A bench power supply is an extremely handy bit of kit to have around for electronics hobbyists, but they can be expensive when purchased from the market. In this Instructable, I will show you, how to make a variable lab bench power supply with a limited budget. It is a great DIY project for beginners as well as any one interested in Electronics.
The main objective of the project is to learn how a linear power supply unit works. In the beginning,to explain the working principle of a Linear Power Supply, I have taken an example of LM317 based power Supply.For making the final Power Supply, I purchased a Power Supply kit from Banggood and assembled it.
This is a high quality stabilized voltage supply with which the voltage can be regulated continuously, and the range in which to regulate the voltage is 0-30V. It even contains a current limit circuit which can effectively control the output current from 2mA to 3A with the ability to regulate the current continuously, and this unique feature makes this device an indispensably powerful tool in the circuit lab.
Input voltage: 24V AC
Input current: 3A maximum
Output voltage: 0 to 30V continuous adjustable
Output current: 2mA - 3A continuously adjustable
Output voltage ripple: minimum 0.01%
Full Project is also available on Instructables
Step 1: Tools and Parts Required
Parts List :
1. Step Down Transformer - 24V, 3A ( Jaycar )
3. Heat Sink and Fan ( Banggood )
4.Volt-Amp Panel Meter ( Amazon )
5. Potentiometer Knob ( Banggood )
6. Buck Converter ( Amazon )
7. USB Port ( Amazon )
8.Binding Post Banana Plug ( Amazon )
9. IEC3 Power Socket ( Banggood )
10.Rocker Switch ( Banggood )
11. Green LED ( Amazon )
12. LED Holder ( Banggod )
13. Heat shrink Tube ( Banggood)
14. Self Adhesive Rubber feet ( Amazon )
15. 3D printing filament-PLA ( GearBest )
Tools/ Machine Used
2.Soldering Iron ( Amazon )
3. DSO- RIGOL ( Amazon )
4. Glue Gun ( Amazon )
Step 2: Basic Block Diagram
Before heading in to the making process, you should know the basic components of a Linear Power Supply.
The main elements of the linear power supply are:
Transformer: The transformer changes the ac mains voltage to a desired value. It is used to step down the voltage.This also serves to isolate the power supply from the mains input for safety.
Rectifier: The power output of the transformer is in AC, this needs to be converted to a DC.The bridge rectifier converts AC into DC.
Input Smoothing Capacitor / Filter: The rectified voltage from the rectifier is a pulsating DC voltage having very high ripple content. But this is not we want, we want a pure ripple free DC waveform.The filter circuit is used for smoothing out the ac variations( ripple) from the rectified voltage.Large reservoir capacitors are used for this.
Linear regulator: The output voltage or current will fluctuate when there is change in the input from ac mains or due to change in load current at the output of power supply.This problem can be eliminated by using a voltage regulator. It will maintain the output constant even when changes at the input or any other changes occur.
Load : Application Load
Step 3: Transformer
Input high voltage AC going into a transformer which usually steps down the high voltage AC from mains to low voltage AC required for our application.For designing the Power Supply,the transformer secondary voltage is selected by considering the the output voltage of the power supply, losses in the diode bridge and the linear regulator. A typical waveform of 24V transformer is shown above. In general we allow about 2V - 3V drop for the bridge rectifier configuration. So the transformer secondary voltage can be calculated as below
Suppose we want to make a power supply with output voltage of 30V and 3A.
Before the bridge rectifier the voltage must be = 30 + 3 = 33V ( Peak )
So the RMS voltage = 33 /sq root (2 ) = 23.33 V
The nearest voltage rating transformer available in the market is 24V. So our transformer rating is 230V/24V ,3A .
Note : The above calculation is a rough estimation to buy a transformer.For accurate calculation you have consider voltage drop across diodes,voltage drop of the regulator, ripple voltage and rectifier efficiency also.
Step 4: Bridge Rectifier
The rectifier bridge convert an alternating voltage or current into corresponding direct Current (DC) quantity. The input to a rectifier is ac whereas its output is unidirectional pulsating DC.
