小heap but ugly variable bench power supply


 Every full-fledged tinkerer in electronics needs a variable power supply, and, as a tinkerer prefers to do, usually builds it instead of just buying. Personally, I did it because I didn't want to spend money on it, eventually discovering that now my PSU has a unique combination of features that no money can by. In this write-up I'll attempt to provide the reader with a comprehensive step-by-step guide on how to build something similar (and probably looking less like crap 馃榾 )


I based my design on this splendid homemade PSU:

 Made by ThomasVDD
You can learn how to make it at:

http://www.instructables.com/id/Variable-ATX-bench-powersupply-FabLab-edition

Since I didn't have access to a laser cutter and couldn't be bothered cutting the box manually with a jigsaw, I just found a plasic box of sufficient size and cut some holes in it.
My PSU was supposed to be a carbon copy of the one on the link above (and I certainly wouldn't write this if it stayed this way). This model has some added features like:
  • Separate ground terminal with ammeter (0-1A and 0-5A range - selectable)
  • 2 additional terminals to measure and display zener diode's voltage
  • A display that shows voltage at variable voltage terminal
  • Additional -5V terminal 
  • A switch that lets you measure current through USB port
  • Awful exterior
If this looks good to you, then please proceed:

What you'll need

  • Working ATX Power supply
  • Some enclosure
  • A matching number of banana plugs and terminals (not exactly matching, based on my experience you never need more than 4 connection leads at once, so probably just 4 plugs and at least 9 terminals)
Banana terminals that I used.
Not the only kind at the market,
you're free to choose something fancier.
Banana plugs.

  • 7 fuse holders and fuses

 

  • 2 voltmeters with 3 wires (Power, Ground and Signal aka measured voltage) - most voltmeters sold on Aliexpress are 2-wired meaning they measure the voltage of the power source. You don't need them - as soon as voltage drops below 4.5V they will switch off.
  • USB port - I found that the easiest in procurement panel-mounted USB connector is "USB Female to Female cable" (BTW most suppliers incorrectly list it as type B):
For a price of 1 item you get 2 connectors after snapping it in the middle! Yay!
  •  2 Ammeters - 0..1A and 0..5A
NEVER buy digital ammeters, they just burn for no reason at all after a month of usage. I personally replaced 2 digitals before switching to analog.
Analog is the way to go!
  •  For variable voltage circuit you will need: LM317, some filtering capacitors, 1.2k Ohm resistor, 10k Ohm potentiometer, 1N4007 (or similar) diode, heatsink for LM317, screw terminals (x3).
  • Miscellaneous: 2 SPDT switches (changeover), 1 SPST switch (on/off), 1k resistor, a couple of screws and standoffs to attach voltmeters to the enclosure.
This "item list" picture was relevant when I started working
on the original PSU, since then I replaced ugly usb ports
and added lots of features. Still, this picture is inspiring so I'll just leave it here.

Building

Here's the schematic for the whole project.

A picture is worth a thousand words, so here's some pictures of build in progress:

1) Attach the box-less PSU to the lid.
2) Fan is essential for keeping the power supply nice and cool.
Don't forget to make some holes for air outflow as well.

3) Build and attach the variable voltage circuit.


4) Plan ahead, make a grid according to which you'll place
your posts and fuse holders. Note a huge analog ammeter
on top center - got fried in less than a month.




Aaaand there goes the cheap chinese ammeter...

At least it works without it just fine.

Finished product:



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