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Why Resistors BURN OUT (and how to choose the right one) This small resistor worth a few cents decides whether a production line worth millions of euros keeps working… …or suddenly stops due to a fault. The problem is that, very often, resistors are chosen or sized in the wrong way: they overheat, change value, and can even burn out. In this video we’ll see together why it happens, how a resistor really works inside the circuit, and how to choose the right one so you don’t destroy LEDs, power supplies, boards, and electrical panels. At the end we’ll also do an extreme experiment: we’ll push an undersized resistor to the limit, so you can see with your own eyes what happens when you get the calculation wrong. And in the meantime I’ll leave you a question: If I have a battery, a resiscostor, and an LED connected in series, should the resistor be placed before or after the LED? You’ll see the answer shortly. 1. What a resistor really does Let’s start from the basics: a resistor is a component that makes it more difficult for current to flow. Let’s take this circuit as an example, the battery provides a voltage (symbol V) measured in volts: it’s the “push”, the total “force ” between the POSITIVE and the NEGATIVE that makes the electrons flow. Instead, the current (symbol I) is measured in amperes: amperes tell you how much “current” passes in the wire, that is how much electric charge flows every second. In simple terms: more amperes = more current flows. Now the resistor comes into play, indicated by the symbol R and measured in ohms: it is the component that limits how many amperes can flow in the circuit. The greater the resistance, the less current can pass. In electrical diagrams you recognize it by its symbol: a rectangle in European standards, a zig-zag in American ones. This formula that was created is Ohm’s law: Voltage equals resistance times current. These three quantities are linked to each other, and the three values influence each other. Here is also its inverse formulas. In practice, what happens is: • If the voltage increases and the resistance stays the same, the current increases. • If the resistance increases and the voltage stays the same, the current decreases. • If voltage and resistance increase or decrease in the same ratio, the current stays the same. When the current is high, the resistor finds itself having to “handle” a lot of energy passing through it: the more energy passes every second, the more power it must dissipate, and that power inevitably ends up as heat. And this is where the problems start. 2. How a resistor protects an LED Let’s take a red LED and a 12 volts and 10 ampere-hour battery; we measure a voltage of 12,45 volts. And here comes the important thing: this battery is not as “gentle” as it might seem. In a circuit like the very simple one that you’re looking at, if nothing limits it, it can deliver for a few seconds enormous currents, on the order of hundreds of amperes. An LED, instead, lives happily and brightly with a few milliamps. So if you connect it directly to the battery you doom it. Here the rule is to protect the LED, with a Resistor. Let’s take these two resistors and, with the multimeter, measure their value in ohms. Later in the video we will also see how to recognize it at a glance simply by looking at it. The first is rated at 47 kilo-ohms and measures 46,2 kilo-ohms. The second is rated at 2,2 kilo-ohms and measures 2,17 kilo-ohms. Here is the answer to the question at the beginning of the video: in your opinion, starting from the negative terminal, do we put the LED first or the resistor first? You’ll be surprised to know it makes no difference! It will work the same either way! Being in series, the current is limited both before and after the resistor, and it is the same at every point in the circuit. We connect in series: positive terminal ? 47 kilo-ohm resistor ? LED ? and negative terminal. The LED, as we see, lights up barely because we are letting a tiny current pass, but by doing so we are protecting it from an enormously oversized battery. We can measure that the red LED has a voltage drop across it of 1,77 volts. We finally measure the current that flows through the circuit, which, limited by the resistor, is 0,23 milliamps. All the calculations match. Now I replace the 47 kilo-ohm with the 2,2 kilo-ohm. The LED is much easier to see because with this smaller resistance we are letting more current pass. ì We can measure that the red LED now has a voltage drop across it of 1,95 volts. ì And that the series current measurement is 4,8 milliamps. Here too, the calculations match. By doing this we are effectively running a small LED with a battery that will keep it on for theoretically 86,8 days. The point is that the battery can give a huge amount, but it doesn’t decide how much current flows. The circuit decides, and with the resistor, you’re setting a limit. 3. What a resistor is made of It is time to see what’s inside the resistor! A classic barrel-shaped resistor (that is, an axial resistor) is made up of three main parts, each with a very precise role: The first (the most important) is the resistive material. It can be a thin layer of carbon, a thin layer of metal, or a wound resistive wire. This is the part that sets the resistance value, that is how much the component opposes the flow of the current. To precisely adjust the resistive value, a spiral-shaped groove is often cut into the carbon or metal layer: in this way, by lengthening and thinning the electrons’ path, the resistance increases. The second part consists of is the terminals, also called lead wires They are the two metal wires for current input and output that allow the resistor to be connected to the circuit. Finally there is the body and the insulation. Inside we see a ceramic core, which provides mechanical rigidity; while on the outside there is an insulating coating that protects the component from humidity, heat, and external agents. 4. Types of resistors: