For our eee100 (Electronics Prototyping course), we were assigned to design an AC to DC supply with 3 outputs. 2 constant DC output voltages: 3.3V and 5V and 1 adjustable from 1.25V to 9V. It also includes 4 LED indicators for the 3 outputs and the ‘ON’ indicator (plugged in outlet). The design must be properly packaged with switches to control the state of the output.

 

After tons of researching, the circuit was actually quite simple… here’s how converting AC to DC works:

 

design

 

 

From the input of 220V AC, a transformer is needed to step down the AC voltage. The rating of the transformer is set to 12-0-12 so that it would be greater than the output voltages and have a current of 1 Ampere.

                The full wave rectifier and a capacitor across give smooth DC output. The capacitor value must be in hundreds micro range. We chose a value of 470uF. The bridge rectifier has a rating of 250V and 1Amp.

                To remove the ripples from the capacitor, the DC output should be passed to a Voltage Regulator. IC voltage regulators were used instead of other voltage regulator designs (zener regulator, etc) to ensure no loading effects and the voltage output is exact. LM317T was used for the adjustable voltage output and for the 3.3V output while LM7805 was used for the 5V output.

                For the LED indicators, they were connected parallel with the voltage regulators. Connected in series with the LED is a current limiting resistor with the typical value of 330Ω and 1/2watts.

                The output of the adjustable IC Voltage Regulator, LM317T.. to know everything about it, CLICK HERE. For the adjustable voltage output, R2 is set to be a variable resistor with a maximum value of 5kΩ. R1 is then computed to be equal to 830Ω (360Ω+470Ω). For the constant output voltage of 3.3V, R2 is computed to be 1.5kΩ and R1 to be 910Ω.

Here is our PCB Layout.. Take note that since there is no available package of LM317 in the library of eagle, we replaced it to 7805 but the connections are still of LM317.. same goes to the bridge rectifier, W045 is not available in eagle so we adjusted the connections of what was available.

But before fabricating the board, prototyping should be done first to ensure correct design

Once it works direct to fabricating, drilling, soldering and lastly, packaging.. ^_^

 

And here are the final results:

 

final

 

 

I would like to thank Triah for her patience and her OC-ness. I hope it is not a bother to post our project here..

Although the stickers were unprofessional, I felt designing it with a pinch of femininity was a must considering both of us were girls.