How do output on/off switches on power supplies usually work?

[Matir]

I feel like this is a stupid question.  Many power supplies have an output on/off switch -- allowing you to set the voltage and current limit before actually outputting anything.  How would these typically work?  I can imagine 3 scenarios:

1) The output linear regulator is brought down to 0V.  (Assuming a regulator that can go all the way to 0V)
2) A MOSFET on the output, but this seems like it would introduce a voltage drop.
3) A relay on the output, which will have the lowest voltage drop and work in all cases.

I'm thinking of building something similar to Dave's uSupply or something as a small 12V/1A bench supply.

[BrianHG]

The nice thing about a 2 pole relay is that you can completely disconnect from the supply's +&-.  No worry about surge in any direction until the mains switch is engaged and the output is stable.  With a mosfet switch, you can blow the mosfet if there is a large voltage on the output before you switch on your supply.  As for the regulator working down to 0v, you just need to make sure you don't drive your output transistor with any type of pulse during power-up.

[timb]

I feel like this is a stupid question.  Many power supplies have an output on/off switch -- allowing you to set the voltage and current limit before actually outputting anything.  How would these typically work?  I can imagine 3 scenarios:

1) The output linear regulator is brought down to 0V.  (Assuming a regulator that can go all the way to 0V)
2) A MOSFET on the output, but this seems like it would introduce a voltage drop.
3) A relay on the output, which will have the lowest voltage drop and work in all cases.

I'm thinking of building something similar to Dave's uSupply or something as a small 12V/1A bench supply.

Typically they use a relay. I've also seen some of the larger power supplies (10A+) use a soft start in combination with the relay (that is to say, when in "off" mode the current limit is internally set to 0; when you switch the supply on the relay is engaged and then the current limit ramps up to your set point over a short length of time (us to ms)).  They do this so the relay contacts don't have to switch a large voltage or current, which can cause arcing and wear them down.