I’m going to explain What is Full Bridge Rectifier and How to choose Diodes and Capacitors for it appropriately in as brief as possible. A step down AC transformer steps down AC voltage like 220 V AC to 12 V AC (or 15 V, 32 V, 48 V etc). We need to convert it to DC to use our regular electronic devices like mobile phone, battery charger, TV, computer, etc. with it. To convert AC to DC efficiently, we need Full Bridge Rectifier which consist of four Diodes. An additional smoothing capacitor is highly recommended for smoothing the voltage. One of the advantages is that it can be used with regular transformers – no need for center tapped (CT) transformers (though full bridge rectifier can also be used with center tapped transformers without any problem).
This is how a full bridge rectifier works. When current flowing through first pair of opposite diodes then another pair is inactive, and vice-versa. When one pair is active, one diode of that pair rectifies the positive charge and another rectifies negative charge (during positive half cycle). Same happen with the diodes of second pair when first pair get deactivated and second pair get activated (during negative half cycle). This way, no cycle is wasted. Further information can be found here and here.
How to choose the transformer?
- If you have a 1.5 Amp load, then choose a 1.5 Amp transformer, if 3, then choose 3 Amp transformer.
- Transformer with 2 terminal on secondary coil is perfect.
- You can also use center tapped (CT) transformer.
- If you have 12-6-0-6-12 CT transformer and suppose you want 12 V DC, then connect inputs of your full bridge rectifier to 12 and 0 point i.e. 1st and 3rd terminal.
How to choose Diodes?
- If you are planning for a 3 Amp power supply, then choose a diode that can handle 3 Amp current. 1N540X (X = 0 to 8) series diodes can handle 3 Amp current.
- If you are planning for a 1 Amp power supply, then choose a diode that can handle 1 Amp current. 1n400X (X = 1 to 7) series diodes can handle 1 Amp current.
There are also other factors like Maximum repetitive peak reverse voltage, Maximum RMS voltage, Maximum DC blocking voltage, etc. You should always check the datasheets.
1N540X series datasheet: https://www.vishay.com/docs/88516/1n5400.pdf
1N400X series datasheet: https://www.vishay.com/docs/88503/1n4001.pdf
Remember: In forward bias, diodes has few resistance (1N4007 has about 570 Ohm). So voltage drop is about 0.6 to 0.7 Volts.
Note: If you add a forward biased diode or resistor in series with a voltage source and try to measure the voltage with multimeter, you won’t get the complete voltage drop on your multimeter screen with a open circuit. Voltage will be dropped only when a load is connected.
How to choose Capacitors?
Electrolytic (polarized) capacitors are used on the output of Full Bridge Rectifier to reduce ripple i.e. smoothing the voltage output.
Simple calculation for calculating capacitance needed
In 60 hZ AC,
60 cycles takes 1000 ms
1 cycle takes 1000/60 ms
½ cycle takes 1000/(60 × 2) = 8.33 ms
So, 60 hZ AC half-cycle time = 8.33 ms.
and similarly, 50 hZ AC half-cycle time = 10 ms.
Assume that peak voltage after rectification is 15 V DC.
Minimum acceptable dropped voltage is 12 V.
So, minimum acceptable voltage drop is (15 – 12) V = 3 V.
Now suppose 1.5 Amp current is drawn.
So, the formula is:
(50 or 60 hZ AC half cycle time × Amp drawn) ÷ minimum acceptable voltage drop
= (8.33 × 1.5) ÷ 3
= 12.495 ÷ 3
= 4.165 milifarad (mF)
= 4165 microfarad (4165 µF or more needed) (1000 µF = 1 mF)
Voltage needed for the capacitor
I you connect your multimeter probes to the output of your full bridge rectifier to get voltage reading, then what you see on screen is the RMS voltage, NOT the peak voltage.
Peak voltage = RMS × √2
If you are getting 15V RMS, then the peak is 15 × √2 = 21.2132 V
So, your capacitor voltage rating must be greater than 21.2132 V. In this case, 25 V or 35 V capacitor is recommended.
Hope you like this post. If you have any question or suggestion, then please comment below.