When I first hear of a Capacitor it was my buddy telling me I could kill myself if I touched one in my Tube Amplifier for my guitar. I didn’t know what they looked like or what they did but the note was stored safely in my brains Grey matter as I have been grabbing my goggles and wearing rubber gloves when going “Deep Capacitoring” ever sense.
So far I have not zapped myself but I’m well aware of their shocking potential *Pun Intended*. I always get a kick out of discharging a cap with my screwdriver by shorting the leads and enjoy the sometimes SIZABLE spark. Only twice have I jettisoned a Polarized Electrolytic off my breadboard and into my face…once drawing blood. and don’t get me started on Tantalum Caps….I have never used one that hasn’t catastrophically failed. I know Tantalum is a conflict mineral so I probably purchased fakes as I can’t imagine they have the terrible design flaw of always exploding even when oriented correctly and within it’s Voltage limit.
My first experience with the real power of Capacitors was when I did my first real electronics repair: my 1985 Fender Telecaster got new pickups (two humbuckers: one that fit my Tele’s single coil Neck Mount and a unique hum-bucking Lipstick Pick-up; going from Single Coil to Humbuckers – Dual, 180 Degree Out of Phase Coils- as a guitar with only two single coil pick-ups and a three way switch is virtually unplayable on stage dsue to interference), Shielded Cloth Wire -it’s very cozy under he control plate and the cloth allows tidy wiring plus Shielded wires that don’t have any grounding issues, Three Way Switch (modified due to going from two single coils to two humbuckers means more wires to the switch), Linear Tapered Pots, new input jack, and new capacitors on the tone pot and the treble bleed circuit on the Volume knob. Now I had no idea what a Low Pass Filter was until that project but once I discovered it could take my $450 Fender Telecaster sound better than my $6,000 Gibson Les Paul I started looking into DIY Electronics to see what other musical devices I could Fix/Mod/DIY/or just learn about. (Here is a link to the Video I made of the Tele’s Overhaul: https://www.youtube.com/watch?v=EBZ6NtohRbk)
That repair got me interested in Elkectronics but I had no Idea how that Capacitor worked. I remember it was a Green Mylar at 47 picoFarads (Single Coils usually use 47 pF, Humbuckers use 22pF and Soapbar style pickups use 15pF traditionally). Unfortunately, at the time I didn’t know that the “Orange Drop” caps sold for $7 each at guitar center would do the exact same thing as any 1 cent capacitor I now currently own thousands of. But what I knew was that it went between ground and the Tone potentiometers Wiper. I read it was an LP Filter and the tone response had to do with the Cutoff Frequency dictated by the Potentiometer and Capacitor’s combined product. So I understood the first elementary thing about Capacitors through that learning experience:
First: the Definition, the Math, the Physical process by which a capacitor can easily be visualized.
Visualizing what goes on inside a Capacitor was an extremely difficult thing for me to do because I visualized it multiple wrong ways until I came across new circuits that didn’t fit within my visualization and then became confused.
First: A Capacitor stores a Charge and Charge is Measured in Coulombs and is equal to a Quantity of Electrons. However, all atoms have electrons and no electrons enter or leave the Capacitor because that would result in the creation of a bunch of ions in the Capacitor which wouldn’t be very stable. So how does it “Store” charge? It more accurately stores an Electric Field. A
What is “inside’ that metal can? What magic is going on in there that can sustain a magnetic field?
- A Capacitor Consists of two plates that are parallel to eachother but separated by a dielectric, like a sandwich.
- Each plate is connected to some part of the circuit by a single lead
- The key property is its ability to form an Electric Field that is spanned across the Dielectric that will store This field as Voltage to the extent that the dielectric has the permittivity necessary for the E field to form Permittivity is like conductivity or susceptibility but Current is not the fluid being exchanged here. It is Electric Field lines.
- Imagine The left input plate being connected to V+. There is one lead in and out and electrons do not flow from the positive lead, they are negatively charged, yet they are our charge carriers
- However due to Voltage being the potential difference between two points, and the dielectric being non conductive yet permeable the difference in voltage will hold the charge even after the voltage source is disconnected BECAUSE, the Dielectric is an OPEN CIRCUIT so that charge can’t flow through the capacitor, and if the positive lead is disconnected then where can that positive charge escape to, same with the negatively charged side.
- Itwill, however discharge through any conductive path connecting the plates that has a smaller voltage drop because the charge will always seek the lowest entropy state and the Electric-Field stored when given a path will take it.
- Imagine lightning. It’s not certain the exact workings but you can think of the clouds acting like a Dielectric between the Ground (where our Negative Charges are their most abundant, and therefore the polarized nature of water will transfer this charge through the cloud into the largely inert sky. Normally Light ( a form of electromagnetic radiation) energizes electrons passing straight through the atmosphere. But with thunderstorms the charge inthe sky collects and will start charge storage due to the clouds now being a dielectric. Eventually the Negative charge above the clouds will find a path where it can find a way to a portion of the ground that has less negative charge, thus discharging.
