Variable Power Supply




Introduction: Variable Power Supply

About: Why would i buy something ready made when i can make it myself with half the features for twice the money? DIY!

 This build is for a variable power supply. I have a bench power supply i made from an ATX power supply and it works great but, it is not variable. Sometimes i need 6v or 9v ect. I cant do it with the ATX supply. So i built this one.
It has a dedicated 5V dc supply for logic circuits. Plus a 1.3V dc to 22V dc variable supply. My volt meter only goes to 20V dc so at 22V the meter is pegged clear over. 

The reason some of this is built this way is because i used everything  from my parts on hand, except the 1K 2 Watt resistor. That i had to buy for this project.


1)  Lm317 IC
1) .1µf cap Mylar number 104
1) 10µf cap 16v
1) 1µf cap 50v
1) 100 ohm 1/4 Watt resistor Brown Black Brown
1) 220 ohm 1/4 Watt resistor Red Red Brown
1) 1K ohm 2 Watt Resistor Brown Black Red
1) 5K pot. A 2k Would be best
2) 1N4007 Diodes
1) Volt Meter 20V dc
1) DPDT Toggle sw
2) SPST  Toggle sw
1) Power Supply or Transformer 24V out
1) Mounting Board or Project  Box
4 ) Header Pins
?) Various Screws, Wire ect
? ) Stuff i Forgot to list

The LM317 voltage regulator has 3 posts. Looking at it from the front, from left to right its 1 2 3. Post 1 is adjust. Post 2 is V out. Post 3 is ground. Its easy to get them mixed up, so be aware of orientation.  

Step 1: Tools and Be-Careful

There are 4 pictures of things you want to use.

There are 4 pictures of things you dont want to use.

Be-Careful so you can choose the 4 of these you want to use.  

You wouldn't want to be forced to choose the 4 you dont want to use. 

                                             !!!      KEEP IT SAFE     !!!

Step 2: Bread Boarding

This is my bread boarded ( second stage i will call it ) power supply. On the lower left is one of my logic probes with pulse injection. Thats  another project to finish later.

The second stage is based on the popular LM317 voltage regulator.
It has input short circuit protection and out put short circuit protection from the capacitor discharge.   You can find this schematic and more info at
While my supply is based off of ladyada, it is not entirely the same.  
I could not get a 10 turn pot so i used a volume control. Not the highest quality, it has skips in the wiper resistance. Mine would not work with the 240 ohm resistor as shown in the schematic, maybe due to the volume control. I ended up using a 100 ohm resistor instead.

Step 3: Primary Power Supply

This is the primary power  supply ( i will call it ), is from a Canon ink jet printer. Specs: 220v in. It has 2 out voltages. First is 5v dc at .2A, second is 24v dc at .55A 

I used this because i didnt have a transformer that was close enough for this voltage range. Plus i have several of these types of power supplies. I wanted to use what i had.
There is 2 little locating nubs on the bottom, they made it not sit flat on the mounting board. I ground these off. Flat now. After i ground these locating nubs off the holes were fairly big. Bigger than i thought they would be. In hind sight it might of been better to drill the board for the locating nubs and then drilled a small hole for the screws. No going back, its done already. It actually worked out ok. The  bigger screws had plenty of clearance for the PCB. 

I marked and drilled the holes for the primary supply to anchor to. Mounted the lower plastic case. Re installed the PCB making sure everything lines up. Cut and tined the out put wires.

Step 4: Assemble Secondary Power Supply

I soldered the heat-sink to the board also to give it more strength. Assembled all the components as per the schematic. 
I used small screws as stand offs, marked and drilled the base with 7/64 holes. They were just the right size so the screws would thread them selves into the wood, keeping the perf board up enough so nothing touches underneath. I used the nut as kind of jam nuts so everything stays in place.

Step 5: Wire Ends

I cut a block of 2 header pins. Soldered the out put leads to it, one leed to one pin then the other leed to the other pin of the same header. Then used individual heat shrink for each wire then one bigger heat shrink to cover it completely, including the plastic part of the header pin.
Red and black is used for the variable 24v side, then blue and red for the 5v side.
This way i can plug direct into the bread board.

Step 6: Switches, Gauge and Wiring

First we want to remove the mains plugin from the primary supply . Then bend the lugs up on the plugin so they dont go through the PCB any more. Soldier wires to the lugs and also soldier wires to the previous spot on the PCB where the lugs used to go through. Drill a hole in the back of the primary power supply box and route all 4 wires through there. Those wires will hook to a DPDT SW, now we can control the mains on and off instead of having to plug and unplug the cord every time. 

I used some lock clips from the backs of computers. Soldiers 2 together as in the picture. Make 3 of them. These are the sw holders.

Use the same lock clip and flatten one out. Then re bend it so it will fit on the 5K pot and also locate on the bottom of the perf board. Soldier it there. This will help to keep the POT from bending at the tabs

Take 2 PC case block out plates. They are the plates you put on to block the slot in the lower back of the case if you are not using a video card or some other plugin card. Then cut them down so they will fit around the sides of the volt meter, secure with washers and nuts. I chose to mount the gauge where you see it. In front and at a slight angle back for easy viewing.

The switch operation goes as follows:
Left switch operates the main power. The middle switch operates the 5v supply. The right switch operates the variable supply.

The LED indication goes as follows:  
The left LED ( green ) is main power at the secondary supply indicator, which is 22V dc. 
The right LED ( yellow ) is 5v supply indicator,
The variable supply has no LED. You will know if its on by the volt meter

The green LED takes a 1k 2 watt resistor.
The yellow LED  takes a 220 ohm 1/4 watt resistor.

I used this calculator for the resistors.

You can see the big 2 watt resistor compared to the 1/4 watt resistor.  Not very common for me. I had to make a special trip just to buy this one.

The last picture shows everything on and working.

Thats it. Not  very professional looking, but it suites me just fine. Also its mine not store bought.
I hope i explained everything well enough for you to understand. If i didn't please ask and i will do my best to clear things up.

If anybody decides to make this or some version of it i would love to see what you came up with. 

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    13 Discussions

    if you change r7 you was have a realy good variabble power supply with curent regulation .I was build this one with 2 transistor so i´ve got 6A power supply.


    Reply 7 years ago on Introduction

    The power supply is rated at .2A at 5V and .55A at 24V.
    An LM317 is rated at 1.5A .

    To answer your question about how many amps this can this hold safely,
    I guess i would have to say the .55A



    Reply 7 years ago on Introduction

    If I used a 3 amp power supply and 2 LM 317 regulators in series, could I use the power supply's 3 amps safely?


    Reply 7 years ago on Introduction

    I don't think that would work. If you need bigger amp regulation i would look at using something like an LM338. It has a 5A rating


    Reply 7 years ago on Introduction

    You can boost a voltage regulator's current output with a pass transistor like I did in my 300 Watt power supply project here.

    By itself I don't think an LM723 can supply more than 150 milliamps.


    8 years ago on Introduction

    I made my first LM317 power supply when I was 13. It wasn't pretty, but I was really proud of it.

    Extra points if anyone knows what the metal frame was before I salvaged it for my project.


    Reply 8 years ago on Introduction

    I too your quiz about the metal frame. I believe i have flunked. I do not know what that frame is.


    Reply 8 years ago on Introduction

    OK I'll give you a hint. It could have been found in every living room across the USA back in the day.


    Reply 8 years ago on Introduction

    A little too far back, but close! It is a TV tuner knob cage. You know, out of one of those old huge console TVs it used to take two strong men to move around.