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Help needed with fuse ratings for atx psu mod to lab psu

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icom128

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Hi, please could some kind soul help me with correct fuse ratings to protect the atx psu
when connected to electical items. Picture is attached.

I:E if any attached items, go bad , I need the fuse to blow instead of the atx psu continuing to
give loads of current.


Thank you.
 

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Before anything else you need to take a look at the 24 volts feeding your LM317, While the +12 volt rail likely has plenty of current note the current available from your -12 volt rail. I would venture there won't be much. Generally less than an amp. Also the -12 becomes the common for that independent supply and can't at any time share the supply common. Your PCB GND and 0V. Fusing would depend on what the -12 volt rail is capable of for the supply, I would venture a guess that the max would be maybe .5 amp.

Yes, in the event of overload, if the PSU is built to ATX design specifications (and new enough) it should shut down with any excessive load.

I would fuse below the rated outputs. The problem with how many ATX PSUs are rated is the currents listed on the case for various voltages don't always consider cross loading. I would fuse maybe 5 amps below maximum ratings. Consider the wires from PSU to jacks are likely AWG 18 so make sure the parallel lines can handle what you fuse for also.

Just My Take
Ron
 
Thank Ron.
The case indicates the -12v is 1A.
So obviously the variable voltage post connections would only have items below this rating
connected for the testing of low power circuits ect. So do I use a 1a fuse on the +
variable posts leading back to the lm317 board ?.

And yes noted on the fixed voltage posts , I will fuse them 5a below thier rated output
current.The casing indicates +5v is 30A , and +12v is 15A.

The ATX is new , never been used as it was emoved from a pc case 2 years ago
when I put a higher wattage psu in. So on the day of purchasing the case the psu
went on a shelf till now.

Is there a safe way of testing the ATX protection , by maybe deliberately shorting
+ & 0v posts to see if the psu shuts down ?.

Thanks.
 
Yeah, you can fuse the variable at 1 amp. Personally, I would like to see 750 mA but let's face it, a 750 mA fuse isn't quite common and I doubt happens in automotive. :)

Is there a safe way of testing the ATX protection , by maybe deliberately shorting
+ & 0v posts to see if the psu shuts down ?.

I always hate that part! The best way to go about it is to slowly bring up a monitored load on for example the 12 volt rail. However, not everyone has a variable 12 volt 30 or more amp variable load lying around.

Next method would likely be to with PSU off, short the output. When you try to power it up it should not come online.

Last but not least is using a healthy switch and placing a dead short across the output with the PSU on. That can get ugly. I don't advocate it.

Ron
 
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