Why would anyone need this? Electricity measured in Amps is and Average of many multiples of trillions of discrete electron movements with charges of 1.392^(-23) and so at any moment the actual value of charge in Coulombs being conducted to the load will never exactly be the Average value. It actually has a Gaussian distribution and so there will be Noise in ever conductor and at ever interconnect. if the noise exceeds the voltage allowable by the Integrated Circuit then a local Ceramic Capacitor will soak up the noise charge for the pivo second it exists, and since that ekectron has become stable its contribution to the current falls so there will be a drop in current of 1.602^(-23) coulombs, which may require The extra charge on the Capacitor to be released and used to supply your OP AMP with the input Bias Voltage it needs to keep it’s inputs equal.
So first and foremost the Capacitor exists to provide Stability voltage levels throughout a circuit where the ever changing current creates ever changing Magnetic fields, creating ever changing Voltage Potential creating ever changing Electric Fields. By storing those incredibly small charges over incredibly short periods of time the circuit can function smoothly as it’s Voltage References are all stable.
Not only does it stabilize the positive voltages but those voltage references have to be referred to Ground and the Capacitor, by form an electric field that retains the Voltage Potential instead of conducting it to ground, it isolates each circuit component (usually only active components or circuit blocks). With a Ground that is constantly fluctuating slightly around 0 volts the entire circuit will be a mess.
SECONDLY, THEY PROVIDE THE MOST COMPLICATED EQUATIONS IN ELECTRONICS THROUGH THEIR ATTENUATION OF TIME VARYING SIGNALS. Capacitors have a Frequency Response dependent on the RC Filter’s Time Constant “Tau” which was simply the product of the Resistance & Capacitance.
Three years later I now finally understand why and how RC Filters works ( I can also do the necessary Complex Analysis to derive it’s properties as well. But at first the Capacitor always had me stumped as I had no idea what it did, how it did it, why they were everywhere, and what were they doing in all those spots. I was going to list “why they are a specific value” but have learned Capacitors have a very large Tolerance Range for Value and also do not have to be precisely calculated outside Complex Active Filtering or Timing Circuitry.
The hard part about learning about capacitors is that in todays world you see them performing one of just a few functions:
- Power Supply Filtering to remove Ripple from a Sinusoidal Signal when Rectifying AC mains to DC (these are the large in size, large in capacitance, and Large in voltage rating Electrolytics you see on circuit boards near where the power cord meets the PCB.
- Decoupling Digital/Analog Integrated Circuits – like the above purpose the function here is to provide a stabilized voltage to the Chip lest the fluctuating Digital Signals destabilize yours Ground or Virtual Ground and thus destabilize your Positive and Negative Voltage Rails.
- Coupling an ac signal from one stage of a circuit to another by eliminating any DC bias or common mode Voltage before an amplifier or output stage. In a broader sense you see them isolating parts of a circuit from other parts by only passing AC signals. This keeps parts of a circuit using different Steady State Voltages from influencing eachother.
I would have said RC Filters but you only see that building block in Analog Music Hardware usually. Or any circuit that is concerned with Frequencies.
Here are uses for Capacitors I have experience with and if I can explain what the cap does in the circuit I’ll try my best!
- Op-Amp Integrator: Imagine an inverting Op-Amp but with a Capacitor in the feedback path. The Non-Inverting node is Grounded so their is a Virtual Ground at the node between the Input Resistor and the Feedback Capacitor. Now Imagine an unchanging DC Voltage is applied to the input through Rin. The input Resistor creates a constant Current that can’t go into the Inverting input and therefore must charge the Capacitor in the Feedback Loop lest the Virtual Ground be disturbed. A Voltage develops across a Capacitor as a function of the Rate of Change in a charging current. At the Inverting input the Op-Amp sees an incoming Current and adjusts it’s output to divert that current through the feedback path. so that both inputs remain 0-volts. So in order to have an input current charging a capacitor while not changing that nodes voltage away from zero volts the output voltage will be heading negative at a slope that is constant so that it creates the inverse constant current on the capacitors other plate evening out the Charge on the across the Cap.
- The Voltage on a Capacitor is independent of the voltage at the capacitors lead. It value is equal to the Charge it holds over its capacitance (makes sense being that capacitance is defined as the ability to hold a charge “Q” when the plates have a specific Voltage Potential formed by the Electric Field created by the charge “Q”.
- In a Time varying signal the Voltage will be related to the rate of change of that charge. This allows for charge pumps and switched capacitor voltage doublers or negative voltage inverters. The change in Voltage as it charges and discharges is relative to the voltage on the other plate. So when the Op-Amp changes it’s output it is doing so to keep that Inverting input at 0-volts, so if a positive voltage flows through Rin then Vout goes negative accordingly.
- So what does the Op-Amp Integrator do? The Capacitor Equation relation Capacitance to Voltage is Coulombs “Q” = voltage ” x “Capacitance”: Q=C*V; or, C=Q/V, that is, 1 Farad (The SI unit of Capacitance) provides that Two