What are “Protections” in a PSU?

(Last edited 5/14/2026)

Since we recently talked about supervisor ICs, we should discuss one of the most well known functions of these chip: Protections.  Those protections are typically SCP (Short-Circuit Protection), UVP (Under-Voltage Protection), OVP (Over-Voltage Protection), OCP (Over-Current Protection), OPP (Over-Power Protection), and OTP (Over-Temperature Protection). We’ll go over these one by one.

First, there’s SCP. A PSU with SCP immediately shuts down if any output rail is accidentally shorted to ground. In most designs this triggers a “latch- off,” meaning the supply won’t restart until it sees a full AC power cycle or a PS_ON reset. A few high-end models even latch off if two different voltage outputs short together. This can be useful  since pinched wires or stray solder blobs often bridge one rail to another rather than to ground.

Next is UVP.  UVP forces the PSU to turn off if an output falls below a preset threshold. Although the Intel ATX spec allows rails to dip to – 5% (for instance, the +12V down to +11.4V) and still be “in spec,” UVP thresholds are usually set around –10% to –15%. If UVP trips, it can indicate an overloaded or failing PSU or simply an unintended short between two outputs.

Then we have OVP.  As the name suggests, OVP prevents rails from rising above safe limits. Even though ATX permits +5% over nominal (e.g. +12 V up to +12.6 V), OVP circuits generally trip at +10% to +15%. Once a rail exceeds its “trip” voltage, the PSU latches off to protect downstream components.   

The next protection is OCP. This one is one of the more controversial protections and can be a little more difficult to explain.   OCP caps the current on each rail or rail group. In ATX terminology, “single-rail” means all +12V current comes from one circuit, while “multi-rail” splits the +12V output into separate circuits, each with its own current limit. If any circuit draws more than its assigned maximum, that rail shuts down. Therefore, if you have a +12V rail with a single OCP rating, you have a “single +12V rail”.  If you have a PSU with a +12V rail with multiple OCP ratings, you have “multipler +12V rails”.

Ideally, every DC output of the power supply (+12V, +5V, +3.3V, etc.) should have OCP, though some budget units skip it and rely instead on over-power protection instead.

Whereas OCP lives on the secondary side, OPP monitors total power on the primary side. Because of this, the protection is not provided by the supervisor IC, but the PWM controller.  If the PSU tries to deliver more than about 115%–130% of its rated wattage for a given time, it will shut down to prevent damage.

And finally, we have OTP.  When internal temperatures climb too high, OTP forces a shutdown. Although some reviews note that the absence of a dedicated OTP pin on the supervisor IC must mean an absence of OTP, most PSUs use the same thermistor-plus-fan-controller circuitry that adjusts fan speed to detect critical heat and signal a shutdown well before components overheat. The fan controller is programmed to spin the fan up to a maximum RPM.  If for any reason the controller feels the need to spin the fan faster than that maximum, a shut down signal is sent instead.

And I would like to add one more protection.  Fan-Failure Protection. Rare but sometimes implemented, this feature watches the cooling fan’s tachometer. If the fan stalls, fails, or is unplugged, the PSU will power off. It’s largely redundant because OTP should already guard against overheating. But it can offer an extra layer of defense in extreme cases.