Mains Linear Power Supply Protection

Question

This is a two-part question:

1. Why do the so many projects that I see that use basic linear transformer power supplies (i.e., mains->transformer->rectifier->regulator) not have any kind of input protection beyond a fuse? Is a fuse really going to help prevent damage to the circuit? Are they assuming that it's going to be plugged into a surge protector, so it doesn't need to worry about excessive input voltage?

2. If it is a good idea to include more protection in a simple power supply, is a simple crowbar-type arrangement like this appropriate?

My thought process was that I just want the fuse to open and the whole thing to become inactive if anything goes out of spec, so during an overvoltage, I basically just want to short-circuit and blow the fuse. The rule of thumb that I have heard is that your device should be able to tolerate during normal operation anywhere between 100-140 volts from a 115v nominal mains supply. This means a peak AC voltage of ~198, hence the 200v TVS diodes. The clamping voltage of common 200v TVS diodes seems to be in the 320-330v range, so a 300v gas discharge tube was chosen to take up the larger surges a little bit before the diodes reach their limit. Anything more than the GDT can handle would probably just arc across the component leads and trip the mains circuit breaker or blow the power supply's fuse instantaneously, right?

Is this enough? Should I also fuse the mains neutral and use a lower voltage TVS diode for D2 in order to open the circuit in the case of reversed hot and neutral? It seems like it might be over doing it to try to protect against grossly incorrect mains wiring like that. Also, now I'm starting to ask myself if it isn't even more dangerous to risk opening the neutral side of the circuit before the hot side fuse blows...

Answer 1

Is a fuse really going to help prevent damage to the circuit?

In general, a fuse isn't intended to protect the circuit. The fuse is there to disconnect your circuit from the mains in the event of an over current. That way, the circuit breaker at your facility (home or otherwise) doesn't shut off and lose power to other branch circuits in your facility. Also, your over current might not be enough to trip the facility circuit breaker, yet enough over current to start a fire.

Other devices such as TVS (rather to use MOV) (after the fuse) ARE desired to protect your circuit from transient (temporary) peak voltages. A transient over voltage can be clipped without blowing the fuse. Fuses take a long time (relatively speaking, look at fuse specifications) before they open.

Should I also fuse the mains neutral

Oh, no, never. Disconnecting the neutral is not a good idea. This leaves the HOT lead still connected, and there are many questions and answers on this EE SE site regarding disconnect of neutral. Like this answer

Answer 2

Generally the old fashoned transformer type of linear PSU is much more resilient to overvoltages than a SMPS .The use of a large electrotytic cap across the DC output terminals of the bridge rectifier limits rise in rail volts when mains spikes occur .The DCR and leakage inductance of the 60Hz transformer keep the peak diode currents to sensible values during mains surges.Hence most of the time you dont need surge protection on a linear PSU.In fact using MOVs could reduce overall reliability .When MOVs blow its really bad ,sometimes they go on fire .I got a coworker to post my exploding MOV demo on U tube.You should still use a fuse on the mains input as stated by Maria.