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lm317 based diy variable benchtop power supply

by:Mingfa Tech     2020-01-27
For any electronic laboratory or anyone who wants to work on an electronic project, power supply is undoubtedly an absolutely necessary equipment, especially a variable power supply.
In this tutorial, I will show you how to build the LM317 linear positive regulator based on variable 1. 2-30V (1.
2 V to input voltage-2. 7V actually)power supply.
These are the features I want my PSU to have.
The power supply does not use any transformer, but in 15-
Many different voltages from 35 V to the output.
Therefore, you can pass the rated voltage 15-
35 V, current 2-
5A or transformer power supply with the same specifications.
The schematic will give you an idea of my plan.
But it\'s not designed to generate PCB files because I usually make boards for my one-time design.
So I don\'t care about component packages.
If you want to create a PCB layout, you must select the appropriate package.
Each transistor has three LM317s and three TIP2955 PNP pass transistors.
Each of these LM317s will reduce the 36 v input to the programming voltage.
U2 will output a constant 12 v, U3 will output a variable voltage, and U1 will generate an auxiliary 12 v for other 5 v and 3.
3 regulators to reduce the amount of heat they consume.
The LM317 can provide an output current of more than 1. 5A.
But in this case, the LM317 must dissipate the excess power as heat due to the large difference in the input and output voltage; so much heat.
So we use the pass element.
Here, I used the TIP2955 power transistor as a transfer element on the front.
You can use the TIP3055 or 2N3055 as a transfer element on the negative or output side.
But the reason I chose PNP is because they don\'t change the output voltage like a PNP transistor (
Output is 0.
7 v high when using).
PNP transistor is used as a pass-through element for low voltage difference and ultra low voltage difference
Low voltage difference regulator
However, they exhibit some output stability problems that can be alleviated by adding capacitors to the output.
2 w resistors R5, R7, and R9 will generate enough voltage to bias through the transistor at low current.
Auxiliary 12 v output is connected to three LM2940 Super inputs
Low voltage difference 5 v 1A regulator, two of which are for USB output and the other for front panel output.
One of the 5 v outputs is connected to the 3 AMS1117 regulator. 3V output.
So it\'s a network of different regulators.
As shown in the figure, the variable output is taken from U3.
I used a 5 k potentiometer in series with a 1 k pot to make a coarse and fine tuning of the output voltage. A DSN DVM-368 (
Tutorials on my website)
The voltmeter module is connected to the variable output to display the voltage on the front panel.
See the wiring section to see modifications to the voltmeter module.
You can use any other V or A module without much modification.
Download the high resolution PNG image of the schematic here.
Or download different sizes from the instructions: plan the position of the connector, switch, etc. and get the correct size for cutting the medium fiber board, aluminum channel, etc, I first designed a 3D model of a PSU box in SketchUp.
I already have all the components.
So it\'s easy to design models.
I used the medium fiber board and aluminum profile with a thickness of 6mm (angle)
Size 25mm, thickness 2mm.
You can download the SketchUp model file using the link below.
LM317 PSU SketchUp 2014 File: download the file below.
You are free to download, modify and redistribute these materials.
These are the materials, tools and components needed.
For the PSU box, for the circuit board, make tool adjustments to the perfboard as per your requirements.
Then the assembly is placed and welded according to the schematic diagram.
I did not make PCB files for etching.
But you can make the PCB yourself using the Eagle schematic file below.
Otherwise, plan the layout, wiring and welding everything with your ingenuity.
Clean the PCB with IPA (
Alcohol)
Solution to clean up any solder residue.
To cut all the dimensions, hole positions of the medium fiber board, aluminum channel, are in the SketchUp model.
Open the file in SketchUp.
I have grouped the parts together so that you can easily hide the parts of the model and use the measuring tool to measure the dimensions.
All sizes are in millimeters or centimeters.
5mm drill for drilling.
Check the alignment of the holes and other parts frequently to make sure everything matches easily.
Smooth the surface of the medium fiber board and aluminum slot using sandpaper.
Once you have checked the 3D model, you will know how to build the box.
You can modify it according to your needs.
Here you can maximize your creativity and imagination.
For the front panel, use an acrylic or ABS board, and use a laser cutter to cut holes in it if you can reach it.
But unfortunately I don\'t have a laser machine and it would be a tedious task to find a laser machine.
