Here's a good explanation from https://www.madbeanpedals.com/forum

 

When a diode is applied as a shunt protection device it is ALWAYS appropriate to have a protective device such as a fuse in the supply line to avoid unlimited power delivered that will destroy the shunt diode and then all of the other polarity sensitive devices..
Thus the much safer scheme is to put the protection diode in series, so that if a reverse voltage is applied the diode will be reverse biased and not much current will flow, and damage would be prevented.

 

Now that I understand this issue better, I’ll be on the lookout for it. A 1N5817 in series does appear to be the strong majority of all the guitar pedal designs I’ve looked at.
Thanks!

 

For power supply circuits, 1N5817 in series will reduce the supply voltage by about 0.3V (depending on current) compared with 0.7V with 1N4001 diodes.

 

For power supply circuits, 1N5817 in series will reduce the supply voltage by about 0.3V (depending on current) compared with 0.7V with 1N4001 diodes.

Then it is a better choice for the series resistor. Thanks for the good advice.

 

There is a much better way to protect against reverse polarity - a MOS transistor.
In normal mode, the transistor is open and there is a low voltage across it. If the power is connected incorrectly, the transistor is closed.

 

Then it is a better choice for the series resistor. Thanks for the good advice.

Diodes are preferred in certain situations because the voltage loss does not vary as much with current (about 0.2-0.3V for 1N5817). With series resistors, the voltage loss follows Ohm’s Law.

 

When a SHUNT diode is used for reverse polarity protection the current limiting device should always be a fuse. When a SERIES diode is used for reverse polarity protection then a diode with a lower forward voltage drop is always the better choice..

 

I have used four rectifiers in bridge configuration so that it works whichever way the battery is connected.

 

I have used four rectifiers in bridge configuration so that it works whichever way the battery is connected.

Certainly that is a useful scheme, but without special measures that puts two diode drops in series with whatever is protected. Thus it does have a cost. But certainly it will be effective.

 

Certainly that is a useful scheme, but without special measures that puts two diode drops in series with whatever is protected. Thus it does have a cost. But certainly it will be effective.

It can have added benefits.
Two 1N5817 diode drops is only 0.5-0.6V.
I have used 1N4001 bridge to reduce 6V battery voltage to 5V.

 

Certainly that is a useful scheme, but without special measures that puts two diode drops in series with whatever is protected.

If voltage drop is critical, such as for battery operation, you can use a MOSFET bridge, at the higher cost of the MOSFETs (below).