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When the battery needs change, it can quickly happen that the new one is inadvertently connected reversed. You will quickly notice, because the contacts won´t match,  but for the electronics it might be too late. To prevent this, some builders have inserted a series diode or a reverse diode to ground. Both have advantages and drawbacks.

R.G. Keens goes beyond that and uses MOSFETs to the rescue. Read more in his article Advanced Power Switching and Polarity Protection for Effects and in an Infineon application note.

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Series Diode

A series diode will produce a permanent voltage drop of some 100 mV dependent on the diode type. In case the circuit is somehow short circuited, the battery´s internal resistance will limit the current to a value below the 1A of a typical 1N400x commonly used for this purpose. Since most designs also have a series resistance in front of an electrolytic cap to filter out any alleged noise from a badly regulated wall wart supply, this also aids to limit the short circuit current.
In case of reverse powering, the current through the load circuit is down to the reverse leakage current of the diode, which will be  microamps.

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Parallel Reverse Diode

A parallel diode (conducts only in case of reverse powering) is better, because it does not continuously produce a voltage drop, while in case of a reverse powering it limits the voltage across the load circuit to -0.6 V. In case of a circuit failure the same applies as above.

The load circuit will survive with both schemes, the battery will survive as well. In case a wall wart is used for supply, it must not deliver more current than the diode can take, but the majority of wall warts sold for stompbox usage are well below 1A.

At first glance a MOSFET circuit as mentioned above will cure all polarity issues and even protect the battery from high current drafts during reverse power conditions.

I ordered some BS250KL which happen to be from Vishay/Siliconix. According to the datasheet those have an Rds,on of 14 Ohms. I took a 20 mA LED as load (which would be awfully much for an average stomp box, even a CMOS gate based effect...) and measured 50mV drop. This viewed isolated looks good.

I also noticed that the maximum current rating is 180 mA (other manufacturers specify 250mA for the BS250 and 800mA for the BS170). In case of a short-circuit on the load circuit (which easily happens during tinkering...) the MOSFET will be history.
Bump.

Since it can easily happen during battery change that the user momentarily produces a short-circuit somewhere, in which case the protection function of the MOSFET mutates into a permanent inhibition function before you think and your pedal refuses further service, while in case of a diode protection the battery would momentarily have encountered an increased current demand and revert to normal as soon as the fault condition is removed. The MOSFET circuit as depicted is therefore not fail-safe.

What is needed, is a fault protection for the MOSFET. A full blown overcurrent protection with shunt resistor and all that would explode in component count and complexity, but a current limiting resistor may do.

At 9V a 56 Ohm 2W series resistor would allow a continuous maximum current of about 160 mA into the load. This would make the protection circuit fail-safe.

For a more typical current demand of say, 5 mA, the voltage drop across this 56R resistor (letting alone the drop the MOSFET produces at this current) would be 0.28 V. With a drop like this the whole idea of the protection circuit vanishes in the lights of increased complexity.

Although the MOSFET circuit has, compared to the simple diode protection schemes, the added advantage of also protecting the battery from current shocks, a series or even parallel reverse Shottky diode might be a better choice and far more efficient after all.

If somebody is determined to use this kind of protection method despite its pitfalls I recommend to bypass the protection during the tinkering phase at least. The final de-bugged board is less prone to short-circuits.