Voltage, Current and Resistance in simple terms

Published March 2, 2018 by Sean

One knowledge area which you are expected to be familiar with for ‘Engineering Operations’ is the basic function of electronic components, and some simple strategies that can be used to test them. You can find many books on basic electronic theory, but these tend to be aimed at people who will be designing circuits – and they rapidly reach the point of needing a good understanding of the theory. My aim here is to explain enough to understand the concepts, and to leave the complete theory to ‘The Art of Electronics, 3rd Edition’. If you can read and understand  Horowitz and Hill, this post will probably be too simplified.

The challenge is to relate voltage and current in an electronic circuit with some concepts that are already familiar, without getting tied up in equations, power, energy and electrons. Particularly not conventional current, holes, etc.

You will probably have seen how a resistor is needed when you connect an LED and a battery together (otherwise, depending on a few things, your LED might go pop). This simple example doesn’t help to explain the concepts that we care about, but I might come back to it later.

The ‘classical’ analogy is to compare electricity with the flow of water in pipes of different sizes. Although this is quite an accurate comparison from a technical point of view, it might be a bit too complicated as a first step. Comparing electricity with water assumes you understand how water works under pressure, or in narrow pipes.

Starting with voltage, this is  the driving force. It can vary in size, and you will come across voltage as the main rating of a battery (1.2, 1.5 4.2, 9, 12 volts being typical for some common batteries). This doesn’t say how long the battery will last, just how strong it is. Imagine a weight lifter, and a long distance runner. The weight lifter can perform a harder task, but does this for a short time. Actually, we can see that how strong they are doesn’t tell us too much about what they can do at all, it’s only part of the story. Depending on the task, you know you need the right sort of person though. If you carry a rock up a hill, you give it more ability to do work.

Current is part of the measurement of going a job. It’s like asking how fast someone is running, or how many people are in a taxi, or how long it takes to read a page in a book. With the rock we carried up the hill, current is a way of looking at how fast it rolls down the hill. Relating current to a battery is complicated.

Resistance is the magic that links voltage and current. For any part of a problem, if you know two, the other is already known. Resistance is a measure of how easy it is to do something. A direct comparison would be to look at crossing a room. When the room is empty, its easy to cross. Replace that empty room with a tight crowd of people, and it will take longer to cross. If our rock is round, it will roll more easily. If we give our runner a heavy weight to carry, they will run more slowly.

Circuits always have some resistance, but it’s not clear what the reason to add the ‘resistor’ component is. To understand that, we need a purpose for our circuit, and this is where things get ugly. Modern circuits hide most of what is going on inside a chip, and use digital logic. The first time that the purpose of a resistor becomes clear is where we need to make a comparison within the circuit. Coming back to the athlete analogy, we can even up any competition by handicapping one of the entrants. Applying resistors in a circuit allows us adjust parts of the behaviour, or start to decide when we reach a certain level. If you want to split a voltage in two, the two resistors will do the job for you. Here, we’d chose high value resistors so that we didn’t waste more current than necessary. If the voltage we have is just a bit too high to do a certain job, we might add a resistor to make the job a bit harder (or maybe more predictable). In some cases, you can view resistors as a kind of marshal, making sure the circuit is well-behaved. The last example of using a resistor is where we want to measure the current – here we need to look, but not interrupt things too much(and we’d want to use a small resistor).

Assuming we have a power supply, this will have a specified voltage, and also a maximum current that it can provide. It won’t force it’s full current into a circuit, but you need to make sure that the circuit doesn’t try to take too much current. You need to make sure that the circuit expects the voltage that the power supply provides (as a minimum). You can connect one or more circuits to the same power supply, so long as they all expect the same voltage, and the current added together is not too much for the supply. What you can’t do is connect two circuits one after the other (in series) so that the voltage is split, and the current re-used. With components, you might see this approach though.

When there is both voltage and current being used, the result tends to be something heats up. You might get light, or a motor moving, but there will be some heat as well. Maybe a lot of heat… This is one reason that it’s important to use the right value components – otherwise things will break. An LED is an example of how things can break easily. With an LED, there is a lot of resistance until it starts to work (and it starts to light up). As soon as it reaches the point it’s designed to work at, it can’t really work any harder, but it can’t push back and will carry on working till it gets damaged. This is a really special case where you need to restrict it, so it can work, but not get carried away. There are three ways to do this:

  • Use a really feeble battery, which can only just power it.
  • Use a special circuit which behaves like this feeble battery.
  • Use a resistor to weaken a normal battery. You still need to pick the right size of resistor.

To make the special regulator circuit, you’ll also use resistors, but probably a transistor or two as well. The difference will be that you can achieve better control with less wasted energy.

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