Capacitors store electrical charge from a power source/supply and then release that charge at a later time. They have 2 close conductive areas called “plates” separated by an insulator that can become equally but oppositely unbalanced with charges. The power supply forces current off one plate while putting that current on to the other plate.
The more charge imbalance, the larger the voltage built up. Those charges still want to return to where they came from, so once there’s a return path (short circuit or through a load) the capacitor will discharge. Capacitors also leak and self discharge over time. Some quicker than others.
You should already know how to use
The charge and discharge process is somewhat similar to that of a rechargeable battery, but there’s many differences to be aware of.
- Voltage smoothing
- RC time constant uses
C=Q/V – Capacitance equals electric charge divided by voltage.
- Acts very similar to a rechargeable battery.
- Conductive “plates” separated by an insulator.
- Charges can be moved off one plate while being moved onto the other.
- An electric field between the plates makes the plate imbalance possible. Atoms missing an electron can pull on an extra electron on the other side of a thin insulator.
- A voltage builds up as the imbalance of the plate charges grows. Ultimately the charges want to return to their original place, once there’s a conductive path.
- A stored charge/voltage can be used to power something later.
- The amount of stored charge for a voltage is determined by the capacitance in units called Farads.
- A Farad is a large amount of capacitance. Supercapacitors are in the Farad range. Most capacitors are in the microfarad (millionths) to nanofarad (billionths) range. Not uncommon for them to be in the picofarad (trillionths) range either. Millifarard (thousandths) capacitors are usually just written as thousands of microfarads. 10,000 microfarad, instead of 10 millifarad. Be careful because the symbol for micro is the Greek letter mu ( µ ) and not m. The m is reserved for milli. Sellers often write 100mF for 100 microfarad, when they should have wrote 100µF.
- Maximum voltage is highly variable by specific component. Electrolytic and supercapacitors usually have their maximum voltage written on the component. Other capacitor maximum voltages will have to be researched.
Smoothing a power supply voltage is the simplest use of a capacitor to understand. The capacitor gets charged directly from a power supply while also powering some other load. The capacitor and the load are connected in parallel. A large enough value capacitor will be able to hold a voltage relatively steadily during brief power supply spikes or drops.
RC time constant uses
Capacitors charge instantly if there is no resistance between them and the power supply. Therefore, resistance is often added to slow down the charge time.
Using just a resistor while charging or discharging, means that the large initial voltage difference between supply and capacitor will cause higher current to flow at the beginning than later on when the capacitor is almost fully charged.
Therefore, there is a lot of current flow/voltage change when first charging a capacitor through a resistor, and very low current flow/voltage change when the capacitor is almost done charging to the supply voltage.
Multiplying the resistance by the capacitance (RC) gives the RC time constant. This is the amount of time it takes to change from the starting voltage, to about 2/3 of the final voltage. It will take that same amount of time to change 2/3 of the remaining voltage. And then the same amount time to change 2/3 of remaining voltage, and so on. Seeing as each time constant only changes 2/3 of the remaining voltage difference, the capacitor is never mathematically 100% charged. So, we settle for 5 time constants being fully charged. By that point, the capacitor’s voltage has reached a state of about 99.99% changed or discharged.
For another example, check out this page Capacitor charge and discharge through resistor protected LEDs circuit
A steady capacitor current will change it’s voltage steadily.
Electronics course 6 capacitor and current source voltage ramp circuit fragment
006 Capacitor current source voltage ramp – LM334 is a more detailed page.
Capacitors pass AC and block DC. Often you want to let current change direction a lot (AC) in part of a circuit, while preventing it from going in just one direction (DC) for very long. You can easily do so by putting a non polarized capacitor, which can be charged in either direction, in series with the rest of that circuitry.
Common circuits using capacitors
- 555 timing
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I used capacitors from this kit in my videos, unless the value I need is not available in this kit.
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