Hello! I certainly hope you did your homework and now know a little bit about voltage, current and resistance because today we’ll be digging a bit deeper into what does what in a basic electronic circuit.
You’ve seen their names and their little symbols on schematics, but have no idea what they really do. Passive components, resistors and capacitors (also inductors but we won’t really bother with them for now as they are much less often used in guitar pedals), are everywhere and are essential building blocks of all guitar effect circuits.
Today, we take a quick look at resistors.
As you should remember from reading about Ohm’s law, applying a voltage to a resistance will generate a current. There is no current without resistance. Actually, there’s is no such thing as “no resistance”. A perfect conducting medium does not exist and zero resistance would mean infinite current in any given amount of time which is physically impossible. There are things like “superconductors” and “perfect conductors” but these don’t apply to us and as such we’ll pretend they don’t exist. For all intents and purposes related to building guitar pedals: wires, guitar pickups, cables, jacks, PCB (Printed Circuit Board) traces, air, you, everything has resistance (and no, it is not futile… nerds).
While resistors have many different practical uses, it is safe to say that they are basically used to control the amount of current, in amperes, going through them. What does that mean for your guitar signal? Not much by itself, I’m afraid. But what if you were to put two resistors in series and take measurement right in between? You get a very simple, but very powerful arrangement: the voltage divider.
Here’s what it looks like:
Let’s say our typical Fender Telecaster with stock single coils and an average strum on the low E string generates a signal of about 100 mV of amplitude. That’s one tenth of a volt. Following the schematics, we can see that our 100mV signal will be applied across R1 and R2, generating current. Remembering Kirchoff’s Voltage Law (you did read about it, right?), we know that the sum of the voltage drops across all series resistor will amount to our source voltage. In plain English, that means that the amount of volts that will drop from R1 + the amount of volts that will drop from R2 will be equal to the amount of volts we put in, which is 100 mV.
Using Ohm’s law, we can deduce that the voltage drops for each resistor will be proportional to their values. What that means is that if we pick resistors of equal values, they will each drop about half of the voltage so, about 50 mV each, regardless of said value.
Now, what happens if we measure our voltage right in between our resistors? We read a voltage of 50 mV. Yup that’s right, we just made our guitar signal half as loud. Crazy. This is the basic principle behind the volume knob on your guitar, your amp and on almost every single pedal in the observable universe.
We will see many other uses for resistors, but for now that’s pretty much the gist of it.