I’ve googled a little bit and haven’t found really good answers and figured you guys would be able to explain this to me.
I’ve decided my new hobby is going to be building tube guitar amps from parts. Hey, it’s Nunavut and I’m bored. Looking at the schematics of various amps, I realized that there’s a lot I forgot from high school physics class.
-How does a transformer work? I get that the number of winds on the primary affects the output voltage on the secondary, but why? Electrons flow around a ferromagnetic core…creating an electromagnet? Why does the windings at the secondary affect the voltage?
-Capacitance and resistance: why do resistors in series add, while resistors in parallel add reciprocally? What are the electrons doing? If I have 3 resistors in parallel, do 1/3 of the electrons coming in go to each resistor? (assuming DC current) Why does capacitance add up oppositely to resistance? (ie capacitors in series add reciprocally, parallel they add up). How does capacitance work? There’s 2 electrodes separated by an insulator, right? How do the electrons jump? Do they need to reach a ‘critical mass’? If so, is the current on the other side come in pulses?
-Is there an analogy explaining amps, volts and resistance? Like in terms of a garden hose or something?
-What’s up with grounding? In amps, it seems everything is grounded to the chassis of the amp. So electrons flow through the circuit, and then to the chassis…then where do they go? Why do AC plugs in houses have 3 prongs? If one blade is + and the other -, why is there a third plug? The ground is literally hooked up the the earth, right? (A pipe stuck into the ground) Why? what happens to the electrons going through my circuit when they reach this pipe in the ground?
-How does inductance work? Guitar strings vibrating in front of a magnet with lots of wire wrapped around it creates an AC voltage. How?
As you can see, I’m pretty ignorant about electricity. Any links or explanations would be greatly appreciated.
I’ll take a swing at these two. In reverse order
An analogy would be the pressurized water system in your house. Volts= water pressure, current flow is the volume of water that comes out of your faucet. Resistance would be a blockage (corrosion in the pipes) or perhaps a flow restrictor.
Or as I guy I used to teach with would explain it, if you drink too much coffee in class, it is voltage that makes you want to go use the mens room, you will leave current in the urinal, and if you squeeze it in the middle, that is resistance.
Resistors in series add. This is due to there being a voltage drop across each resistor. So in a circuit with two resistors in series R2 will see a lower voltage than R1. Parallel resistors used to drive me bat shit until I came up with an analogy that I can wrap my 15 watt brain around.
Let’s say we go to Home Depot and buy one of those 5 gallon plastic buckets. We will drill a hole in the side at the bottom. We cover the hole and fill the bucket with water. We uncover the hole, and time how long it takes the bucket to empty. For the sake of this discussion let’s say it takes 10 minutes to empty the bucket. Now if we drill a second hole, and repeat the experiment, it will take 1/2 the time or five minutes for the bucket to empty.
Why? Simple, before there was one path for the water to reach the ground. With two holes, there is less resistance and therefore the water will run out faster.
Or let’s take this back to electronics. We have an injector circuit on a car that has 4 injectors wired in parallel each of which is 16 ohms resistance. Assuming a 12V circuit and solving for the current flow you have I=E/R
I=12/16
I=.75Amps.
Now this particular engine management system is on a 4 cylinder car and all injectors are wired in parallel. So: #1 injector draws .75A #2 injector draws .75A #3 injector draws .75A #4 injector draws .75A
For a total of 3.0A
Now let’s plug that back into Ohm’s law and solve for resistance. R=E/I
R=12/3
R= 4 ohms. is our total circuit resistance.
Unlike like a series circuit each resistor in a parallel circuit gets the same voltage, so there are multiple paths for the current to flow.
Yes, that does help. A lot. So if resistors were in series, it’s like the first bucket with a hole drains into a second bucket also having a hole. The flow rate through the first bucket through the second bucket will add up to decrease flow rate. Excellent! I have trouble with abstract concepts unless I can analogize them.
Transformers and pickups work on the same principles - this is a simplified discussion of the basic ideas, so don’t go out and try to build a substation.
A conductor not moving in a magnetic field has no current induced in it. If the conductor moves through the magnetic field, or the magnetic field varies around the conductor, a current will be induced. The inverse is also true - if a current flows through the conductor, it will try to move through the magnetic field (but only while the current is changing). Upon these principles are built generators (where a magnet is rotated within a coil to produce a current) and electric motors (where a current is passed through a coil in a magnetic field to produce motion).
In a transformer, the primary coil and magnetic former is fed with an AC current. This produces a varying magnetic field (the frequency of the magnetic field is the same as the AC frequency in the primary). The secondary is a static coil within a varying magnetic field, so a current is induced in that coil. The output voltage and current depends on the relationship between the number of turns and cross-sectional area and the former - it’s been a while since I did all the maths. Also, the geometry of the former and the coils has an impact on losses, as does Back EMF (where the current flow through the secondary creates a magnetic field in opposition to the primary).
The pickups have a static magnet and a static coil - so no current should be induced. However, the ferromagnetic string vibrates within the magnetic field, and affects the flux through the coil windings. It is this very small change in magnetic field that is converted into the electrical signal that an electric guitar produces.
Nope. The electrons don’t jump from on side to the other, they are attracted/repelled by the field created by the electrons on the other side. As an electronic engineer you think of capacitors as components either (1) Storing charge (2) Blocking DC and passing AC. (Here we go with the simplifications) A capacitor can’t pass DC because there is no path for the electrons to flow continously, but if the voltage at one plate is varied the other side will be dragged up/down until it reaches the same voltage. If the voltage is always changing (AC) then the current/signal passes through the capacitor. The impedance (AC resisance) decreases with higher signal frequency. The impedance drops for parallel capacitors because you are increasing the overall plate area.
Inductors (coils) are the same but the other way round
The ground is not an operative part of the circuit. It’s a separate path to Earth provided for safety; it only carries current when there is a fault such as a hot wire contacting the grounded chassis. It can also be used for mitigation of EMI (electromagnetic interference): for example, by grounding a metal enclosure around the circuit, noise signals will preferentially go to ground, rather than into or out of your circuit. Both the neutral and the ground eventually go to Earth via a ground rod connected to your load center, but they are kept separate until then. the neutral carries current, and may have significant voltage present on them. The safety ground does not carry current and is, or should be, always at zero volts.
I’m not actually designing an amp from scratch. I bought a kit. You don’t really need to know much to assemble the kit, I’m just curious. As to it being time consuming, that’s the idea. Thanks for all the responses so far!
