Reverse Polarity Protection with Series Diodes

To protect an effects circuit against reverse polarity there are two common options: a diode is either wired in series or it’s wired in parallel. This post is about series diodes.

Series protection diodes

A diode is a semiconductor that (ideally) passes current in only one direction. Wiring it in series with the positive pole of the battery or DC jack ensures that no current flows when the polarity is reversed.

Unlike their parallel counterparts, series protection diodes don’t have to pass any fault current, just the regular current drawn by the effects circuit. One important characteristic then is the DC blocking voltage VR that states how much reverse voltage a diode is able to block. For most designs a small diode like the 1N4148 should be sufficient with a blocking rating of 75 V.

Forward voltage V_F

A diode doesn’t just conduct when current flows in the forward direction, it also needs a certain minimum voltage applied before it conducts — the so-called forward voltage VF dropped across the diode.

At the low currents guitar pedals usually draw, most silicon diodes like the 1N4148 or the 1N4001 drop about 0.7 V. As the current increases, so does the forward voltage.

Another drawback then is that the voltage drop also leads to power loss. If a pedal draws for example I = 10 mA then P = 10 mA × 0.7 V = 7 mW. This may or may not be acceptable when running off a battery where the risk of experiencing reverse polarity is low.

Reverse polarity

A series protection diode is an almost perfect protection against reverse polarity as its very high resistance will open the circuit and block reverse current. Any reverse leakage of modern diodes is unlikely to destroy even sensitive circuits.

Only with excessive reverse voltage VR > 75 V applied for too long will the diode break down and likely be destroyed by thermal effects.

Reverse overvoltage

If a circuit can handle a higher voltage input in the forward direction it should withstand it in reverse direction as well. The DC blocking voltage VR of the diode is also called the breakdown voltage. Breakdown can happen in two ways: either a diode shorts or it opens. (It might also short first and then open up.)

A short would obviously be the worst outcome as it would let all reverse current through to the effects circuit. However, even if the diode opens up there’s no guarantee it breaks down fast enough before the reverse current reaches op-amps or transistors.

Melted diodes often go short, at least for a while. Sometimes they eventually burn open sometimes they don’t — depending on the fault current.

AC tolerance

An effects circuit designed for DC is not likely to withstand negative peaks from an AC power supply for too long. At the very least it will destroy or degrade performance of transistors and op-amps relatively quickly. A series diode protects against that.

Current of any half-sine wave in reverse direction is blocked, it’s passed in forward direction only. As long as you’re not overvolting the pedal you’re good. Keep in mind that a 9 V AC adapter usually specifies the VRMS (root mean square or effective voltage), so the actual peak is at about 12.7 V.

Bonus: Half-wave rectification

With an AC source and current passed only through a forward-biased diode we get a half-wave rectifier. That essentially provides DC — although with an excessively large ripple.

If the effects circuit has a power supply filter network behind the diode, its capacitor(s) would reduce the amount of ripple voltage and smooth the resulting DC.

The pedal might periodically switch on and off or its characteristics pulse. That’s likely a good indicator to the guitarist that something’s off.

Lowering voltage drops

Smaller voltage drops can be achieved via Schottky diodes, trading off against a lower blocking rating and higher reverse current leakage. An 1N5817 is rated for VR = 20 V DC blocking voltage, which is good enough for most 9 V pedals. Its forward voltage is about 0.18 V at 10 mA.

In many (not all!) pedal designs series diode voltage drops probably won’t matter. A well-designed circuit doesn’t just stop performing correctly at 8.8 V or it would use only a small fraction of a battery’s capacity.

Not to forget, there’s quite a lot of variance between power supplies and what voltage they’ll provide — sometimes that’s 10 V, sometimes only 8.5 V. And that can again change depending on how much current is drawn and how linear the supply’s output is.

Alternatives

There are a few alternatives providing reverse polarity protection with no or almost no voltage drop and lower power consumption. Reverse-biased parallel protection diodes are probably the simplest approach.

A slightly more advanced idea uses a MOSFET as a switch that’s closed only with the right polarity supply voltage applied. Its characteristics are similar to a series protection diode, although incurring only a tiny, negligible voltage drop.

Yet another idea combines a PNP and an NPN transistor.