power supply - 'Voltage Fuse' or better

I have a power supply with multiple outputs (5v, 12, 24v) According to the manufacture (and after testing) there must be a minimum load on the PSU for it to supply the stated voltages. Without a minimum load the voltage can go up to 37V (which is very bad)

As I can't garentee that the system I'm building will use minimum W all the time, I've added 'dumb' resistors to do the job. They get really hot but I used a 20W resistor when I just need to 'consume' 2W.

After this, everything works fine.

But in order for me to sleep well at night I would like to guarantee that if one of these resistors would fail for any reason, the high voltage would not burn the electronics.

I have used a PTC for limiting current in the past but I would like to use a similar device to limit the voltage.

Is there such device? Can you recommend on a different solution? I can't change the PSU now as it was the only one available in a very short notice.

Answer

Power supply in this circuit is +12V. Zener voltage and current rating of the PTC will depend on the specific power supply and load, of course.

In case of overvoltage, the Zener diode starts to conduct and clamps down the supply voltage. At the same time, power supply current goes up. Then either power supply becomes regulated, or PTC fuse opens.

For normal operation, this circuit should have margins. The Zener voltage should be slightly higher than the nominal power supply voltage. The max voltage, which load can tolerate, should be somewhat higher than the Zener voltage.

(I really like this circuit, because it protects against: overcurrent, overvoltage, reverse polarity. And it does that with just 2 components.)

edit: related post about this type of overvoltage protection.

Unlike the clamp circuit above, the crowbar circuit pulls the voltage below the trigger level.

There are also overvoltage and overcurrent protection ICs. These are typically controller chips, which work with external FET switches. Some examples can be found here and here.

The ICs called hot swap controllers sometimes have overvoltage and overcurrent protection functions too.