Capacitors store energy in the form of charge.
They are like electronic buckets filled with electrons or perhaps more like compressed gas cylinders.
The potential difference across a capacitor changes slowly unless large currents are flowing.
Electrolytic capacitors use thin rolled up foil plates separated by a liquid or gel electrolyte. The insulation between the plates relies on a chemical reaction. If the capacitor is connected up the wrong way round, this chemistry fails and the capacitor works as a conductor instead. It gets hot and can explode!
The negative pin is indicated by the arrows and minus signs.
Electrolytic capacitors have a very large capacitance for their size.
The working voltage is low. 12 to 160 volt ratings are common. The image above shows a 35 Volt capacitor.
They have a significant DC leakage current, sufficient to upset some timing circuits
They are not manufactured with good tolerance / accuracy and this can be up to 50% out
Their capacitance is not stable and can change with time
They are not suitable for high frequency radio applications because the coiled up foil roll acts as an inductor blocking high frequency performance
They work well at audio and ultrasonic frequencies.
Their uses include ...
DC power supply smoothing - This is a particular example of decoupling.
Audio signal coupling - Block DC and pass AC.
Audio signal decoupling - Remove unwanted AC signals.
Timing in 555 or other timer circuits - RC resistor capacitor circuit
They can not be used ...
In AC circuits where the polarity across the capacitor reverses
In logic gate astable circuits because the polarity across the capacitor reverses
These have the value printed in plain language like 4700 µF and there is no problem.
Tiny capacitors might be labelled with a number up to three digits long. These are values in picofarads (puffs) and the third digit (if present) is the number of zeros you need to add. If there is a letter, this is the capacitor tolerance (accuracy of manufacture). J = 5%. K = 10%. M = 20%. Google for the other tolerance codes if you need them.
8 = 8 picofarads
22 = 22 picofarads
121 = 120 picofarads (add one zero)
332 = 3300 picofarads (add two zeros)
473 = 47000 picofarads or 47 nanofarads (add three zeros)
On big capacitors, this is clearly labelled. On tiny capacitors it might not be labelled at all. Usually small capacitors will be safe in any sensible school project because they have a high enough voltage rating. Electrolytic capacitors are more of a problem. You must take care that the voltage rating is higher than any voltage the capacitor will experience at any time in your circuit. It is a good idea to allow a safety margin so use a 16 Volt capacitor in a 12 Volt circuit. High voltage capacitors tend to be physically large so, if possible, it is a good idea to design circuits to work on lower voltages. Alternatively design circuits that avoid the use of capacitors.
This is a 0.1 microfarad or 100 nanofarad capacitor rated up to a maximum of 63 Volts.
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