Smoothing Capacitors
Smoothing Capacitors
Find out more about the uses of smoothing capacitors in renewable energy circuits
electric circuit | educationA
capacitor is a device which stores
electrical charge. In the most simple terms it can be thought of as being similar to a rechargeable battery in that it stores power to be used at a later time.
Charging a capacitor is simply a matter or putting voltage across its legs until current stops flowing.
Capacitors are an essential component in the majority of electronic circuits.
There are many uses of
capacitors in
renewable energy generation, however one of the most important is in smoothing the output of a
bridge rectifier after it has converted AC electricity (typically generated by
wind turbine generators etc) into DC (used to charge batteries and power many low voltage devices).
Bridge Rectifier
Alternating current (AC) electricity
alternates - i.e. the voltage cycles from a positive maximum to a negative maximum and back again repeatedly. When this is
rectified by a
bridge rectifier all negative voltages are
rectified into positive voltage as shown below:
If this direct current (DC) voltage is used to light an LED for example, then it would flash on and off repeatedly. This is usually not considered to be desirable and there are many cases where a fixed voltage is required. In these cases a
smoothing capacitor is used.
The smoothing capacitor circuit shown above requires just one
capacitor placed across the output from the rectifier. As the output voltage increases, the capacitor is charged, and as the output voltage falls back to zero, the capacitor releases its charge (stored energy). The resulting voltage across the capacitor looks like this:
Capacitor Ratings
Capacitors are rated according to their
voltage and
storage capacity. The larger the capacity of a capacitor, the more charge it can store and the longer it will take to discharge. However, a very large capacitor will take a long time to charge initially and so it will take a while before a constant voltage is output. If the current drawn from the circuit is large, then the capacitor will discharge quickly and the amount of
ripple in the voltage will increase.
If the voltage in the circuit is above the rated voltage of the chosen
capacitor, the capacitor will be damaged and may even
explode. Therefore it is essential to choose a capacitor with a voltage in excess of the maximum output voltage from the bridge rectifiers.
Choosing a Capacitor
Typically in
renewable energy applications a
1000 micro Farad (1000uF) capacity smoothing capacitor is chosen. These are available with a range of voltage ratings including 10V (pictured above), 16V, 25V, 35V, 63V with higher voltage capacitors costing more than those with lower voltage ratings. If the amount of
ripple in the voltage is more than 10% then a capacitor with a higher value - e.g. 2,200 micro Farads - should be used.
NEW 1000uF 16V capacitors are now available at the
REUK Shop.
Example Smoothing Capacitor Circuit
At the top of this page is pictured a very simple circuit using a
smoothing capacitor. Two AC inputs from a small
stepper motor wind turbine are rectified by a
W005 1.5A bridge recifier into DC voltage to light an LED. Without a
smoothing capacitor the LED would flash on and off, so a
1000uF capacitor is used to supply a constant voltage to the
LED.
The output from a
Sturmey Archer Dynohub wind turbine once rectified should also be passed through a
capacitor if it is to be used for anything other than battery charging.
Article Last Modified: 11:47, 4th Apr 2007
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