Thursday, February 25, 2016

New ATX Bench Supply, Importance of fresh multimeter batteries

A few years ago I converted an old PC power supply into a fixed voltage, bench power supply. They are very handy to use considering the common voltages they produce. But I have been in need of another one due to me leaving my old one on my bench at a friends house. So about months ago I procured an old ATX supply, an enclosure, various parts to mount onto the face and back, and a new tool to add to my collection called a "nibbler". Unfortunately, after wiring everything up and flipping the switch, nothing happened. I started probing everything I could and snipping wires hoping to break a short that occurred someplace. I felt like I failed and shoved everything aside, telling myself I would deal with it later. It sat up there on the graveyard for 3 months. The graveyard is what I call the top shelf of my bench. It's where projects go to die. It sat up there taunting me until my friend and I needed to purchase a higher wattage supply for our server we were building. So, figuring that the problem was with the ATX supply, I took it back down from the graveyard and started fresh.

Hit the jump for more.

Above is the old bench supply I made a few years back. I built this one in a couple hours and even made a little circuit board to plug a new supply in if the other burns out. It has worked really well and continues to work today. Every shop should have an easy to find source for these common voltages.

Every ATX power supply has the same pinout with standard Molex connectors. A few options are available to use with this pinout and a couple things must be done in order to make it operate.

Brief description of the pinout:

+3.3VDC ----- Positive 3.3 Volts DC

+5VDC ----- Positive 5 Volts DC

+12VDC ----- Positive 12 Volts DC

COM ----- Common, 0 Volts DC

-12VDC ----- Negative 12 Volts DC (IIRC this is used for addressing with RAM. Is not capable of handling very much current. Some people have used this in conjunction with +12 to get +24 though I wouldn't recommend this. I haven't tested it's limits myself though. I should do that sometime.)

PWR_OK ----- This is a 5 volt signal that is output from the ATX supply after the supply passes all of it's internal self tests. When you first turn on a power supply, the voltage outputs will fluctuate up and down before finally stabilizing at a nice clean DC current. Basically this signal will turn on after the supply stabilizes. A PC uses this signal to prevent various hardware damages by not allowing current to flow from the rails until the motherboard sees this signal. This signal could be used to light an LED indicating that the rails have a stable voltage.

+5VSB ----- Standby Positive 5 Volts DC. This is a 5v source that is on as long as the supply is plugged into the wall. It will remain on even when the rest of the supply is switched off. I used this on my old supply to light an orange LED. I don't recommend having an LED always on shining at you even when the device isn't being used.

PS_ON ----- This is a PNP signal line used to turn the power supply on. All that is needed is to connect this wire straight to any COM wire and Boom! The power supply switches on and a voltage will be present at the rails. I usually do this with a switch on the front of the supply.

I used the "nibbler" tool (nothing like the old game) to "nibble away at circular holes I cut out in order to get nice 90 degree angles. I mounted that fan using a jig saw. That would have taken way to long for the little nibbler to do.

Here's the inside before I started wiring everything up. The ATX shown in the picture is actually the old one that didn't work. Which brings me to why it's important to keep fresh batteries in your multimeter. I know what  you're thinking and no, the old supply really was dead. There was nothing wrong with my meter at the time. Now when I was testing out the new supply to make sure it worked I was reading 5.4 volts on the 3.3 rail, 8.6 volts on the 5v rail, and 15.6 volts on the 12v rail. I was troubled, and thought that perhaps this ATX was bad as well. I remembered I had another ATX in my arcade powering lights and pulled it out and still, it had some odd readings. Then I remembered that sometimes these old supplys needed a minimum load of around 1 amp to operate. So I attached a 10 Ohm 10 Watt Resister on the 5v rail and again, strange high readings. I looked up at my variable voltage supply which has it's own built in meter and turned the pot to 3.3v and measured with my meter. The meter read 5.4 volts. Well wtf. I pretty much new by now that my meter was wrong and used my bench meter to test the outputs of the ATX. All the rails were fine. After finishing up installing the ATX supply and 24V supply into the enclosure, I replaced the 9v battery in my meter and it was back in business. I hope I remember this next time...

I have always been under the impression that all that was needed to combine multiple power supplys is to connect the 0v references with each other. Basically, connect the grounds. In the past, when running into certain issues at work I have suggested doing this with various things and my coworkers were always reluctant, saying they don't think you can do that. I even read somewhere before that in certain cases it's not this simple. So I have started to second guess myself about all of it. Well, it is that simple and that is the way it works. 0v isn't necessarily a real thing. 0v is a reference to all other voltages. just like earlier with people using +12 and -12 to get +24. The math is the same. Voltage is nothing more than a potential difference in electrons. My new supply has a dedicated 24V supply that is now sharing the same common(0V) as my ATX and everything is reading fine.

The red switch was originally a 120v switch with a 120v lamp inside. I took it apart and wired in an led and 330 resistor. The switch connects the PS_ON and COM and the LED is connected to the 5V rail as opposed to the PWR_OK. The display is a combination volt/amp meter and isnt currently attached to any rail. I'm not sure yet how I want to utilize it. The banana plugs on the far left are also currently not connected. I'm thinking about routing them to screw terminals on the back in order to have sort of a contextual power source. for instance, I have a large 40V supply I'm using for my pinball machine and it would be nice to store it off my bench and up on the top shelf. By routing it through my bench supply I can conveniently have +40v at my disposal until I finish with the project that's using it.

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