Power Jack Protection Schemes for Effects Pedals

Reverse polarity protection can be realized with either shunt or series diode protection. You can use either or both.

Shunt diode wiring

Shunt Diode Protection

Connecting a diode "backwards" across your power jack (anode to negative, cathode to positive) means it will (hopefully) do nothing when the correct power is applied, and short circuit the power adapter when the wrong polarity adapter is used.

The pros are: easy to install (see picture) especially when modifying a pedal that never had a dc jack (i.e. you don't need to drill the circuit board), and no loss of power supply voltage.

The cons are: no ac protection, no over-voltage protection, short circuiting the power adapter is unpredictable and you don't know if the diode will fail or which way it will fail (open or short).

Series diode wiring

Series Diode Protection

An alternative method is to include the diode in series with the supply voltage. You can do this on either the positive or the negative terminal. The chief drawback is that you lose a "diode drop" of voltage (~0.6V) from the power supply. A neat trick to only apply the diode drop to the wall wart is to put the series connection on the negative terminal of the dc jack. The battery, which uses the "ring" terminal of the input jack for a negative connection, is unaffected when the diode is placed on the dc jack's negative terminal. Look up early '80s Boss schematics designed for "ACA" (12V) adapters and you will see an example of this.

Pros: ac protection, and safer polarity protection than shunt.

Cons: no over-voltage protection, and much more awkward to add to a dc jack than the shunt diode (as shown in illustration above). Series diodes are much more at home when mounted on the circuit board.

You will also note that the old Boss pedals will use both the series and the shunt diodes together, to get the best of both worlds.

MOSFET Diodes

R.G. Keen has some tips on using MOSFETs as low voltage drop series diodes here:

http://www.geofex.com/article_folders/mosswitch/mosswitch.htm

Over-Voltage

One problem neither diode scheme presented thus far handles is the issue of over-voltage, i.e. expecting +9V and getting +24V by mistake. Since most wall warts are going to output less than 50V, one method of protection is to make sure that all components used can withstand 50V being applied. This means calculating currents at 50V at making sure the resistors wattage ratings are high enough as well.

A voltage regulator, like a LM78L09, will accept up to +35V on the input, and output +9V. You have to accept a 2-5.5mA idle current draw. Not too bad, but may hurt your battery life spec (if that matters much...).

If all components have a 50V or higher rating, and a voltage regulator is used, it should be able to withstand most over-voltage situations I can imagine.

Protect your inputs and outputs too...

The real sneaky over-voltage killer is static electricity. That little spark you get from walking across carpet in the winter is the discharging of kV. That's 1000s of volts. No kidding. Of course, it carries very little current so we don't really get hurt from this (usually). If a device really can't stand even a instantaneous pulse of 1kV, then shunt diode protection is about the best thing you can offer it. Static electricity discharges can destroy components connected to the input and output jacks as well. Read over the datasheets of anything you use with inputs and outputs and add protection diodes there as needed. Many ICs will include protection diodes within the IC itself, and other ICs need you to take care of that little detail.

For instance, I put protection diodes in front of any TL0XX op amp inputs that have a path to a jack.