A basic full wave rectified power supply is shown below. The
transformer is chosen according to the desired load. For example, if the
load requires 12V at 1amp current, then a 12V, 1 amp rated transformer
would do. However, when designing power supplies or most electronic
circuits, you should always plan for a worst case scenario.
With this in
mind, for a load current of 1 amp a wise choice would be a transformer
with a secondary current rating of 1.5 amp or even 2 amps. Allowing
for a load of 50% higher than the needed value is a good rule of thumb.
The primary winding
is always matched to the value of the local electricity supply.
Notes
An approximate formula for determining the amount of ripple on an unregulated supply is:
Vrip = Iload * 0.007 / C
where I load is the DC current measured through the load in amps and C
is the value of the capacitor in uF.The diagram below shows an example
with
a load current of 0.1 amp and a smoothing capacitor value of 1000uF.
The calculated value of ripple is (0.1 * 0.007) / 1000e-6 = 0.7 volts
or 700mV. The value of peak-peak ripple measured from the graph is
628mV. Therefor, the equation is a good rule of thumb guide for choosing
the correct value for a smoothing capacitor in a power supply.
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