Smoothing Capacitors

Find out more about the uses of smoothing capacitors in renewable energy circuits

A 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 (1) 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 (2) are now available at the REUK Shop (3).

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 (4) wind turbine are rectified by a W005 1.5A bridge recifier (5) 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 (6) wind turbine once rectified should also be passed through a capacitor if it is to be used for anything other than battery charging.

Web Link References

(1) http://www.reuk.co.uk/Bridge-Rectifier.htm
(2) http://www.reuk.co.uk/buy-1000UF-CAPACITOR-16V.htm
(3) http://www.reuk.co.uk/REUK-Products-For-Sale.htm
(4) http://www.reuk.co.uk/Stepper-Motor-Basics.htm
(5) http://www.reuk.co.uk/buy-W005.htm
(6) http://www.reuk.co.uk/Sturmey-Archer-Dynohub.htm