Power Supply AC – DC 12V
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The term power supply is more commonly abbreviated to PSU, this will be used from hereon in. Telecommunications equipment is designed to operate on voltages lower than the domestic Mains voltage. In order to reduce this voltage a PSU is used.
The main function of power supply is to convert AC to DC, the first stage is to make full wave rectifier to the AC signals by using bridge rectifier, filter the rectified wave by using filtering capacitor and finally select the appropriate voltage regulator to generate pure DC signal.
To provide a useable low voltage the PSU needs to do a number of things:-
* Reduce the Mains AC (Alternating current) voltage to a lower level.
* Convert this lower voltage from AC to DC (Direct current)
* Regulate the DC output to compensate for varying load (current demand)
* Provide protection against excessive input/output voltages.
Current rating for bridge rectifier should be suitable with load current, also; the designer should consider the voltage drop across each diode, which is normally equal to 0.7v.
Note that only two diodes are required when using center-tapped transformer.
Calculation for the filtering capacitor:
5i
C = ——–
Vp. f
C: Capacitor value.
Vp: Peak voltage. (“Bridge output max voltage”)
f: Frequency of the AC supply.
i: Load current.
Note that the above equation for 10% ripple voltage.
To calculate the voltage required and the transformer secondary winding we first determine the input voltage for the regulator, which is 15v, plus a 10% of this value for ripple. For a regular transformer we have to consider a bridge rectifier, as a result; we will add 1.4v. So the secondary winding should be 15+1.5+1.4=18.9 lets say 18v @ 5 Amps. Now we will calculate the capacity of the filtering capacitor. By using equation number 1 and assuming that f=60Hz, we will get C=5 x 5 / ((18-1.4) x f) =25,100µF
The following power supply circuit uses LD1084#12 voltage regulator, which can provide up to 5Amps with only 1.3v dropout. Input voltage must be around 15v. It is highly recommended that you follow this value exactly, if the input voltage is more, the regulator temperature will increase more.
Tip: It is possible to build your power supply without adding a regulator, when you want to power up an electrical motor, a small ripple in voltage will not affect the performance, unlike the electronic circuits and devices!
You may be interested in reading: Power Inverter 12VDC to 220VAC
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