Soft Switch ATX Power Supply - Page 1
Forum section: electronics / projects

author: BruteClaw
author_rank: Newbie
timestamp: February 24, 2020, 01:32:42 am
content: Hello All,I have a project for a friend that he wants to run a cluster of Raspberry Pi 4s from an ATX power supply in a computer case. He is working out the mounting by 3D printing parts. I have been tasked with power distribution.Design is fairly simple. Either a group of buck converters to step voltage down to the 5 VDC from the power supplies 12 VDC rail. Or good old LDOs. Just concerned about heat dissipation inside the computer case for the second option. Still trying to decide on heat vs cost.Using this because he wants 16 total of the Pis and at 4 amps each, that is 64 amps. Not many PC power supplies offer that on the 5 VDC rail. Now I know the 4 amps is max draw at full load, but might as well design for worst case.Anyways, I was trying to come up with a circuit to latch the power supply on using the computer case original button. I came across an older video:and thought that might be useful. But I know to turn on a PC power supply, you drive the PS_ON pin to ground. So I dug a bit further and found this:https://www.edn.com/latching-power-switch-uses-momentary-pushbutton/Specifically Figure 2 for the high side load.My question to the forum is then: Would there be an issue using the EDN high side load circuit to directly drive the PS_On wire to ground? Or should I just play it safe and use either Dave's circuit or Figure 1 to drive a relay to drive the wire low?Thanks for having a look.

author: Prehistoricman
author_rank: Regular Contributor
timestamp: February 24, 2020, 02:04:09 am
content: Both of those circuits are intended to switch a load. In your case however, you're switching a signal. Certainly those circuits would work, but they are inappropriate.You can use a standard NPN to short PS_ON and a latching switch circuit to drive that.Your latching circuit could either be the old-school way such as this:https://circuitdigest.com/electronic-circuits/soft-latch-switch-circuitor use a digital latch chip (or make one out of gates).Edit:Here's a bunch of circuits for inspiration:https://bipedu.wordpress.com/2013/08/12/power-soft-switch/

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 24, 2020, 02:41:05 am
content: Back when Perk mining was profitable, quite a few built mining rigs with dozens of cheap smartphones. The most common way to power them was with salvaged ATX power supplies, using multiple on larger racks. Keep in mind most coiners were not very electronics knowledgeable (just basic electrical knowledge of how to connect wires) hence why buck converters were rarely used. (Plus they would have to buy the buck converters, as such standalone modules are rarely found in scrap PCs.)As for the power button handling, take a look at Pi drivers for handling soft power control. Generally there would be a GPIO that outputs a high to keep the supply turned on and another GPIO accepting input (usually active low) requesting the Pi to shut down. For the cluster, the stay on signal can be commoned with diodes and the shutdown request connected via resistors to prevent overcurrent from incorrectly configured GPIOs. The power button connects to both the shutdown request and the power supply enable with diodes to prevent conflict (will need a pullup on the shutdown request side) and a transistor can act on the stay on signal to keep the (active low) power supply enable pulled low. If you want the ability to hold the button to force a shutdown, one way to do it without a microcontroller is to have a timer that delays a few seconds, then disables the keep on signal. Or just wire the reset button to short base to emitter on the transistor that holds the power supply on, thereby overriding the stay on signal.

author: BruteClaw
author_rank: Newbie
timestamp: February 26, 2020, 12:23:45 am
content: Prehistoricman and NiHaoMike,Thank you both for your replies.I think I might just breadboard up that old school method and test it.And if that fails, maybe the Pi that monitors the heat and controls the fans will pull double duty as a soft power solution.And while this cluster is not being used for mining crypto exactly. We are going to be playing around with Gluster FS and distributed computing/storage. Basically trying to see if we can use some off the shelf parts that someone could pick up at your local big box store and this custom board with some 3D printing to make a highly available and modular storage or compute cluster. Which if you wanted could be a miner. But he is going to test bed it by putting his companies web-page on it with docker containers to see if we can get the load balancing to run right.