The voltage drop across a general purpose diode is around 0.7V and schottky diode is 0.4V. At any instant two of the diodes in the rectifier bridge are in operation.But since the diode conducts heavily, it may effectively be higher. A good safe value is twice the standard or 0.7 x 2 = 1.4V.
The DC output after the bridge rectifier is approximately equal to the secondary voltage multiplied by 1.414 minus the voltage drop across the two conducting diodes.
Vdc = 24 x 1.414 - 2.8 = 31.13 V
Step 5: Smoothing Capacitor / Filter
The rectified voltage from the rectifier is a pulsating DC voltage having very high ripple content. The large ripples that exist in the output makes it almost impossible to be used in any powering application. Hence a filter is used.The most common filter is by using a large capacitor.
The resultant output waveform after the Smoothing Capacitor is shown above.
Step 6: Regulator
The output voltage or current will change or fluctuate when there is change in the input from ac mains or due to change in load current at the output of the regulated power supply or due to other factors like temperature changes. This problem can be eliminated by using a regulator IC or by a suitable circuitry consisting of few components. A regulator will maintain the output constant even when changes at the input or any other changes occur.
IC’s like 78XX and 79XX are used to obtained fixed values of voltages at the output.Where as IC’s like LM 317 we can adjust the output voltage to a required constant value.The LM317T is an adjustable 3-terminal positive voltage regulator capable of supplying different DC voltage outputs other than the fixed voltage power supply.The above example circuit uses a LM3 17 voltage regulator IC. The rectified output from the full wave bridge rectifier is fed to a LM317 regulator IC. By changing the value of potentiometer used in this circuit, the output voltage can be controlled easily.
Till now I have explained how a Linear Power Supply unit work. In the proceeding steps, I will explain the building of Bench Power Supply by assembling a DIY kit.
Step 7: How the Power Supply Kit Works
The working of the kit can be understood by following the schematic diagram shown above.
To start with, there is a step-down mains transformer with a secondary winding rated at 24 V/3 A, which is connected across the input points of the circuit at pins 1 & 2. (the quality of the supplies output will be directly proportional to the quality of the transformer). The AC voltage of the transformers secondary winding is rectified by the bridge formed by the four diodes D1-D4. The DC voltage taken across the output of the bridge is smoothed by the filter formed by the reservoir capacitor C1 and the resistor R1. The circuit incorporates some unique features which make it quite different from other power supplies of its class. Instead of using a variable feedback arrangement to control the output voltage, our circuit uses a constant gain amplifier to provide the reference voltage necessary for its stable operation. The reference voltage is generated at the output of U1.
The circuit operates as follows: The diode D8 is a 5.6 V zener, which here operates at its zero temperature coefficient current. The voltage in the output of U1 gradually increases till the diode D8 is turned on. When this happens the circuit stabilises and the Zener reference voltage (5.6 V) appears across the resistor R5. The current which flows through the non inverting input of the op-amp is negligible, therefore the same current flows through R5 and R6, and as the two resistors have the same value the voltage across the two of them in series will be exactly twice the voltage across each one. Thus the voltage present at the output of the op-amp (pin 6 of U1) is 11.2 V, twice the zeners reference voltage. The integrated circuit U2 has a constant amplification factor of approximately 3 X, according to the formula A=(R11+R12)/R11, and raises the 11.2 V reference voltage to approximately 33 V. The trimmer RV1 and the resistor R10 are used for the adjustment of the output voltages limits so that it can be reduced to 0 V, despite any value tolerances of the other components in the circuit.
Another very important feature of the circuit, is the possibility to preset the maximum output current which can be drawn from the p.s.u., effectively converting it from a constant voltage source to a constant current one. To make this possible the circuit detects the voltage drop across a resistor (R7) which is connected in series with the load. The IC responsible for this function of the circuit is U3. The inverting input of U3 is biased at 0 V via R21. At the same time the non inverting input of the same IC can be adjusted to any voltage by means of P2.
