This instructables will show how I built a variable voltage power supply based off of the lm338 variable voltage regulator.
The lm338 circuit is very simple and can be made with only the voltage regulator, a resistor and a potentiometer. the voltage regulator relies on the voltage given to its adjust pin, this voltage is adjusted using a resistor and a potentiometer to change the voltage given to the adjust pin. the capacitors are used for smoothing the input voltage and output. The lm338 can output a constant 1.2-30vdc at 5 amps (adjustable volts), input of 33 vdc or lower (input has to be a couple volts higher than output) and a peak current of 7 amps. the full datasheet of the lm338 can be seen here!
I will be working on building a pcb model so I can make a few pcbs and have these to use for future projects but until then if you would like to make your own pcb for this project there are many examples online and if not they are sold as kits! and as prebuilts!
Step 1: Understanding the Circuit
here is the circuit I made on circuits.io .The inductor in the front is supposed to be the secondary winding of a transformer, the transformer will put out 24vac, after the bridge rectifier changes the ac to dc the signal needs to be smoothed out to pure dc. the large capacitor smoothes out the signal to pure dc with the added bonus of raising the voltage. the voltage is raised from 24vac to 33vdc, this is because ac is not measured from the peaks of the sign wave, meaning the net voltage is actually higher than the voltage we usually see. we can find this rise in voltage by dividing the ac(24) by .707 . 24 / .707 = 33vdc. the .707 is not accurate for all ac voltages but it works for us mains well enough.
after the rectifier and the caps the voltage should be about 33vdc, perfect input voltage for the lm338 variable voltage regulator. The regulator looks at the voltage on its adjust pin and sets the output to that same voltage so to change the voltage on the adjust pin two resistors are used in series connected between positive and negative out, this means that the adjust pin is looking at the difference of the pos and neg. This can be seen this picture I found on the googles .
after the voltage regulator there are two capacitors to help with voltage smoothing and two diodes in case of any voltage feedback into the circuit, and thats it. not really much to it and I really suggest trying to build it as a beginner project because its good solder practice and a power supply is a good tool to have for any tinkerer so why not build one yourself.
Step 2: Parts!
so now that we know a bit more of how it works we have to build it, I built mine out of an old pc case because I wanted room to add more projects inside of it, which im currently doing. Below is the parts list but I used parts I had at home like the power plug from the old pcs power supply but I will include those into the parts list because not everyone will have the same parts laying around.
power plug in: power plug female fused $6.27
extra fuses: 10 amp fuses $4.99
large input smoothing cap: 10000uf 50v cap $4.95
overkill bridge rectifier: 1000v 50A rectifier $6.92
prebuilt adjustable lm338 circuit: 1.2-36v 5A lm338 $9.66 can be replaced with next item
replacement power supply (more complicated so you couldnt work/fix/build it yourself but it has variable volts and amps so it can be more useful but you will definitely learn a lot from the other power supply
drok buck converter 0-55v 0-5 amps $35 not needed, can be added along side or later on
toroidal transformer :6/12/24/48 volt toroidal transformer $45
banana plug female : 10 total, used for outputs $7 not needed but nice to have
volt amp meter : nicer one than I got $15 there are a bit cheaper ones but this one is nice
banana plug cables : 5 total $9 not needed
this looks like a really good transformer because it puts out 24vac which is what this project but it also puts out many other useful voltages and at high amps so this will be handy for other projects, I personally have one and I use the 24 for this power supply and the 48 for another variable power supple.
large project inclosure( I used a pc case)
picture is of the center tapped 48v toroidal transformer and the two bridge rectifiers but minus the two smoothing caps, the two outputs are not needed but I used both the lm338 and the drok buck converter so i needed one output of 33vdc and 66vdc (drok actually takes 55vdc so i need to lower it)
Step 3: Putting It All Together!
ok in total the circuit will go
variable voltage circuit
I dont have many photos of putting it together but in the next step ill just dump all the photos of interior and exterior of the power supply
Step 4: Pictures.....
Thats it guys I really hope you enjoyed the project, I sure know I did. its come in very handy for many other projects and was a great way to learn a bit. even though I could have bought a prebuilt lab power supply for $70 ish i think this was far more worth it since I can repair it, add more to it and have learned so much while building and researching for it.
if you have any questions please feel free to ask and any tips are greatly appreciated :)
Participated in the
Circuits Contest 2016