Introduction: ATX (computer) Power Supply Soft Start


I recently had the need to find a power supply that could provide in excess of 40 Amps at 12 Volts. For those that have looked before, you will know that these are

  1. Very expensive to buy
  2. Difficult to make (well).

My solution turned out to be a standard Computer Power supply for about $65 (Every time I mention $$, you can assume I am talking in $AUD). The only problem was creating a way to power it on and off without plugging in a motherboard, and, I kinda liked the push to start/push to stop so I made this on some strip board with about 10 components.

You will need at least beginner level of electronics experience to get the most enjoyment from this project.

A brief history of Power supplies.

Original (AT) power supplies did not have a soft (electronically) controlled on/off. They just used a switch. The newer style lets the motherboard control the power so it can wake it self up when needed, and so it can finish installing those incessant updates while you go to bed, knowing that it will shut itself down when finished.

Newer versions of the ATX power supply have more power concentrated in the 12 volt outputs than the 5 volt outputs, and also come with 3.3 volt outputs. Usually there are multple voltage and ground outputs of each colour.

  • yellow -> 12 volts
  • red -> 5 volts
  • orange -> 3.3 volts
  • black -> ground/0 volts

Control lines

  • purple -> standby power (always on)
  • green -> on/off control
  • grey -> power OK indicator

Note that some older power supplies do not like starting with zero load on them. For these, you may wish to put a 5W, 10ohm resistor between the Red and Black output wires to provide dummy load. The Corsair PSU shown in the video did not have that problem, and was happy to start with no load connected.

Step 1: How Does It Work

Every time you press the button, a pulse is sent to the next stage of the circuit, which is a divide-by-two counter. So, you press is once, the counter goes high and turns on the power supply through a transistor. You press it again and the counter goes low (it's dividing your button presses by two). So for every two presses you do, the output does one on/off cycle. Keep it Simple.

Tools you will need

  • soldering iron
  • side cutters

Parts you will need, approx cost $5, + $5 more for a nice looking switch.

  • square of copper vero (strip) board min 11 x 16 holes
  • 1 x bc548 npn small signal transisitor (or an small npn transistor really)
  • 1 x 74LS76 JK Flip-Flop with Set/Clear
  • 1 x 100uF Cap (over 10V)
  • 4 x 1K resistors (1/4 watt)
  • 1 x 10 resistor (1/4 watt)
  • 2 x 0.1uF Cap, (either ceramic or surface mount, we dont care)

plus a momentary on switch with both normally open (no) and normally closed (nc) contacts.
plus some hookup wire, in various colours and solder of course.

A new ATX power supply will start around $50 for a cheap $450W one, and go to $200 for a beefy 1000W one.
You can get these at your local PC parts supplier. If you have a couple laying around ... well, all the better.

Step 2: Cut Yourself a Piece of Stripboard.

This will be 11 x 16 holes in size. (about 1 x 1.5 inches)

Turn it copper side up and drill away the copper where X is shown on the diagram. The diagram included here is showing the copper side looking from the bottom side up, so it will match the cutaway diagram. Check the example and don't make any mistakes.

Step 3:

When we solder a circuit board, we want to do it based on the height of the components, so the first thing to be soldered in will be the wire links. With regard to the to bent wires, my apologies to all the orthogonally challenged OCD peeps out there, the good news being that when the Integrated Circuit is in place, you wont see those two wires.

Step 4: Solder in the Resistors

For those electronically astute among you, you will have noticed that the diagrams all say 1K resistors, and yet, I have used 1.2K in the photo's. The truth is that it doesnt matter as these are all just pull resistors for the logic pins. Anything between 1K and 4.7K will do the job just fine. The secpnd truth is, I ran out of 1k resistors in my junk box and I had plenty of 1.2K ones left ....

Step 5: Solder in the IC (JK Flip Flop) and Other Bits

The integrated circuit and the transistor are both prone to heat damage if you take too long to solder them in.

Be quick but not rushed.

Hint. If you think the components are getting too hot, wait 10-15 second between doing each leg to give it time to cool off each time.

Step 6: Connect Up the Switch

If you have an integrated LED in your switch (like mine), the switch has 5 tabs on it.

Two of these will be marked + and - and will be the LED which must be connected around the right way (polarity sensitive).

The other three will be the actual switch.

  • One tab will be common (you can find out using a multi-meter and by pressing the button).
  • One will be normally open (not connected when the switch is in the un-pressed state)
  • And the other will be normally closed (connected in the un-pressed state).

Step 7: Connect the Switch and Test.

Connect the purple and black wires from the ATX power supply. This will power up your circuit. Do not connect the green wire until all verified working OK.

Connect up the LED and switch to the circuit board and press the button.

Place a logic probe or multi-meter at the base of the resistor shown as output (in blue), and you should see it toggle between 0 and 5 volts. If all that is working OK, connect up the green wire from the power supply, and repeat.

You should now see the power supply and the indicator LED turning on and off when the button is pressed.


ps. Feel free to add comments and let me know of any corrections required.


rafununu made it! (author)2017-02-13

When I saw "soft start" I said to myself, "he's gonna destroy everything". Fortunately, this isn't what I call a soft start. Ther's an error in the schematics as the 2nd flip flop stays in the same state whatever happended on the clk input, there should be a loop between either /Q and D or JK.

AndrewG166 made it! (author)AndrewG1662017-02-13

Rafununu, the video clearly shows it working, so I'm not sure how you can say it doesnt work. It's a bit like telling someone they cant ride a bike, while they are riding past you on a bike. If you look at the truth table for a 74LS76, you will clearly see "toggle" listed for the combination of J,K high and clock pulsed. You are correct for a D type FF, but that is not what this is. (google JK flip flop divide by 2 to see dozens of examples the same as this one.)

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