So I decided to stick to the traditional method.
I found the plastic frame and box of the old refrigerator in a scrap shop.
In fact, I bought them at an unreasonable price.
One of the frames is thick enough to be flat enough to be used as a front panel;
Not too thick or too thin.
I cut it with the correct measurement method and drill and cut holes in it to accommodate all switches and output connectors.
My main tools are hacksaw and rig.
Due to the specific design of the box, there may be some problems connecting the front panel to the rest of the box.
I stick the plastic sheet of ABS plastic to the back of the front corner and screw it directly on it without the need for nuts.
You need to do something like this or better.
For the radiator, I used one of the old CPU coolers.
I drilled holes on it and connected all three through transistors with a motherboard insulator (
This is important! )
Electrical isolation between them.
Realizing that the radiator alone could not do the job, I later added a cooling fan from the outside of the radiator and attached it to the auxiliary 12 v.
First of all, you have to polish the fiber board with 300 or 400 grain size sandpaper.
Then apply a thin, uniform Wood primer or a mid-fiber board primer.
Apply a layer after the first layer is dry enough.
Repeat once as per your request and let it dry for 1 or 2 days.
You have to polish the primer layer before spraying the paint.
It is easy to paint with compressed paint cans.
Fix the circuit board welded in the center of the bottom plate and tighten it using small machine screws and brackets between them.
I used the wires for the power of the old computer because they are of good quality.
You can weld the wire directly to the board, or you can use a connector or pin head.
I did the PSU in a hurry so I didn\'t use any connectors.
But it is recommended to use connectors as much as possible to make everything modular and easy to assemble and disassemble.
I had some rather strange problems with wiring and initial testing.
The first is the unstable output.
When we use the PNP channel element, the output oscillates, thus reducing the effective DC voltage on the meter.
To solve this problem, I had to connect the electrolytic capacitor with high value.
The next issue is the output voltage difference between the board and the output connector!
I still don\'t know exactly what the problem is, but I fix it by soldering some high value resistors for 1 k, 4.
7 K, etc. , directly at the output terminal. I used 2K (1K+1K)
Program the value of the resistor for the auxiliary 12 v and the main 12 v output.
All we need is dns-DVM-
When all other outputs are fixed, the 368 voltmeter of the variable output.
First, you have to disconnect (IMPORTANT! )the jumper (Jumper 1)
As shown in the figure, then use the three lines in the schematic.
There are already 5 v regulators inside the voltmeter.
Supplying 12 V directly to it causes unnecessary heating.
Therefore, we use a 7809, 9 v regulator between the AUX 12 v and the Vcc input of the voltmeter.
I have to make the 7809 a \"float\" component that I added after soldering the board.
Connect SMPS with rated voltage between 15-
Current of 35 V and minimum 2A, input into the circuit board through the DC bucket Jack.
I have used SMPS for 36 V 2A
Current Protection (shutdown)built-in.
See the measurement table for load testing.
Due to the output power limit of the smps I use, the load adjustment here is not very good.
It will limit the current and shutdown of the high current flow.
So I can\'t do the surge current test.
Load adjustment seems to be good, up to 14 v.
But set voltage above 15 v (8, 9, 10)
, When I connect the load, the constant current is 3, the output voltage will drop to about 15 v. 24A.
At 10, the load voltage is half of the set voltage at 3. 24A current !
So it looks like my SMPS don\'t provide enough current to hold the set voltage.
The maximum power I can get is 11, 58 W.
Therefore, as long as you keep the low output current, the output voltage will remain in the position it should be.
Always pay attention to the voltage, current and temperature of the radiator, because it will consume a lot of power there.
After completing the test, assemble everything and label the front panel the way you like it.
I painted the front panel with silver paint and marked things with permanent marks (
Not a good way).
I put a DIY sticker in front that I bought with the first Arduino.
This power supply design has many advantages and disadvantages.
It is always worth studying them.
The advagesadvagesa digital multimeter is the best tool to solve the power supply problem.
Check all the regulators before welding using the breadboard.
If you have two DMMs, it is possible to measure the current and voltage at the same time.
This is a basic linear power supply.
So you can improve a lot.
I rushed to build this because I needed some kind of variable power supply very much.
With this I can build a better \"precision digital power\" in the future \".
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