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.


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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.

crowbar circuits

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

overvoltage and overcurrent protection ICs

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.

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  • \$\begingroup\$ Is the loading resistor necessary for the indicated purpose? Unless the power supply is just plain broken, the power dissipated by a simple zener should never exceed the minimum load power requirement, and the supply should not be able to supply any significant current above the zener voltage. \$\endgroup\$ – supercat Oct 13 '13 at 18:49
  • \$\begingroup\$ @supercat The load resistor was mentioned in the O.P. The concern was that it could fail open and the power supply would go out of regulation. That's why I've added the load resistor to this schematic snippet. The load resistor is not required for the Fuse+Zener scheme, obviously. Come to think of it, a better place for the load resistor could be downstream of Fuse+Zener. Then the load resistor itself would be protected, if the power supply fails and generates output voltage that's too high. \$\endgroup\$ – Nick Alexeev Oct 13 '13 at 19:26
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    \$\begingroup\$ Effort that could be spent protecting against the possibility of the passive load device failing open could be better spent simply ensuring that doesn't happen. Unless the desired regulation voltage is extremely close to the maximum acceptable voltage, a zener won't have to conduct any substantial sustained current and thus shouldn't fail unless the supply malfunctions in a fashion much worse than the no-load-overvoltage scenario. \$\endgroup\$ – supercat Oct 13 '13 at 19:55
  • \$\begingroup\$ I tried this on the bench just now with a 1A fuse (ATO automotive type, if anyone's curious) and a SMCJ43 TVS diode (43V standoff voltage). I hit the 61V limit on my supply and got just 100mA of DC current. Needless to say, the fuse is still healthy. There are a few lessons here: 1) This circuit won't work if the overvoltage is of the slowly rising form. 2) Whatever you're protecting should have plenty of margin for overvoltage on it. At least x2. \$\endgroup\$ – PKL Nov 27 '17 at 22:09

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