Voltage and Current Sources

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Every circuit has a power source, or else it won’t do much. While the actual power source can vary wildly, they all fit under the two general titles of “voltage source” or “current source”. From a lightning strike to a USB port, any source can be modelled as one of these two things. As such, they will be found in nearly every circuit you come across.

Voltage Sources

Voltage source schematic symbols
Voltage source schematic symbols

A voltage source is anything that creates a constant voltage differential between its two outputs. In general, you assume that one side is tied to ground, or provides the reference voltage level. The other side is maintained at the rated voltage, whatever that happens to be. An ideal voltage source will source whatever current is needed to maintain the voltage differential you’re aiming for. With a voltage source, it’s important to never short circuit it as it will provide as much current as it possibly can to maintain that voltage differential.

Let’s look at Ohm’s Law.

Ohm's law formula
Ohm’s law states that the voltage across a resistor is directly proportional to the current flowing through the resistor.

We can see that current is voltage over resistance. If resistance goes to zero, current correspondingly becomes infinite. This is not an ideal situation in most cases. Even though real voltage sources, such as batteries, won’t create infinite current, they will produce as much as they can, which can cause burns or small explosions and most likely destroy the circuit itself.

Voltage and Current Sources Resistance vs Current Graph
Resistance and current relationship

Voltage sources are much more common than current sources. Frankly, most anything you think of will be a voltage source. Batteries, DC power supplies, electrical outlets, the USB port on your computer, even things like solar panels are voltage sources. While these are all voltage sources, they do have their own unique concerns in how to work with them.

Different types of voltage sources
Different voltage sources - batteries, DC power supplies, electrical outlets, USB ports, etc.

Current Sources

Current source schematic symbols
Current source schematic symbols

While current sources aren’t that common to most people, they do play an important role in semiconductor circuit design with things such as current mirrors and are even used in LED drivers and battery chargers. Current sources vary their voltage in order to meet the design requirements for the current needed by the circuit. They have the exact opposite safety problem of voltage sources.

Let’s look at Ohm’s law again, but arranged in a different way.

Ohm's law formula solving for voltage
Rearranged Ohm's law formula

In this form, you can see that if resistance goes to zero, and current is held steady, then the voltage will also go to zero. However, if resistance goes to infinity, an open circuit, then the voltage will go to infinity. Again, this is a problem. In essence, just not connecting the two halves of the circuit creates dangerous voltages. Although real current sources will not reach an infinite voltage, it is still a dangerous or destructive situation.

Resistance and voltage relationship graph
Resistance and voltage relationship

For both voltage and current sources, with good circuit design (and, as a consequence, higher prices), these dangerous situations are controlled. Still, if you’re designing a circuit, you need to make certain that you have something to prevent those situations.

AC Voltage Supply

AC voltage sources follow all the same rules as a DC voltage source, and as they are extremely common, there are well established notations to represent them.

Voltage and Current Sources Voltage Source Symbols with Notation
A 5V DC voltage source and a 120V AC voltage source with 60Hz frequency.

The symbol and notation shows that the voltage source on the right is alternating (with the curvy line) and that it has a peak to peak voltage of 120 volts. It explicitly states that it alternates at 60 Hertz, which is the assumption in the US unless indicated otherwise. It also assumes that the voltage source is a sinusoid, which is always the case unless stated. Much as how that DC voltage source will provide whatever current is necessary to maintain 5 volts, the AC voltage source will provide whatever current is necessary to maintain the voltage *at that moment*.

Time in Schematics

If you think of certain situations, like lightning, sparks (miniature lightning!), opening/closing a switch, you may be confused by the thought that this is producing a “constant voltage differential” as the voltage changes quickly and frequently. It also is dependent on time, whereas we just assume a steady state with everything else we’ve been looking at. There are ways to deal with this as well.

Unit step function shows how a switch behaves
Unit step function shows how a switch (OFF/ON) behaves.

With step functions, such as a switch opening or closing, all of the pertinent information is written in the schematic next to the voltage source representing that switch. You then use time constants and differential equations to see how the voltage changes over time. This can get mathematically intense but it’s not our intention to explain how this works, just that it happens.

With impulses, something to represent lightning and sparks, you can model these in a few different ways and there are usually capacitors used in the model as well. This is a, perhaps only surprising to me, rather large field of study. Mostly due to engineers out there who dedicate their lives to making electronics more robust when facing static discharges.

Ideal versus Real Power Sources

To this point, we’ve been assuming that these sources are ideal power sources for both the voltage and current sources and just want to clarify that, we usually assume that our sources are perfect or “ideal” when we’re running simulations or performing calculation. This makes life significantly better and usually is quite close to reality. This means that they can source as much current or voltage as necessary to do their job and don’t have any built-in capacitances, resistances, or inductances. But we do need to state this: Real power sources do have limitations and unwanted impedances and they may be incredibly important. But, again, we can also very frequently ignore them.

Summary

So, we discussed voltage sources and current sources and briefly went over the potential dangers of both. We also touched on the difference between an ideal and real source and how, to simplify things, we ignore certain factors at times. If you have any questions, leave it in the comments below!

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