When dealing with electronics you need 1 key thing, electricity. For quite a while now if I needed 5 volts I would just grab my homebrew arduino, but that is not always handy and its tethered to the pc and it does not have 3v. If I wanted 3 volts, now I am digging around looking for my UBW32 which does have 3v3 but now I have a 50$ microncontroller with very small regulators (so therefore only small loads) dangling around just for power, and its a mess.
So I need just a board that takes some DC from a wall wart and regulates it to usable voltages, and I set about to make it. This regulator board puts out +5, +3.3, variable and negative variable voltages, is pretty easy to make, and make a nice addition to the bench. (until I can get a real bench supply someday)
Now I know this is not ground breaking hackery, but I hope it helps someone out there, join us after the break to see what’s going on.
First up, parts! I started by reviewing the datasheets for the 3 regulators, and pretty much throwing most of their advice away as most of this was scavenged parts. Beggars can’t be choosers.
That works mostly okay, but due to my resistance choices on the LM317T I am loosing a bit more (about an extra volt!) than I should be in the regulator. I used 100 ohms and 1khoms for the voltage adjustment … 120 and 2k or 240 and 5k would have been a much better choice, if I had those parts on hand.
But this is what I ended up using
1*SPST toggle switch
1*LED of your choice and appropriate resistor for 5 volt operation
3*Silicon rectifier diodes (I am using 1N4001′s but most would do fine)
2*47uf capacitors with voltage ratings higher than ~16 volts (I am using 50 volt caps)
1*10uf capacitor (its for the 3.3 volt line so anything more than 3v will work)
1*4.7uf capacitor (again I am using a 50 volt)
6*100nf ceramic capacitors (code 104)
2*100 ohm resistors
1*1k ohm resistor
1*1k ohm trimpot
1*7805 in a TO220 package
1*3 volt regulator in a TO220 package (I am using a ST “LD33V” though its not common)
1*LM317T in a TO220 package
4*Dual terminal/ screw blocks
1*Large 3 position heatsink or 3 normal TO220 heatsinks
Wire, solder, and a 2 position jumper
More or less voltage comes in is filtered a bit with capacitors and is fed to the 7805 and 317T, the 3v3 regulator is wired to the output of the 7805 (along with the LED) so it does not have to drop a bunch of current and not run hot with minimal loads. Keeping in mind there are 2 regulators connected to a wall wart this thing in theory could suck up to 2.5 amps out of your supply so you either need a wall wart that can handle it or be very mindful of how much current you are drawing.
Next is the schematic .. not much to say here its a schematic… (click for large version)
Update: fixed the backwards diode
Finally I have to make the thing, I choose perf board with copper pads on one side, and uninsulated 22 gauge solid tinned copper as I find this way to be pretty easy as long as the layout is not too complicated.
Construction starts by mounting the regulators to the heat sink. I got this heat sink and many of the capacitors out of an old dead PC power supply. It is pretty big and should do the trick just fine, but there is a catch. Each of the regulators mounting tabs go to different functions, so unless I want to connect the outputs of the 3v3 regulator and the LM317T to the ground of the 7805 I am going to need either separate heat sinks or some silicon pads and nylon washer isolation hardware which my power supply was gracious enough to provide. (only after I threw them away and had to dig in the garbage for 20 min)
Here is a shot of the power input and 7805 all wired up and tested working.
Another shot with the 3v3 regulator wired up and tested working.
Another shot with the 317T wired up and tested working.
And finally the 555 voltage inverter is in place and working. While they make voltage inverters and I have a 5 volt one somewhere I rather like the idea of having one that can be hooked up to the 317 because I really don’t know if I will need -12, or -5 or anything in between.
Keep in mind this is a kludgey way to make a negative voltage, and it will not be balanced with the positive output of the 317 (you loose 0.7 volts or more in the diodes). Also it takes a few seconds before it stabilizes on a voltage, so start low and stop a little short as it will continue rising for a few seconds. You might be able to minimize that by using a lower value capacitor on the 555′s output, right now I am using a 47uf but I have seen everything from 10 to 470 depending on which schematic your looking at.
Anyway here is the final product and I am ready to rock. You might notice a jumper block on there and that is to cut power going to the 317. With the losses due to my resistance choice and the 555 timers output, that part of the circuit draws about 37ma, the 7805, led, and 3v3 regulator only draws 36ma (20 for the led). So if you’re not using it why bother adding the additional load to your power supply? I may go back and tweak that.
Thanks for looking!