This was a weekend project that I did so I can use a breadboard and get power to it. Some of my projects involve using 5V for a logic supply, while for others I may need a higher voltage. In this instructable I'll show how you can use a fixed voltage regulator as a adjustable one. The advantage of this is method is that your minimum output voltage isnt 2V, such as in the LM317T, but your minimum is what ever voltage regulator you used. For example, in this project I use a LM7805 as a adjustable Regulator, It gave an adjustable Voltage from (Vin-2) to 5V. I say Vin-2, since there is about a 2V drop across the regulator.
The Main reason for making this is because I wanted to make a portable breadboard with a Power supply in case I was bored at someones house, had a circuit with me, and wanted to play with stuff. This orignally stemmed from me salvaging a BUNCH of Mosfets/Transistors and not being able to test them when I was away for a weekend. Thus, this thing is powered from a wallwart or anything that has a 2.5 mm Plug
Step 1: The Design
I needed a PSU that gave both 5V and was adjustable in case I needed 12V. Granted, I could have used a LM317 AND an LM7805, but because Of board space, I couldn't use both. I could have used Just a LM317 Adjustable Voltage regulator, But I didnt feel like breaking out my volt meter if I needed Just 5V.
Then I looked around the web and found that a Fixed voltage regulator can be used as an adjustable version. The reason why this works is that it makes ground "float". By adjusting the pot, your adjusting the ground point seen by the regulator.
The switch shown in the schematic is optional. I removed it from the final design (but its still on the PCB). I later figured I can just remove the power Plug and use that as my switch.
Step 2: The Board
Believe it or not, but the board was the hardest part. Why? I had a major size restriction on purpose. The two connectors at top fit exactly into the top two rails of my breadboard. Those parts Couldnt move at all, and the size restriction was based on the scrap PCB I had lying around from other projects, since I try to use Every Small piece of PCB that Ive cut.
Now, if you use a different design, yours may be different. Note that the PCB didnt come out so well. Ive been having really bad lucky lately with making stuff, and failed 3 times before this before I got it right. Normally Im pretty good at making PCBs with Staple Photo paper and a Laser jet, but this one just kept slipping whenever I applied the iron (Even with tape!). Most of the traces came Out OK, except for the bigger traces on the top and bottom of the board.
When your done making the board, I HIGHLY recommend applying a thick coat of clear coat. This is to protect the copper and to prevent shorts during soldering. You can probably use a thin coat of color if you wanted to and get the same effect. I didnt coat this board with CC (clear coat) because its raining out.
After that Check the board for shorts where there shouldnt be shorts. Take note of opens, as these can be fixed during soldering. If you find a unwanted short, use a exacto knife. These things are thin enough to get between even the smallest traces and sharp enough to get rid of extra copper-BUT be careful, cuts from a exacto hurt like a paper cut.
Step 3: The Parts
Here are the parts per the schematic
1 LM7805 5V regulator (blue box)
1 1k Resistor - this is for the feedback (green box)
1 182 Ohm resistor -this is for the LED (green box)
1 1k Pot - The adjustable resistor used for setting the voltage (red box)
1 LED - any color that isnt too bright
1 680uF 16V Capacitor -(Optional)-Used because the source is more than 6 inches away from the regulator (purple box)
1 1uF 16V Capacitor (optional also) - Same reason, used to help regulation. (purple box)
2 2.54 mm Headers I salvaged from a Old motherboard- These are used to plug in to the board
1 2.5mm Plug. I also salvaged this part, but got it from a old Speaker power supply (the grey box).
Also optional is a small TO-220 heatsink. I added this on for the hell of it since its adjustable (a bit of insurance). If you dont have a heatsink the LM78xx series will shutdown if it gets too hot or if you short the output. You can also use a screw and a peice of aluminum (not aluminum foil!-its too thin) or extra Copper PCB as a heatsink. The heat transfer may not be as good, but its better than nothing.
Step 4: Putting It All Together/Getting It to Work
I usually solder the smallest parts to the biggest parts, from smallest height to tallest. This usually works best but even then parts can get in the way of each other. To get the headers like I did, I had to do a little finagling. Place the headers in their designated place, and apply heat to the base of the board closest to where you want it soldered. You will melt some of the black plastic stuff, but thats OK. as long as you get it soldered relatively neatly your OK.
Once you get all your parts soldered, Once again, CHECK FOR SHORTS! Opens arent a problem since those should have been taken care of when you soldered. One Short can Ruin your whole project and you'll look at it for hours wondering whats up with it. Use the Continuity function on your meter so you can probe around. Having the schematic nearby is also handy to have, but usually common sense says that Power and ground shouldn't be shorted together. Remember KISS- Keep it stupidly simple.
However, Unless your caps are bad Thats the only problem you should encounter. If the output is shorted the IC shuts down and you get 0 volts on the output. If the input shorts Out you also get nothing on the input BUT your Wall wart voltage drops (which is what I am powering this from).
Now once all THAT is settled, plug in power (if you havent already done so to trouble shoot) and your LED should turn on. Oh, One more thing. If your LED isnt connected it wont light up. Its intensity will vary when you vary the voltage. Now behold, your completed breadboard Portable Power Supply
Step 5: Cost
Now you might be wondering, What if I didnt have all these parts? Well sir (or madam) you need to get a blow torch and start salvaging! But before you do that, go to your favorite supplier and order the parts from the parts List. My favorite distributor is Mouser or Digikey, but I usually order in Odd or even amounts, usually rounding UP. For example, if I need 3 of one part, I'll order 5 in case some break or if I need more.
But for the sake of example, I'll order 1 of each. I am also leaving the capacitors, headers and LED out, since you should have the LED and caps are optional. The caps and Headers are salvageable from old electronics
Price: $0.26 Each
There is no doubt in my mind you'll use more of these, so order 3 or more.
Price $0.02 each
Freaken Cheap. 1% Resistor. 2 cents each, order 50 Of these as you'll use them also.
Acceptable replacements for the 182 Ohm resistor are 180 to 220. These will cost the same as the 1k,
A bit expensive but I couldnt find the same type of pot I used.
Total Price $1.82!!
Even if we included the Headers + capacitor, The total wouldnt be more than $2. For a linear Breadboard PSU, I call think this is pretty dam cheap considering Ive seen others being sold for $30
Step 6: Schematic/PCB
Here are the schematic and PCB files for use in Eagle PCB