB:
equation for the real voltage supplied of a voltage source, which is the EMF - the voltage drop across the internal resistance
  • Ohm’s Law also applies on the internal resistance r. So, the larger the current is the more voltage will drop across r. Therefore, Vab will be smaller. This is not what we want.
  • We want a large load compared to the r!

In the image above you can see the general representation of a voltage source. The graph on the right shows that if we increase the current, AB voltage will drop. We can prevent this by choosing r << Rload

  • Below there is a calculation showing mathematically why we should choose a load with a much higher resistance than the internal resistance of the source.
  • Ideally, Vab is equal to the EMF. But we can’t achieve this due to the voltage drop across r. The only way to achieve this is to have a high Rload value, so (Vab/Rload)*r is very small. This way we subtract a small number from E so Vab is almost equal to E.
proof for choosing a large load to have a stable voltage source

Current Source

  • Good (stable) source of current irrespective of the load
  • Voltage varies
  • An ideal current source I is in parallel with the internal resistance r.
  • Since r and Rload are in parallel, the current will split between these two branches.
    • I = I1+I2
    • where I2 flows in the Rload branch
  • Ideally, we want I1 to be super small so most of the current flows to the load
    • So we want I = I2 but can never achieve this due to r.
    • But we can make r big so I1 is minimal
  • From Ohm’s Law: if there is a resistor and a current flows across it, there will be a voltage drop.
    • So there will be a voltage drop across Rload
    • r is in parallel, so the voltage drop across R is the same.
    • Thus, V1=V2
  • From common sense, if a resistor resists the current flow, and we want minimal current through r, we can achieve this by r >> Rload
  • Here is also a quick mathematical explanation:
proof for choosing a large load to have a stable current source

optional reading: Success in Electronics book by Tom Duncan
NEXT TOPIC: Norton’s Theorem