Incorrectly Using CapacitorsWe recently published a Capacitor Circuit Note and as always we received a lot of great feedback from our readers. In order to address your questions we asked our Technical Support team to give us the lowdown on capacitors. They've provided some valuable knowledge and stories from their own personal experiences. In the mean time, our Product Marketing team decided that showing you exactly what happens when you reverse polarity of a capacitor or expose a capacitor to overvoltage would be a great learning opportunity.
What are capacitors and how do they work?A capacitor is a passive electrical component that has two terminals. It's basically two conductors, typically having conduction plates, separated by an insulator known as a dielectric. It also has connection wires that are connected to the conducting plates. The dielectric is what determines the type of the capacitor. The dielectic material can vary, but it must be a poor conductor of electricity.
The purpose of a capacitor is to store energy. The negative terminal accepts electrons from a source of power, while the positive terminal loses electrons. The capacitor releases its stored energy when required. It works similarity to a battery but can release its entire charge in a fraction of a second.
Common types of capacitors are ceramic capacitors, paper or film capacitors and electrolytic capacitors. There is also a family of super capacitors which offer high capacitance.
Capacitor Applications:Capacitors have an array of applications. They play a critical role in digital electronics as they protect microchips from noise on the power signal by decoupling. Since they can dump their entire charge quickly, they are often used in flashes and lasers along with tuned circuit devices and capacitive sensing devices. Circuits with capacitors display frequency-dependent behavior so they can be used with circuits that selectively amplify certain frequencies.
Selecting a Capacitor:Selecting a capacitor is largely dependent upon the electronic device you're working with and which current is being used (AC, DC, etc.). You should determine if you need a polarized or non-polarized capacitor. In order to do so, check the schematic of your project. If the capacitor is indicated with a plus sign (+), then a polarized capacitor is required.
Farads (F) are the amount of charge a capacitor can store. Since one farad is fairly large, most capacitors will have values marked in picofarads (pF) or microfarads (µF). A picofarad is 10^ (-12), or one trillionth of a farad, while a microfarad is 10^ (-6), or one millionth of a farad.
The voltage of a capacitor is proportional to the charge stored in the capacitor. They are capable of blocking DC signals while passing AC. Capacitors can also eliminate ripples. If a line carrying DC voltage has ripples a capacitor can even out the voltage by absorbing the peaks and filling in the valleys.
The voltage on a capacitor is not the rating, but rather how much voltage you can expose the capacitor to. For example, if your voltage source is 9 volts, you should choose a capacitor that is at least double the voltage, 18 volts or even 27 volts to be safe.
AC or bipolar electrolytic capacitors have two anodes connected in reverse polarity. DC electrolytic capacitors are polarized by the manufacturing process and therefore can only be operated with DC voltage. Voltages with reverse polarity, or voltage or ripple current higher than specified can destroy the dielectric and the capacitor. The destruction of electrolytic capacitors can have catastrophic consequences such as a fire or an explosion. If a polarized capacitor is installed incorrectly, the capacitor whistles then explodes. On the other hand, non-polarized capacitors are primarily used for filtering out harmonic noise in almost every circuit, are more friendly to handle.
"Some large, electrolytic capacitors can store a charge for a long time. Some can even self-charge to some extent," a Jameco Technical Support Engineer explained. "An electronic design engineer I used to work with was prototyping a power supply, tuning the circuit, testing parts, etc. As was his habit, he took a cap out of the circuit to replace it and without thinking stuck one of the leads in his mouth. The capacitor discharged its entire load more or less instantly and actually caused him to fall out of his chair. He was OK but it was scary. A few months later he had to have the tooth pulled right where the cap had discharged. He'd electrocuted that tooth."
Remember to work safely when handling capacitors and always follow the specifications for your device or project. A capacitor can be an essential component but it can also result in damaging and dangerous consequences if not used appropriately.