Understanding Power Supply Basics
Majority of Electronics enthusiasts and hobbyists wants to build their own AC-DC power supplies to test the electronics circuits built by them. The objective of this easy to build and understand circuit is to familiarise the constructor or the hobbyist with the effect of changing the rectifier-filter configuration on the DC output voltage produced in a conventional mains operable AC/ DC power supply.
By opening or closing an appropriate combination of switches (SW-1 toSW-9 in the circuit diagram), this circuit could be configured into any one of the popular power supply rectifier circuits like (i) Conventional Half Wave Rectifier with positive DC output voltage (ii) Conventional Half Wave Rectifier with negative DC output voltage (iii) Conventional Full Wave Rectifier with positive output DC voltage (iv) Conventional Full Wave Rectifier with negative DC output voltage (v) Full Wave Bridge Rectifier with positive or negative DC output voltage. The effect of adding the filter capacitor at the rectifier output on the DC voltage produced can also be seen. In addition, the effect of increasing the size of the filter capacitor (i.e. the value of filter capacitance) on the DC voltage produced can also be observed.
Undoubtedly, it will be a good starting point for a hobbyist who happens to be a beginner, but it would simply be excellent for a hobbyist studying at senior secondary level or in the first year of Diploma in Electronics to try to build this simple but informative circuit as a part of their project because Transformers, Rectifiers and Filters are a definite part of their curriculum too.
CIRCUIT DESCRIPTION
The input transformer steps down the AC mains voltage from 230V (nominal) to 12V (nominal) between the center tap and either of the two ends of the secondary winding. The transformer being used in the circuit has a capability to deliver a current of 250 mA. The 12VAC appearing across the secondary is the RMS value of the waveform and the peak value would be 12 x 1.414 = 16.8 volts. Diodes D1 to D4 rectify the AC waveform appearing across the secondary with the number of diodes and the configuration depending upon the combination of switches chosen to be closed. Capacitors C1 to C4 provide filtering or the smoothing of the rectified waveform which although is a unidirectional one yet it is far from being a pure DC, more so in case of a half wave rectified waveform. Meter (M) with the help of DPDT switch (SW 11) has been used to monitor both positive as well as negative voltages thus eliminating the need to have separate meters. The fuse on the input is there to protect the circuit from inrush of heavy current in case there is any accidental short on the output side. Table 1.1 shows the position of various switches (OPEN or CLOSED) and the corresponding rectifier-filter configuration we get as a result of that.
CONSTRUCTION GUIDELINES
The PCB layout, as seen from components’ side, and the components’ layout are respectively depicted in Figs. 1.2 and 1.3. The project is so simple that it can very easily and conventiently be assembled on a general purpose PCB. In that case, different components need to be interconnected on the copper side of the PCB with the help of wires as per the circuit diagram.
PARTS LIST
| Capacitors
C1, C2, C3, C4 |
220uF, 50V (Electrolytic) -Axial leads (Fig. 1.4) |
| Transformer
T1 |
Mains Transformer,230VAC input, 12-0-12 V output, 250mA (Fig.1.5.) |
| Switches
SW1, SW2, SW3, SW4, SW5,SW6, SW7, SW8, SW9 |
ON-OFF type miniature toggle switches or a DIP switch (Fig. 1.6). |
| SW10 | Power ON switch (Toggle switch rated for mains voltage) (Fig. 1.7) |
| SW11 | DPDT Switch (Fig. 1.8) |
| Meter
M |
DC Voltmeter (0-15VDC) (Fig. 1.9) |
| Fuse
F1 |
500mA |
| Miscellaneous
Multistrand wires, solder metal |
Testing Guidelines
A conventional half wave rectifier produces a DC output (on no load) that is 0.318 times the peak value of the voltage appearing across rectifier input(same as that appearing across the transformer secondary). This peak value would be 16.8 volts (nominal) forthe transformer chosen for this project. Remember that this value would fluctuate with fluctuations in the mains input. The DC output (again on no load) for a full wave rectifier (conventional or bridge) is 0.636 times the peak value. The filter capacitor smooths the rectified waveform and has the effect of increasing the DC output. Larger the size of the filter capacitor, better is the quality of DC produced. All these effects can be observed by monitoring DC output.
NOTE; The photograph of the meter shown in Fig. 1.9 is only a representative photograph of a typical analog voltmeter to give an idea as to how the meter may look like.
Written by arjun on December 7th, 2009 with
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