Converting a PC power supply (ATX) to a Variable Power supply.

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madjackghengis:
Hi Bernd and interested parties, I've built two different Vellman electronic kits which use solid state chips to make variable speed "chopper circuits" specifically for driving motors with full torque and variable speed. One is specifically designed for twelve volts, and is limited to 1 amp of load current, the other is useful from five to thirty five volts, determined by the power supply used. Both are easy to build and quite reliable and responsive. They were built to use to drive a surplus wiper motor for a power feed on the Y axis of my mill, and one to drive a drill-driver motor whose batteries gave up the ghost, to drive the top slide of my lathe. The wiper motor works fine, but I have not yet connected the other, as I was looking for a power supply for other than twelve volts, and now have an answer. Both kits are very easy to assemble, and are well proven out circuits that are reliable, and give repeatable performance. I will be trying out the second one on power out of an old computer power supply, and wish I'd thought of that first. Vellman also makes an electronic ignition kit for converting points to electronics, which allows one to use micro switches for "points" without having the ignition current burning out the micro switches. It also works on differing voltages, although I have not tried it to find how low one can go and still have reliable sparks. I have a couple Jap motorcycle coils which are about 0.9 ohms, and will try them on five volts through the vellman kit and report back as soon as I have five volts to try it with. Mad Jack :beer:

raynerd:
Bernd, sorry for the delay, I read your question this morning but I wanted to check something on the power supply before I answered it and I`ve only just got into the shop now.

Green - PWR_ON is an active high and must be grounded for the psu to turn on DC. For testing I just connected to gnd and it boots the supply right away however on the conversion it is the green that the switch is connecting to - grounding it to turn the supply ON.

Purple is +5v Standby - this line is ALWAYS 5v whilest you have a AC power supply to the AXT unit. It can be ignored or connected to an led as I have to show power is being fed to the PSU.

Grey - PWR_OK - this goes to +5v when there is a supply to the DC power lines. I`m going to add a second LED here as this is a confirmation that the power lines have a feed.

To test the PSU, I simply grounded green and purple and attached a load across the +5v. So that you don`t have to have anything connected on the actual thing when it is up and running, I`m using the 10w, 10ohm resistor.

Hope that is of some use.

Chris

Bernd:
Thanks Chris. That's exactly what I needed. Thanks also goes out to the others that answered. Much appreciated.

Bernd

DMIOM:

--- Quote from: craynerd on February 18, 2010, 10:55:30 AM ---.......
Green - PWR_ON is an active high and must be grounded for the psu to turn on DC. For testing I just connected to gnd and it boots the supply right away however on the conversion it is the green that the switch is connecting to - grounding it to turn the supply ON.

Purple is +5v Standby - this line is ALWAYS 5v whilest you have a AC power supply to the AXT unit. It can be ignored or connected to an led as I have to show power is being fed to the PSU.

Grey - PWR_OK - this goes to +5v when there is a supply to the DC power lines. I`m going to add a second LED here as this is a confirmation that the power lines have a feed.

To test the PSU, I simply grounded green and purple and attached a load across the +5v. So that you don`t have to have anything connected on the actual thing when it is up and running, I`m using the 10w, 10ohm resistor. .......
--- End quote ---

Chris - a couple of points :

1/ If the Purple is the +5v standby (which is what is often used to power the Ethernet port and was used for the serial port UART, so you can have "wake on LAN" and used to have "wake on Ring") then that +5v is, as you say, live all the time - so grounding the Purple is in fact putting a dead short across one of the supplies! :zap:

2/ You mention the Green is grounded to turn the PSU on, but describe it as Active High? I suspect it has a pull-up so it floats high, but is actually Active Low? (i.e. take it low / to ground) to operate.

3/ Have seen in photos you're fitting terminals etc. in through sides of the case - I'm sure you're careful but just in case anyone else doesn't know, switched mode power supplies can have lethal hundreds of volts DC in them .... :zap:

Dave

raynerd:
Dave, 1. Yes, in retrospect, think I should have connected across LED even when testing. 2. Green is called Active High on a few sites but I see what your saying.

OK, carrying on today, I realised that in actual fact, once I`ve made this I don`t want to keep pulling it apart and adding to it as was my initial thoughts. So, if I`m going to try and build a circuit that gives me a variable supply as well, I need to do it now so that I can correctly position the rest of the components and front binding posts and have this finished. I wanted to add a fully variable output and followed these notes to build a suitable circuit on breadboard for testing purposes:

http://www.wikihow.com/Add-Variable-Voltage-to-Your-ATX-Based-Bench-Power-Supply

The main component is the LM317T voltage regulator. Here is the trial circuit:

The pot is quite sensitive and I need to sort this out as it is difficult to control the voltage but it is stable once you get it there. This is the lowest voltage possible 1.26v

At 10.00V

At maximum - 19.81V. Really this should be 24v so I think I need to change the load resistor.

Again, this is just a log of what I am doing…google and Do Your Own Research first!

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