Polarity Protection

POLARITY PROTECTION FOR GUITAR EFFECTS PEDALS

Guitar pedals are full of components, some of which are so sensitive to voltages of the wrong polarity that they will explode - electrolytic capacitors in particular.   Exploding caps is not good.  

And to really start at the beginning, the wrong polarity means V+ and ground (or V-) have been swapped the wrong way around.  This usually happens due to people using random 9v adaptors, not realising that most effects run off centre-negative adaptors.  

How to protect circuits?

There are several ways to protect polarity.  I'll start with the most common two and describe some pros and cons for each.

Both methods below rely on a key characteristic of diodes - they only let current flow in one direction. On schematics, this is the direction that the triangle in which the diode symbol is pointing.   

Diode across the power supply (parallel)

The diode sits across V+ and grounds with the cathode (negative side) facing the positive voltage.   Some people are really quite vocally opposed to this method, but you will see it used by some major builders.

V+ can't travel through to ground, as the diode prevents this from happening - but if the polarity of the power supply is reversed, the diode acts as a short between V+ and ground, protecting the circuit.

• Allows maximum voltage to pass through to the circuit

• While it protects the circuit from damage, it essentially short circuits the pedals power supply to do this - it could damage your pedals external power supply, if the power supply has no short-circuit / reverse voltage protection

• Does not protect against overvoltage.  i.e.  plugging it into 24v when it can only take 9v

• 1N400X series diodes are commonly used.  A 1N4001 is rated to 50v, which is plenty for our purposes.

Diode in series with supply voltage

Being in series with the power supply means that current has to travel through the diode, and being a diode it only lets power through one way.  So it's a really simple way to solve the problem.  

• There will be a voltage drop matching the forward voltage of the diode used, which is why 1N5817 Schottky diodes are seen so often, as they have a lower voltage drop compared to a 1N4001 (about 0.6v with a normal diode, and 0.3v with Schottky)

• Does not have any impact on external power supplies

• Does not protect against overvoltage.  i.e.  plugging it into 24v when it can only take 9v

NOTE:  Vintage pedals were designed to run off batteries, so you rarely see polarity protection on schematics, as it was not required because it's impossible to connect a 9v battery backwards.