A Smarter PSU Converter Leaves The Magic Smoke Inside

Over the years, computers have become faster, but at the same time, more power hungry as well. Way back around the 386 era, most PCs were using the AT standard for power supplies. Since then, the world moved on to the now ubiquitous ATX standard. Hobbyists working on older machines will typically use these readily available supplies with basic adapters to run old machines, but [Samuel] built a better one.

Most AT to ATX adapters are basic passive units, routing the various power lines where they need to go and tying the right pin high to switch the ATX supply on. However, using these with older machines can be fraught with danger. Modern supplies are designed to deliver huge currents, over 20 A in some cases, to run modern hardware. Conversely, a motherboard from the early 90s might only need 2 or 3A. In the case of a short circuit, caused by damage or a failed component, the modern supply will deliver huge current, often damaging the board, due to the overcurrent limit being set so high.

[Samuel]’s solution is to lean on modern electronics to build an ATX to AT adapter with programmable current protection. This allows the current limit to be set far lower in order to protect delicate boards. The board can be set up in both a “fast blow” and a “slow blow” mode to suit various working conditions, and [Samuel] reports that with alternative cabling, it can also be used to power up other old hardware such as Macintosh or Amiga boards. The board is even packed with extra useful features like circuitry to generate the sometimes-needed -5V rail. It’s all programmed through DIP switches and even has an OLED display for feedback.

It’s an adapter that could save some rare old hardware that’s simply irreplaceable, and for that reason alone, we think it’s a highly important build. We’ve talked about appropriate fusing and current limiting before, too – namely, with LED strips. 


24 thoughts on “A Smarter PSU Converter Leaves The Magic Smoke Inside

    1. Work on more telecom gear.
      They come in lots of ratings, are impossible to find locally, and are “fun” to replace in the back of whatever rack your gear is bolted into…

  1. They are Littelfuse OMNI-BLOCK with very fast acting fuses (10A for 5V, 5A for 12V). I added them because this kind of tool is useless if you can’t trust it 100%. So, if something fails in the electronic overcurrent protection (blown MCU or firmware bug), they act as a last line of defense against damages. The only drawback is their prices : $2.50 each.

    1. I never encountered a situation where I could fix a serious “short” problem and required only one fuse. So considering that for a serious problem you’ll need more fuses, this will only mean that you can buy these fuses in bulk. Meaning that the costs of a single fuse drops dramatically.

      1. Sure, but that fuse is only needed in case of serious failure from the ATX2AT Converter itself. It should never blows because the adapter will trigger overcurrent protection way before.Anyway, these fuses are not very expensive, the most expensive part is the fuse holder.

  2. ATX supplies tend to trip out if you draw too much power.
    Then requiring a power cycle to work again.
    An irritation if you need a lot of initialization current for say an indicative load or you’re doing electrolysis and trying to balance the conversation of electrolytes and solvent.
    The trip out is a bit too slow to be of use in protecting anything delicate from shorts and bad coding.
    Capacitor powered power cycling on output fail can be good. As can additional protection between the PSU and the device. Depending on the application.

  3. Call me crazy but if the old motherboard is drawing huge currents due to a short circuit then I’m unlikely to care about it being damaged.

    Actually on the upside the trail of black makes the short circuit easier to find. I remember fault finding a poweramplifier which kept tripping the control fuse this way. Couldn’t find the transient fault with my normal fault finding methods, so I bridged out the fuse and hooked up a huge powersupply to it and turned it on.

    2 hours later when the smoke detector went off I found what was left of the tantalum capacitor which was causing all of the problems.

  4. with today’s pcs with demanding graphics cards like gtx 1060 and up power supplies can easily push 40+ a.

    a 1000 watt power supply for extreme gaming or even crypto coin mining can push 80 amps on the 12 volt rail

    1600 watt power supplies witch is the max you can get for 110 lines are like 150 a on the 12v rail.

    most of the power on the 12 volt rail is in the pcie connector for the graphics cards

  5. Methinks we do not understand the purpose of fuses. Fuse are intended to mitigate fire and shock hazards in electrical equipment with little or no consideration to system reliability. They are not intended to prevent equipment failure. Littelfuse published a well-written tome called “Fuseology 101”. Look for versions that were published prior to 2010. It should be noted that the ratings and implementations of fuses (I^T curves per the nationally-scoped standards) and fuse holders differ significantly between North America and most other places.

    Digital detection of overload and digital control of overload is a silly game to play. Analog control of the current detect and current interrupt is the only reasonable and reliable solution.

    And for those that think they understand the design and limitations of ‘electronic’ fuses, please note the IEC60730-x stuff.

    1. Sure, fuses are not intended to prevent equipment failure, but they can limit damages (up to fire) if a failure happen. This tool was designed to prevent more serious damages if something goes wrong. Replacing a (almost) short tantalum on a motherboard is not a big deal as long as you don’t have burnt PCB traces, melted connectors or other disasters.

      Let’s take a real-world example. You just got a nice 486 motherboard and you inadvertently mounted the CPU reversed (rotated 180°). If you put the power on with a passive adapter, the 3.45V voltage regulator will try to deliver the maximum current supported due to the short on CPU. Let’s say 5A (not enough to trigger a modern ATX PSU OCP). It will only shut down on overtemperature after 5-10 sec. Enough to destroy the CPU and probably other components. With an electronic fuse on the line, current will be switched off within 350 ms (or much less in fast blow), quick enough to save both the CPU and the motherboard (tested!). Mounting a 486DX4 reversed is quite stupid, but I must admit I did that myself. A failed cap will have the same effect.

      About the digital detection/control of the overload, I don’t really understand your point. This implementation (shunt -> op-amp -> ADC -> MCU) is used on many new PSU (especially the latest Seasonic DC-DC PSUs) without issues.

      1. I think he means that a bug in the MCU code could render your OCP ineffective. Whereas an analog circuit with an opamp or two will keep working no matter what, barring a hardware failure. Which is a valid point to make when we are talking about safety equipment meant to prevent damage/fire. Also the reaction time of an analog circuit is likely going to be quite a bit faster than of an MCU. Which may or may not matter in the application.

    1. small step up dcdc to ~15V and 7912 will solve it for under $5 free shipping.
      -12 was afaik only used for ancient logic chips (rams, cpus) and maxed out at 200-300mA

        1. Ha, you are absolutely right, I was thinking -5V. I suspect so was the OP.
          -5V was used for ram and ISA slots(dual supply opamps in sound cards mainly?), removed when ISA died.
          -12 is still on PCI slot, its days are numbered, modern motherboards dont even come with PCIs anymore.

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