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I found many pages where people convert an ATX power supply to a benchtop power supply to give 12, 5, 3.3V.

However the current output can be very high (several amps).

When using this and you NEED high current, that is great. However, when using this for typical benchtop hobbyist needs (e.g. supply 5V to a microcontroller) the current supply needs are small. I understand that it will draw as much current as it needs -- but my question is one of safety. Since the current "compliance" is essentially set to several amps, what happens if I accidentally touch + and -? Or what about if something in my circuit suddenly blows up and shorts + to -? Then I suddenly have several amps running thru my circuit. I believe then an internal fuse in the ATX unit will probably blow (right?) but at least for some short length of time there were several amps running thru my resistor right? or Wrong?Would anybody have a simple adjustable current limit circuit that I could build and add to my PSU?

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    \$\begingroup\$ ATX power supplies usually use foldback or hiccup current limiting rather than fuses. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 31 '15 at 3:20
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    \$\begingroup\$ I used a PC power supply for years. My experience with accidentally shorting it out on the breadboard was that there's a little spark, and then the power supply shuts down. To reset, just turn the mains off for a few seconds. Even a thin wire can take 50 A for a short time, and the protection circuits are very fast. The only dangerous thing might be a long thin wire, with enough resistance not to trip the supply. \$\endgroup\$ – tomnexus Dec 31 '15 at 4:30
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It's quite far from certain death (indeed, it's voltages "generally recognized as unlikely to be hazardous", but we just had a big long discussion of that starting from a more hazardous and less useful premise.) Summary - you're a lousy conductor, you won't carry several amps at those voltages. Doesn't mean you can't make a big messy spark, or start a fire, but it won't electrocute you if you leave the box closed, unless you walk off into very marginally possible or probable scenarios.

The simple answer if you want a small-amperage supply is some in-line fuse holders with fast-blow fuses of whatever size floats your boat. If a quarter-amp suits your needs, it does not matter if the supply can provide 50 amps. You won't pull much more than 1/2 amp for a very short period before the 1/4 amp fuse vaporizes.

The short-term probably more expensive, possibly long-term cheaper option is DC circuit breakers.

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  • \$\begingroup\$ Nice simple idea with the fuses! Surprisingly, of the first 7 such ATX-to-benchpowersupply tutorials on Google, only one of them tells you to add a fuse, and even then he says it's optional... :-/ \$\endgroup\$ – dnh37 Dec 31 '15 at 4:32
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    \$\begingroup\$ There is a LOT of poor advice and poor examples and outright wrong explanations of how things work on the internet. Just learn to filter it, and make wise choices, and if you miss a wise choice, learn from it if you survive it. And when a little voice says "this is not smart", stop and listen to it, rather than forging ahead. \$\endgroup\$ – Ecnerwal Dec 31 '15 at 4:35
  • \$\begingroup\$ Note that 'a very short period' can mean several seconds, if the current is only double the fuse rating - fuses are good for protecting against fires, but not so great for protecting sensitive circuits against overcurrent. Also a resettable 'polyfuse' is cheap, if you expect to blow your fuses regularly. \$\endgroup\$ – nekomatic Dec 31 '15 at 13:50
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A typical 600W ATX power supply is specified to supply up to 25A from its 3.3V and 5V rails (and 46A at 12V). This is an immense amount of current — if shorted, it will probably melt your wiring.

When short circuited, the behavior of the power supply will be unpredictable. A good power supply will shut off. However, this behavior is not guaranteed; less expensive power supplies may attempt to continue supplying voltage, or may fail dramatically. In short: do not short-circuit computer power supplies.

If you need a bench power supply for experimenting, I highly recommend that you purchase a "real" lab power supply — a 30V/5A single-rail model can be had for under $100. Such a power supply will allow you to monitor current, and explicitly set a current limit beyond which the supply will either limit the voltage or shut down. It's a worthwhile investment.

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  • \$\begingroup\$ Does that mean certain death if you happen to be in the path of the current, or is it just a flesh wound? And if so dangerous, why do so many people seem to take death in their hands and make them? (Just touch the terminals with your hands and you're dead!) Is there some general trick to permanently cut the current compliance (by modifying jumpers inside or something) to something more reasonable like 0.1A? \$\endgroup\$ – dnh37 Dec 31 '15 at 3:58
  • \$\begingroup\$ Thankfully, no. The resistance of the human body is sufficient (>1kΩ) that you can't really electrocute yourself on 12V. The risk is more that you'll heat wires/components to the point that they explode and/or cause burns. \$\endgroup\$ – duskwuff Dec 31 '15 at 4:18
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    \$\begingroup\$ @dnh37 Unless your skin is cut or pierced, you won't be shocked by 12V, no matter how much current the supply can provide. For example, car batteries are 12V and can put out hundreds of amps, but nobody is ever careful around them :) \$\endgroup\$ – bitsmack Dec 31 '15 at 4:52
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    \$\begingroup\$ Enthusiast-class supplies will current limit normally, as it's required in the ATX spec -- the only time you'd ever see weird behavior is on a Shenzhen-market-special "ATX" supply, I reckon \$\endgroup\$ – ThreePhaseEel Dec 31 '15 at 12:37
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For safety reasons, I'd suggest getting a used bench top power supply on eBay that you can use current control on. It's just the easiest solution.

If you're really set on using an ATX power supply you can build an external fuse box and then connect the input port of your project's network to the output port of the fuse box. If something shorts in your network then the fuse would blow before the supply can source enough current to cause serious problems. If you choose to do this, make sure you use wiring that won't melt when carrying current at or slightly above the fuse's rating. It's probably best to use higher rated (thicker gauge) wire coming from the power supply and going into the fuse box. This way you can use the same box setup for projects that require higher current..just make sure to change out the lower-rated fuses for ones with the correct rating.

I believe most Arduino's will use up to 500mA (USB limited.) Fuses are relatively expensive so if you're looking for a cheap solution in the grand scheme of things and plan on doing a lot of projects, it might be worth it just to cave in and get a bench top supply, depending on how many fuses you plan on blowing :).

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