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Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
anyone here knows how to bypass the short circuit protection on ATX power supplies? regular ATX power supplies can be modified to make a bench power supply but they're not nearly as fun as non-protected power supplies :( would be pretty awesome if the chort circuit pretection can be bypassed. if its not practical to bypass, where can I find a schematic for an adjustable current limiting circuit.
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
Since all ATX PSUs are different (layout, design details) and then again they're all the same (the earlier generations seemed to follow the TL494 /KA7500 design example with little variations) you can start searching for housekeeping circuits, dedicated monitoring ICs and comparators such as the LM339 / LM393, then trick them into believing everything is fine. I had to remove a TO92 thyristor to keep my hacked AT PSU from tripping. It can be that simple.
have a look at this circuit to get familiar with the standard way these PSUs are built.
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
awesome, i just bypassed the short circuit protection. my ATX psu has a fan7680 IC for protection and I just short pin 3 to ground which is the Fault Protection Output pin.
i wonder what would happen if I short the 12V rail for more than 10 seconds. would the SMPS tranformer burst into flames or would the switching transistors just pop?
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374
I've seen SMPS controllers being powered by an extra winding on the filter choke so the voltage is quite low while in idle mode but rises up to 28..30V when the PSU puts out 150% of its rated current (I backed up the Schottky rectifiers with a matched second diode, the holes were already on the PCB). Maybe you'll have to check that all voltages are within operating conditions. The switching transistors do not bother the higher output current much. First smoke may be seen coming from the transformer, rectifier or filter caps
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
hboy007 wrote ...
I've seen SMPS controllers being powered by an extra winding on the filter choke so the voltage is quite low while in idle mode but rises up to 28..30V when the PSU puts out 150% of its rated current (I backed up the Schottky rectifiers with a matched second diode, the holes were already on the PCB). Maybe you'll have to check that all voltages are within operating conditions. The switching transistors do not bother the higher output current much. First smoke may be seen coming from the transformer, rectifier or filter caps
ok i just added a toggle switch to enable/disable the short circuit protection feature. now it looks almost sexy
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ATX power supply short circuit protection bypass