Let us assume that for a given output of several volts, P2 is set so that the input of the IC is kept at 1 V. If the load is increased the output voltage will be kept constant by the voltage amplifier section of the circuit and the presence of R7 in series with the output will have a negligible effect because of its low value and because of its location outside the feedback loop of the voltage control circuit. While the load is kept constant and the output voltage is not changed the circuit is stable. If the load is increased so that the voltage drop across R7 is greater than 1 V, IC3 is forced into action and the circuit is shifted into the constant current mode. The output of U3 is coupled to the non inverting input of U2 by D9. U2 is responsible for the voltage control and as U3 is coupled to its input the latter can effectively override its function. What happens is that the voltage across R7 is monitored and is not allowed to increase above the preset value (1 V in our example) by reducing the output voltage of the circuit.
This is in effect a means of maintaining the output current constant and is so accurate that it is possible to preset the current limit to as low as 2 mA. The capacitor C8 is there to increase the stability of the circuit. Q3 is used to drive the LED whenever the current limiter is activated in order to provide a visual indication of the limiters operation. In order to make it possible for U2 to control the output voltage down to 0 V, it is necessary to provide a negative supply rail and this is done by means of the circuit around C2 & C3. The same negative supply is also used for U3. As U1 is working under fixed conditions it can be run from the unregulated positive supply rail and the earth.
The negative supply rail is produced by a simple voltage pump circuit which is stabilised by means of R3 and D7. In order to avoid uncontrolled situations at shut-down there is a protection circuit built around Q1. As soon as the negative supply rail collapses Q1 removes all drive to the output stage. This in effect brings the output voltage to zero as soon as the AC is removed protecting the circuit and the appliances connected to its output. During normal operation Q1 is kept off by means of R14 but when the negative supply rail collapses the transistor is turned on and brings the output of U2 low. The IC has internal protection and can not be damaged because of this effective short circuiting of its output. It is a great advantage in experimental work to be able to kill the output of a power supply without having to wait for the capacitors to discharge and there is also an added protection because the output of many stabilised power supplies tends to rise instantaneously at switch off with disastrous results.
Credit :This section is not written by me rather it is taken from electronics-lab.com.The full credit goes to the original author.
Step 8: Identify the Components in the Power Supply Kit
Before stating the assembling of the kit read the Instruction manual carefully.
The kit come with all the components mixed together in a single packet. So, it is recommended start working by identifying the components and separating them in groups like : transistors,Opamps,Regulator,Potentiometers and the connectors.This really save a lot of time during the building of the kit.
Step 9: Identify the Resistors
In the power supply kit, the highest number of components used are resistors of different values. In the kit the resistors packed in a bunch and their values are not labeled. So we have to manually measure the resistance value by using a Digital Multi-meter.I measured the values and written on the small strip of paper in the resistor leg.
Step 10: Soldering the Components
The thumb rule for soldering the components on the PCB is " Solder the components according to their height". Always start with smaller height components.First I solder all the resistor,then diodes,then ceramic capacitors,then transistors then Opamps and so on.Use a pliers when bending bridge diodes legs to avoid breaking. In the kit there is no DIP Base for Opamps, so I used the base from my own stock.
Note : Do not solder the 3mm LED, as we are going to connect wires from LED to be mounted on the front panel.
Step 11: Soldering the Power Transistors
The metal part of the high power transistor ( 2SD1047 ) and medium power transistor ( 2SD882 ) is attached to the heat sink, making it suitable for devices dissipating several watts of heat.The heat sink for transistor 2SD882 is included in the kit. So you have to buy a separate heat sink for the other transistor ( 2SD1047 ).But good thing is that the perfect size heat sink matching with the PCB outline and along with a Cooling fan is available from the same manufacturer. You can buy it from Banggood.
Thermal compound is used to improve heat transfer between the device case and the heat sink.
Step 12: Prepare the Potentiometer
The potentiometer could be placed directly onto PCB, and could also be lined onto the board through its socket and wires. Potentiometer tagged with A is the current limit potentiometer and V is the voltage potentiometer. The voltage potentiometer could be substituted by 10K multi turn wire-wound potentiometer at your own will, with which you could adjust in a more accurate way.
As we want to install the potentiometer in our enclosure, we have to solder the the PCB board through the JST connectors in the kit.
