I have a transformer that convert 240 VAC to a 9-0-9 output.. I have built a bridge rectifire.. which works perfectly well with a normal 2 terminal connection.. however i donot know how to connect this in the 3 terminal transformer.. also if possible, i would like to convert it to a supply which can provide +ve & -ve voltages.. ( either 18 V or +-9 V )

 

You are closer than you think.

http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

 

thanks for that link whatsthatsmell.. but if i need two 9V supplies, then i will have to use 2 bridge rectifires, is that right ?

 

 

well.. thanks for the verification kermit2

 

Here's another way to draw the same thing:

A bit easier to visualize how to use a single bridge rectifier to get dual polarity output.

 

thanks Sgt Wookie.. & the 2 capacitors are place parallel to the resistance.. to remove the kinks ?

 

Yes, but the caps are just shown for placement. They should be quite a bit larger, depends on your load and how much ripple voltage you can tolerate.

With 50Hz in, 10,000uF with a 1A load will result in 1v ripple.

 

ok.. thanks.. but what if i plan ot use this to supply an IC.. ltes say like the 555 and a 4017.. will they work satisfactorily with this 1v ripple.. & how calculate the ripple voltage & the required capacitance ..lets say for 60 Hz

 

s long as the circuit doesn't try to pull more current than the power supply can give, the voltage fluctuation should not be very bothersome.

Yet again, it will ultimately depend on what application you put the circuit to. If small variations in timing or voltage would affect the operation, then the ripple should be further filtered out(read - a regulator should be used)

 

A simplified formula for ripple is:
Ripple_Voltage ~= Output_Current/(Ripple_Frequency * C_in_Farads)

The ripple frequency in this type of rectifier is double the input frequency.
The formula doesn't account for many things, such as transformer impedance, capacitor ESR, etc. However, it's still useful to get in the proverbial ballpark.

 

This page is good, for unregulated power supply design:

http://www.zen22142.zen.co.uk/Design/dcpsu.htm

If adding a regulator, keep in mind that you need to keep the regulator's input voltage higher than its output voltage by at least its "dropout voltage" spec. Otherwise the output gets rather nasty.