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:
- 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.
- 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.
- 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.
- 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:
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:
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 :-)