First insert the heat shrink tubes in to the 3 wires of the JST connector and then solder the wires to the legs of the potentiometer. Then cover the soldering joint by heat shrink tube and apply heat all around to give the final touch.For better understanding you can see the above picture.
Step 13: Wiring Diagram for Input Power Socket
I used a IEC 3 Pin 320 C14 Socket for power input. It has inbuilt power socket,fuse for protection and a switch.The connection diagram is shown in the above image.The red and blue wire in the diagram is connected to the primary side of the transformer.I have left the ground connection ( green wire ), if you have a metal enclosure, you can connect it.
Step 14: Volt-Amp Dual Display Meter Wiring
My Volt-Amp meter display had thick black, red and blue wires. Thin ones are red and black for power supply for chip itself.The wiring diagram is as follows :
● Black line (thin): vacant or module negative
● Red line (thin): power supply positive
● Black line (thick): Common measuring (GND )
● Red line (thick): Measuring terminal voltage input positive
● Blue line (thick): Current input+
Please refer to the wiring diagrams for more details
Step 15: Make the USB Circuit
Another optional connector that you can add is a USB outlet. This will let you run any device that is powered by a USB port.USB output voltage is 5V which can be get by step down the 24V DC.First adjust the trimpot in the buck converter to set the output voltage to 5V.Then soldered the buck converter input terminal to the 24V linear regulator output or the terminal provided for the fan connection.The output of the buck converter module is connected to the USB port.
Step 16: Complete Wiring Diagram
The wiring is pretty straight forward.I added an extra Volt-Amp meter and an USB circuit in the circuit.
1. Inlet Power Socket : Connect wires as explained in the earlier step.
2. The output wires from the Inlet Power Socket is connected to transformer primary (220V ) side.
3.The secondary (24V ) side is connected to input terminal of the Power Supply kit.
4. Volt-Amp Meter : The wires from output screw terminal is connected to Volt-Amp meter as explained earlier .
5.The post terminal is connected to the Power Supply PCB output through a rocker switch as shown in the above wiring diagram.
6. USB Connection : The DC power after the diode bridge is tapped off to provide supply to the USB through a buck converter module.
Step 17: Making the Enclosure
The enclosure is designed based on the Thingiverse design "The Ultimate box maker".I used Customizer to get the exact size of the enclosure according to my requirement. First I measure the PCB and transformer size and then finalized the enclosure size ( 200 x 140 x 80 ).
I have designed the front and back panel separately in Autodesk Fusion 360.After design, I printed all the components ( Top Shell , Bottom Shell, Front Panel and Back Panel ) separately.
I used my Creality CR-10 3D printer to print all the parts.I have printed with 0.3mm layer height and 50 mm/s speed.The print quality is really excellent.
The .stl files for the enclosure is attached below.
Step 18: Install the All Components
Insert the components on the front and back panel slots as shown in the picture.
Mount the PCB board by screwing at the four corners.
Place the transformer on the base provided on the enclosure and then mount it.
Mount the buck converter module on the lower shell of the enclosure by applying hot glue.
Route all the wires properly.
Then place the top shell and secure the nuts on the two sides.
Step 19: Warning
When all the external connections have been finished make a very careful inspection of the board and clean it to remove soldering flux residues. Make sure that there are no bridges that may short circuit adjacent tracks and if everything seems to be all right connect the input of the circuit with the secondary of a suitable mains transformer. Connect a voltmeter across the output of the circuit and the primary of the transformer to the mains.
Note :Do not touch any part of the circuit while it is under power.
Step 20: Testing
I will recommend to test the power supply before closing the enclosure.If anything went wrong you can rectify it easily.Once finished,plug in the power cable, power up the power supply using switch in the back of the supply and LED should turn on along with fan. Now turn the voltage knob clockwise, you will notice the gradual increase of the voltage reading on the display unit. To see the current reading you have to connect a load at the post terminals.If everything goes perfect,then congratulations !!! You have done your bench power supply.
You can add more features as well as modify the kit to get desired output according to your requirement.Have a look to the following link where lots of discussions is going on regarding the modifications of this power supply kit.
Modification : https://www.eevblog.com/forum/beginners/bangood-ps...
I hope you have enjoyed and learned how to make a linear power supply.