ATXPSUKIT ATX Bench-Top Power Supply Adapter Board Kit

 

1. ATX PSUs are designed with extremely tight clearances inside them. If you go putting binding posts in there, you may be closing in on safety clearances (and these are important - the mains input side of most switching supplies see voltage spikes up to 1000+ volts, and DC busses around +370 volts. It's not worth risking life and limb!)
2. You have a perfectly good computer power supply, and if you're anything like me you may want to use it to power op-amps one day, and a computer motherboard the next - why modify it beyond it's original use?
3. You still need a soft-switch circuit.
   
4. ATX power supplies have built-in protection, but there's also a Power Good output that tells the motherboard when the PSU is ready, or if there's a fault condition. It would be nice to have this displayed on an LED.

This ATX adapter board offers all these features, has standard 0.75" jack spacing for dual-banana plugs, and has color-coded binding posts for each of the voltage rails: 5V standby, 3.3V, 5V, +12V, -12V. This project comes as a complete kit, with the PCB, all components including binding posts, components, and ATX connector, plus comprehensive assembly instructions which includes the schematic diagram and PCB assembly diagram, as well as drill templates for panel mounting.

 

 

Let’s start with the basics: for any electronics or embedded system tinkering, you have to have a good power supply. Now I don’t know about your personal lab equipment budget, but mine is rather, well… let’s just say that I’m married to the head of the finance department, and she’s not exactly an electronics engineer. If you’re anything like me, you’ve probably crafted a few linear regulated power supplies over the years, but it’s no trivial task to design and build a variable high powered lab supply. And also, if you’re anything like me, the majority of what you tinker around with is digital circuits and low-voltage analogue stuff, like audio preamps and such.

In most cases, I am messing around with a microcontroller and a few opamps, so typically it’s handy for me to have a 5V rail for the micro (and any glue logic I might have), and +/-12V rails for the opamps. Increasingly, the sample devices I’m working with (for example, the Freescale DSP56367 DSP, or the NXP LPC2101 ARM-7 microcontroller) require low voltage rails for internal core and IO, namely 1.8V and 3.3V. This is a pain in the butt if you’re having to build a supply with all these rails for each project, but most of them (except 1.8V) can be obtained from off-the-shelf PC ATX switching power supplies. This is great, because somehow (I don’t exactly know how) over the years I have accumulated several of them. They all have 3.3V, 5V, 5V Standby, +/-12V supply outputs.

Granted; on many, cross regulation of the 12V rails is not fantastic. For the vast majority of opamp circuits it is more than adequate. I have seen others converting ATX power supplies to benchtop lab supplies and putting minimum load resistors in them – in my experience this is not necessary since the cooling fan is generally enough of a minimum load, and most modern switching power supplies are designed to run down to zero load anyway. I thought I’d give it a go – I need to volts and these are just lying around.

One thing I didn’t want to do was simply hack up ATX cases to put binding posts on them, for a few good reasons:

• You could really be messing with safety clearances inside the case. I don’t want to risk getting electrocuted because of the location of the 5V binding post output.
• I want the ATX supplies to remain ATX supplies – there is a chance I’ll want to actually plug them into a motherboard again at some point, so I don’t want to hack off the ATX connector.
• I wanted a more elegantly designed solution, that had not only the binding posts, but also some clip points I could clip alligator leads onto.

So I set about designing a PCB for adapting the ATX supply to benchtop use. As with anything, a good place to start are some specifications.The adapter shall:

1. not alter the power supply itself,
2. use an ATX connector,
3. have binding posts for each ATX power supply voltage and standby,
4. have a matching return (GND) binding post for each voltage output,
5. be capable of handling heavy supply currents,
6. have a switch circuit to make use of the ATX on/off controller,
7. have LEDs to monitor standby and Power Good signal, and
8. use through-hole PCB design so you, dear reader, can make one for yourself easily.

I had also considered a built-in panel meter so you could monitor the output voltage of each output, but quickly realized this would be unnecessary feature-creep for this, since the outputs are fixed and fairly well regulated, and most users (like me) can plug in their multi-meter and get a more accurate measurement anyway.

The switching circuit is a simple latch based on a two inverters from a 4049 HEX inverter, powered from the ATX standby 5V rail. The other gates in the inverter are used to drive the power on control signal to the PSU, and the LEDs. Pressing S2 turns on the power supply while S1 turns it off again. I used a yellow LED for D1 which indicates when Power Good goes high. This way, you get some warning of ATX power supply fault conditions. D2 is a bi-color LED, which indicates standby mode (green) or powered-on mode (red). I must say I think the boards came out rather well. The component and silkscreen side (front) and back of the boards are shown above and here: