wylbur wrote: if I understand correctly, you've put the diode between Vin and GND such that it blocks when the circuit is wired correctly, but allows current/shorts Vin-GND if the polarity is reversed.
That's correct.
Forward-biased diodes have an exponential voltage-to-current function: if you increase the voltage across a diode by about 60mV, it lets ten times as much current through. That relationship applies all the way down to 100mV or so, at which point the current flowing through the diode is down in the femtoamp range (1e-15). By convention, we usually assume the current through a silicon diode is about 1mA when the voltage across the diode is 645mV, and that below about 400mV, the current through the diode is so low that we can safely ignore it.
The current through a reverse-biased diode stays down in the femtoamp range.. the 1mA @ 645mV estimate is based on about eleven +60mV->x10 current jumps. Again, for most purposes, that rounds down to 'none'.
wylbur wrote:Should I put a fuse in series before the diode and ground?
That wouldn't hurt. Connect the fuse between your circuit and the power supply's positive terminal. That way when the fuse blows, it cuts off the connection to the power supply completely.
wylbur wrote:Otherwise if the polarity is reversed, the power supply could be damaged when the diode shorts, right?
Not if the power supply is well made.
Power supply designers know the risks of having a shorted connection too, so any reasonable design includes protection measures. The power supply will probably have its own internal fuse, but it will also have 'foldback current limiting', which makes the maximum output current proportional to the output voltage. It might also have a thermal foldback circuit that reduces output current if the main control transistor starts to get hot.
That doesn't mean you want to short a power supply's output for the fun of it, but does mean most supplies can survive a momentary short without being damaged. A good power supply with a datasheet will probably specify the maximum short-circuit current and the amount of time it can maintain that kind of short.
wylbur wrote:In the correctly-wired case, the diode is simply blocking, right?
Yes.
wylbur wrote:So it has no effect on Vin as long as Vin > GND?
Also yes.
wylbur wrote:If that's true, I'm not worried about the diode's Vf.
This isn't quite correct.. most ICs have their own reverse-biased diodes connected to all the IO pins for protection against static electricity.
The average doorknob spark has a voltage around 1.5kV, which is more than enough to blow holes through a mosfet's gate insulation. That was a big problem back in the 1980s when mosfet devices first hit the market, and people got obsessive about ESD protection. The spark only has a couple nanocoulombs of charge though (enough to charge a 1nF capacitor to 2V), and the spark only lasts a few nanoseconds. The actual current during that time is less than 1A.
IC designers add small reverse-biased diodes from the pin to VCC and GND to give the spark's current a safe path through the chip. That leads to a party trick where you can power most ICs through an IO pin while the official VCC pin is disconnected. The power to run the device comes in through the IO pin's upper protection diode.
The protection diodes start to conduct at about 300mV, so you have to take those Vfs into account when you think about external protection diodes. If you use a regular silicon diode whose Vf is about 825mV @ 1A, that's enough for a pair of 300mV diodes to turn on, plus about 110mV of extra bias voltage for each one.. enough for maybe 50mA to flow through them. That's less than the current from a spark, but it lasts longer, and gives the diodes time to heat up and suffer thermal damage.
wylbur wrote:so would I prefer some non-Schottky diode that has better blocking properties?
Nope.. Schottkys are the way to go. They have about the same forward voltage as the ESD protection diodes inside a chip, and still follow the same +60mV->x10 current rule as silicon diodes. The vast majority of externally-applied power will go through the Schottky before a chip's internal protection diodes reach a voltage high enough to conduct more than a couple milliamps.
The parameters you want to look for are low Vf and high Id (maximum diode current). Schottky diodes are used in power applications all the time, so you can find versions rated for a few hundred amps.
In practice, look for one whose continuous current rating is about 2x the value of the fuse you select. That way you can count on the fuse blowing to protect the diode, instead of the diode blowing to protect the fuse.