We've all been there: grab a power cable, plug in a pedal, and *poof* the magic tone smoke escapes from the pedal. The smoke is usually hard to put it back in, though the pedal can often be repaired.
But why did it happen in the first place?
In all likelihood: a reverse-polarity power supply. Almost all pedals use a center-negative power supply (for more about power supplies) and if a center-positive supply is used, the flow of electricity through the circuit components is reversed, and well, there goes that magic smoke. In the best case, it was a shunt diode that can be replaced. In the worst case, something crucial in the pedal circuit was damaged and may not be repairable.
As a builder, how can I then keep the magic smoke where it belongs? By using a system for reverse-polarity protection. I'll discuss three options and their drawbacks below.
Shunt Diode
The simplest, least effective, and arguably most dangerous system of power protection is a shunt diode. The diode (D1) is placed with the cathode to the incoming power supply, and the anode to ground
Under normal operation (center-negative power supply, ~9V), the diode does not conduct electricity and the pedal circuit is powered. If the polarity is reversed, say via a center-positive power supply the diode will (hopefully) short circuit the power adapter and (hopefully) prevent the reversed-power from reaching the rest of your circuit.
There are strong Buyer Beware™ connotations here:
the diode could fail in an unpredictable way. e.g. it may burn out and create an open circuit, thus doing nothing to prevent the reverse-polarity from affecting your circuit.
short-circuiting a power supply is unpredictable as well. more modern, isolated power supplies should be able to handle this (hopefully) without issue, but older, cheaper or non-isolated power supplies could react negatively.
While you may see this circuit on a lot of DIY boards and internet-schematics, it is not recommended in any capacity here at Swamp Witch for the aforementioned reasons.
Series Diode
A more robust option is the series diode. The diode (D1) is placed with cathode to the incoming power supply, and the anode to the positive supply of your circuit. In this way, reverse-polarity is prevented by normal operation of the diode. This also provides AC input protection.
The drawback to the series diode method is that the circuit's supply will suffer one forward-voltage drop (e.g. the commonly used 1N4001 has a forward-voltage of 1.1V!). This can be ameliorated by using a diode with a smaller voltage drop, such as the 1N5819 (approx. 0.7V at pedal supply currents).
MOSFET/Transistor
The most robust option is a reverse-protection MOSFET. In this instance, a P-Channel MOSFET (Q1) is connected with Drain to incoming supply, Source to the circuit's supply, and the Gate to ground. The MOSFET is held ON when the Gate is low - passing signal with limited voltage-drop, and turned OFF when the Gate goes high (that is, when the "Ground" has a higher voltage than the "9V" as in the case of reversed-polarity). R1 and D1 are used here to protect the gate from power surges. (For more on this, see R.G. Keen's excellent writeup).
Compared to the shunt diode, we get:
Repeatability -- once the shunt fails, it fails
Predictability -- the MOSFET always turns OFF, and when its OFF, the rest of the circuit is safe
Compared to the series diode, we get:
Better performance -- if the MOSFET is selected correctly, the voltage-drop is negligible
MOSFET protection is the main option in Swamp Witch power supply circuits (as well as some of your other favorite boutique builders, like Electronic Audio Experiments -- whose knowledge and information was valuable in understanding how this all works and why it's important).