How to not get shocked by Capacitors
I like to keep myself hydrated as I am working. Since I don’t work right next to my kitchen tap sometimes I find myself too occupied to go for my water from the kitchen. After a ending up with a few headaches I found that keeping a two litre bottle of water on my desk lasts me much of my work day.
Capacitors are useful devices that store electrical energy from a voltage source. Capacitors can hold on the this energy until part of the circuit demands the energy from them.
Capacitors allow you to manipulate the shape of electrical signals in a current. Something that would be impossible to do with resistors alone.
Capacitors are made by having two metal plates separated by an insulator. The insulator is also known as a dielectric. Capacitors are represented in circuit diagrams by the following symbol.
Capacitors in Circuits
You use a capacitor in a circuit by charging the capacitor to fill it up. Then when the time comes discharging the electrical energy to a part of the circuit that needs it.
Let’s explore how to charge and discharge a capacitor
Say for example you have a simple circuit containing a
- Direct Current(DC) source
- A capacitor
- A light bulb
Assume the capacitor is in a series arrangement with the light bulb as in the figure below
Current flow above isn’t sustainable as a path through the capacitor doesn’t exist. Yet, if we explore this kind of circuit we find that electrons flow through it in a very interesting manner.
The negative terminal of the battery above contains a surplus of electrons. As they move towards the capacitor, the electrons find themselves blocked by an insulating core. This results in a saturation of negative charges on plate.
At the same time. The positive terminal of the battery attracts electrons from the capacitor’s other plate. As they pass through the light bulb they will light it (although very briefly). This results in a saturation of positive charges on that side of the capacitor.
Now we have a net positive charge on one plate of the capacitor and a net negative charge on the opposite plate. This build of charges results in a voltage difference across the two plates. This voltage difference represents the electrical energy stored the capacitor.
When the voltage drop across the plates equals the battery the capacitor is said to be fully charged. If you replace the the battery with a wire. This provides a complete path for surplus electrons to flow from one plate to the other. As the flow of electrons happens the capacitor discharges by powering the light bulb.
Hence you observe the bulb lighting up for a small while as the capacitor discharges and goes off.
Maintaining Voltage Levels
When you apply DC voltage to a capacitive circuit the capacitor plates take time to charge. The same happens when you remove the DC source the capacitor takes time to discharge. For this reason capacitors are said to oppose voltage change.
Which in essence means capacitors take time to catchup to source voltage.
Allowing Alternating Current (AC)
As you have seen in the example capacitors can pass DC voltage but only for very shortly. Since the dielectric provided a barrier to sustained electron flow.
Yet as an AC source flows it varies it’s direction of flow every so often. I explored this in a previous post about sourcing and channeling electricity.
Now if we used an AC source in a circuit such as the one below
As the current rises from 0 voltes to the peak voltage the capacitor charges up as it does in with the DC source. When the current reaches it’s peak voltage it will change direction. As the current changes and the voltage decreases causing our capacitor to discharge. And then this happens all over again in the other direction.
The AC source changes direction and thus the capacitor goes through a charging, discharging and recharging cycle. As a result electrons move back and forth through the circuit lighting up the bulb. This happens even though there is no complete conductive path through the capacitor.
Hence capacitors are said to pass AC current even though they block DC current.
Did I miss anything about Capacitors?
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Next time we cover Identifying and Choosing Capacitors
