Are these the Capacitors you are looking for?
When choosing capacitors for your circuits it’s important to choose carefully. Else you risk building a malfunctioning circuit. In extreme cases you could end up with an accident that could be painful and expensive.
A capacitors characteristics and its behaviour is affected by:
- How the capacitor is built
- The material used for the capacitor’s plates
- The material used for the capacitor’s dielectric
- The surface area of a capacitor’s plates
How much Charge can a Capacitor store
The ability of an item to store any amount electrical charge is called capacitance.
A capacitor’s capacitance depends on three things
1. The surface area of the metal plates
2. The thickness of the dielectric
3. The of dielectric between the plates
While it would be awesome to learn how to calculate capacitance, it would be beyond the scope of this blog. Most of the capacitors you will encounter will have a documented capacitance value
You measure capacitance in units called Farads and denoted by the letter F.
A farad is that the capacitance needed to hold one coulomb of charge with a potential difference of 1 Volt. Yet, it is more to common encounter capacitors in the microfarad and picofarad range.
A microfarad is a millionth of a Farad represented by µF
1µF = 0.000001 Farads
A picofarad is a millionth of a millionth of a Farad presented as pF
1pF = 0.000000000001 Farads
Identifying Capacitor Values
Most capacitors will have their values printed on them in Farads or parts of a Farad. Capacitors with enough space will have their capacitance and working voltage on them.
Capacitors use a three digit system to denote their values. The system is in picofarads and works as follows
A capacitor marked with three digits for example 123.
This would mean a capacitive value of 12 followed by 3 zeros. That is 12000 pF. Some capacitors will have two digits on them such as 34, this means 34pF no third digit means no zeros to add at the end.
Although not as common there is a different system that uses both numbers and letters
7Rl
In this case the placement of the R denotes a decimal point.
7R1 = 7.1
In the system the units could be microfarads or picofarads.
Some capacitors use a single letter to state the tolerance of the capacitor such as
123Z
The Z here means that this capacitor has a tolerance of +80% up to -20%, see the table below for common tolerances
Working Voltage
Sometimes indicated as WV is the amount of voltage you can drop across a capacitor.
Excess of which may damage the capacitor allowing unwanted current to flow.
Capacitors for DC circuits are usually rated for a WV of between 16 to 35 volts. Plenty considering most DC sources range from 3.3V to 12V.
It’s important to keep a keen eye on the working voltage. Always be sure to select a capacitor with a WV that is 10–15% more than your supply voltage.
Polarity
Larger value capacitors may care about how they are inserted into your circuit. Designed for use in DC circuits. In these the positive terminal will need to have a higher voltage than the negative terminal.
Polarised capacitors will usually have a minus sign pointing towards the negative terminal. In these capacitors the negative terminal will usually be shorter.
By now you should have an understanding of
- How to define capacitance
- Sizing up capacitance and knowing how it relates to your needs
- Reading a capacitors capacitance and tolerance
- Identifying polarized capacitors
- Choosing capacitors with the correct working voltage for your needs
Next time we explore combining capacitors and understanding capacitive reactance
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