Computing stuff tied to the physical world

TK – Cheap power supply

In Hardware on Jun 7, 2012 at 00:01

Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.

(I won’t call this a “lab power supply, for reasons explained below)

In a weblog post a while ago, I took apart a standard computer power supply unit (PSU). Now, instead, let’s do the opposite and turn it into a useful tool for experimentation with electrical circuits:

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What you see here is a neat little way to repurpose any standard ATX power supply. Just snip off most of the wires, except for that 20-pin connector, and assemble this neat little ATX adapter board by Benjamin Jordan:

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It’s available as a simple kit with a few basic components and all the connectors and binding posts.

The reason this is convenient is that it makes it somewhat easier to work with an ATX power supply (especially if it gets mounted on or near that power supply). There are push-buttons to toggle the supply on and off (except for the 5V standby voltage on the rightmost blue post, which is always on). There’s a LED to indicate whether the power supply is on (red) or in standby mode (green), and there’s an orange LED to indicate that power is OK.

All the main voltages are nicely arranged on binding posts, with matching ground return posts (all tied together internally), and there are holes to get to those same voltages via alligator clips – this is clearly for experimentation!

The is no high voltage anywhere, so the stuff is completely safe in terms of voltage. But there is still a risk:

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The currents available in most PC power supplies are phenomenal: 25 and 35 Amps on the 3.3V and 5V voltage rails, respectively. That’s what modern CPU’s and memory chips and all the supporting logic need, nowadays. The +12V supply is also pretty powerful, and normally used for all those terabyte disk drives people seem to be using.

This means that no matter how we touch it, it wont hurt us – anything under 40V is considered safe since our skin resistance prevents any serious amount of current flowing. But an electrical short circuit can (and will!) still easily vaporize thin copper traces on a low-power PCB. In other words: this is totally safe in terms of voltage, but the currents caused by shorts can generate sparks and enough heat to destroy components, wires, and PCB’s.

The above PCB itself is ok – its wide and thick gold-plated copper traces were designed to carry heavy currents.

What this means is that this setup is indeed a very cheap way to get lots of useful voltages for experimentation, but that it’s not the same thing as a “laboratory power supply” which also needs to have adjustable current limits.

Here are the voltages I measured coming out of this thing:

  • +5V standby, actual value, unloaded: 5.16 V
  • +3.3V, actual value, unloaded: 3.39 V
  • +5V, actual value, unloaded: 5.19 V
  • +12V, actual value, unloaded: 12.01 V
  • -12V, actual value, unloaded: -11.35 V

Close enough, and more importantly: most are slightly high. That means we could add very precise low-dropout regulators to get the voltages exactly right or we could add a current sensing circuit and limiter, to get that extra feature needed to turn this into a cheap yet beefy lab power supply.

  1. Beefy, yes, but have you looked at the quality of the power coming out of these things? Forget about using them for anything analog, audio, or data acquisition related. Unless, of course, you add said regulators to clean them up. My company’s latest data acquisition device (128 channels @ 40 MHz) requires 1.8V @ 11A: prohibitive to supply with a conventional linear supply (10 kg of transformer and heatsink!). A switcher by itself is way too noisy, but with a 3.3V ATX supply turned down to 2.8V, and line filters and linear regulators to drop the last volt, the whole supply is <1kg, and the wallplug-to-board efficiency is still almost 50%.

  2. Neat idea for getting some raw amps to the bench and nice touches on the PCB (croc clip docking and those radial vias for the mounting post pads). Some caution on the design though – many switchers require a minimum load on at least one output rail (automatically in place when connected to a motherboard).

    Depending on vendor, the unloaded case can lead to significant over voltage spikes as the feedback path goes out of control. A substantial size 10Ω across the 5v rail is a wise precaution until the unloaded behaviour can be ‘scoped out.

  3. Somehow, the sunrise simulation idea JCW posted some time ago pops to my mind when reading this.

    You could use an ATX power supply to power 2 40W 12V car lights and dim those with PWM throught a MOSFET plug. There is a stable standby voltage powerfull enough to power a Jeenode and the jeenode can be used to control the state of the PSU. So when it’s wakeup time, the jeenode powers on the PSU and starts PWM dimming the 2 car lights, gradually rising their voltage. When you’re awake, the Jeenode can also power the PSU off again, sending it into a powersaving state. (I’m mentioning car lights, because their light is more simmilar to natural sunlight, but hey, led strips will work too!)

    Of course, we would need a modified PCB for this. But it seems to solve two problems at once because we can automatically poweroff to low power mode when not using a device, still keep minimal logic running and power up when we need it!

  4. @vliegendehuiskat: (off topic: you have gotten famous lately Orville) ! Why would you waste 70W on incandescent bulbs and a power supply when this LED with this or that powersupply could achieve the same? Even though it is for a brief illumination time, these days there is no time to waste.

    • It was just an idea in the direction of simulating sunrise. (And yes Orville is way younger than me!)

      I’m not to scared of the amount of volts, amps and power available. A simple car battery can supply 12V at 60 Amps for about 15 minutes with ease and it has no protection either. So if you’re scared of this, you should even be more scared of a typical car. Yet I’ll be the first to hook the jumper wires when needed.

      Besides, people wearing rings or other metal on their bodies when fuzzing with any kind of electronics are eligible for a Darwin award.

  5. Also look at this project: He added an adjustable output with voltage and current display. The only thing that is missing…. An adjustable current limiter:

  6. I’m not so convinced this is all that safe for humans. If you, for example, accidentally got a ring welded across that 5 V, 35 A supply you could do your finger a lot of harm very quickly. Would you like to have a 175 W soldering iron wrapped round your finger?

    The chances are that in practice you’d draw too much current and the supply would shut down but a) you probably don’t want to bet on that and b) quite a few joules might get transferred before that happens.

    I think people obsess on voltage too much and don’t worry enough about current and particularly energy and power available.

    • Some high currrent equipment I worked with in the ’80s (with multiple 5v 100A power supplies) had prominent “Remove rings and watches before servicing” labels everywhere.

      If you are dealing with any PSU with a hefty current capability you really should never have anything metal below the elbow.

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