DIY DC Power Supply

DIY DC Power Supply

As most of us keep working on simple circuits at our home electronics lab. As anyone with a home electronics lab would say it is never complete without a good bench power supply. When I went to ask about the power supply I was shocked to hear that the price was about 2000Rs. for a circuit which would cost about 200rs at max.

So I built my own DC Power supply which had a range from 0 to 25V of regulated DC power output.

CHECKLIST:

Here is the list of components you’ll require to build your own DC power supply:

Item                                         Quantity (Nos.)                       Cost (Rs)

AC plug ( 2 Pin)                                   1                                  10

A 10:1 1A Transformer                        1                                  110

LM 317 Voltage Regulator                   1                                  30

Heat Sink                                            1                                  4

1N4007 Diodes                                    4                                  2

100KE Resistor                                   1                                  0.25

1uF electrolytic capacitor/100V            2                                  15

240E Resistor/0.2% Tolerance             1                                  1

100KE Potentiometer                          1                                  10

Wires                                                 A few                          10

PCB                                                   1                                  10

Hence the total cost comes out to: 200Rs.

THE PLOT:

Here is the basic block diagram of the circuit:

It consists of 4 stages:

1. The First stage is a 10:1 Transformer to step down the 230V mains to 23-0-23V. This gives us a max voltage of 23 - (-23) = 46V of RMS value which comes upto 65.05V.
2. The Second stage is to convert the stepped down sinusoidal voltage to a rectified output. To achieve this we’ll use a Bridge rectifier made by 4 1N4007 diodes.
3. Next the rectified output is a modulus sine wave, As we want a perfect DC wave at the output, we’ll remove some part of the AC using a Capacitor Filter, A high value will help but will cost more. One thing to note here is the voltage rating of the capacitor, it is always good to get a high voltage rating capacitor to avoid blowing up one, let me remind you blowing up a capacitor is no less than an exploding BOMB. So get hold of a 100V capacitor, be very specific on this.
4. Now we have got a DC wave with some ripples, to eliminate these and reduce the effect of loading we’ll use a voltage regulator. If you want to make a constant DC power supply use a constant voltage regulator. IC numbers are 78xx. Where xx is the value of the regulated output. As we are making a variable voltage supply we’ll be using an LM317 voltage regulator. Mind you that the value of the output voltage is always less than the input. So don’t go on expecting a 30V output for a 25V input.

CIRCUIT ANALYSIS:

Here is the circuit diagram we’ll be using:

STAGE 1:

If you want to get more information on a transformer and it working refer this link: 

Transformer

Now as you got to know that the transformer works on EMI, let me ask you a simple question. What is the output of a transformer for a DC input??

 The answer is it’ll be zero as the voltage should be varying with time for EMI.

What are the ratings of a transformer??

It has 2 ratings:

• The turns ratio: N1/N2
• The current rating

The turns ratio N1/N2 gives the ratio of the voltage increase / decrease and current decrease / increase according to the formula:

N1/N2 = V1/V2 = I2/I1

Where all variables with 2 refer to secondary transformer parameter and with 1 refer to primary transformer parameter.

The current rating is the maximum current that can be driven out by the transformer. Here we’ll be using a 1A one.

STAGE 2:

Diodes have 2 ratings:

• Forward voltage: Here it is 0.7V, it is the turn on voltage of the diode i.e., if the voltage across the diode is less than 0.7V the diode doesn’t conduct.

• Max. reverse voltage: If the reverse voltage across the diode exceed this, the diode might burn off, so we’ll be using a diode with a max. reverse voltage rating of 700V.

This stage is a bridge rectifier.  To get more information on the working of a Bridge rectifier refer to this link:

Bridge Recifier 

The waveforms of the bridge rectifier are as shown:

As you can see it is not a DC waveform, but a (+) sine waveform.

 STAGE 3:

Capacitors have 2 ratings:

• Capacitance:

This is the representation of the max. value of charge that can be stored on the capacitor and it given by the formula:

C=Q/V

• Voltage Rating:

This is the max. Voltage that can be applied to the capacitor without burning it. It is always good to get a high voltage rating capacitor to avoid blowing up one, let me remind you blowing up a capacitor is no less than an exploding BOMB. So get hold of a 100V capacitor be very specific on this.

For more information on capacitors refer this link:

To reduce the variation of this AC, we use a capacitor filter as shown:

 The output is also shown in the above diagram.

 STAGE 4:

The main idea of a regulator is to obtain a perfect/ regulated output from an unregulated output.

Given below is the block diagram of the regulator which we’ll be using LM317: 

For more information on voltage regulators refer this link:

The output even now is not a perfect DC. It contains some unwanted AC components called “RIPPLES”. To further decrease these ripples we’ll use a DC voltage regulator. However the use of a regulator has one more advantage, it decreases the effect of loading on the transformer. Be sure to use a heat sink on the regulator.Adjust the potentiometer the vary the output voltage and use a voltmeter or multimeter to measure the output.

BUILD YOUR VERY OWN DC POWER SUPPLY AND ENJOY :-)