A reverse voltage can easily damage electronic circuits. Let's look at simple reverse polarity protection circuits using diodes.
Reverse Polarity Protection
The right polarity matters. Almost all semiconductor components and electrolytic capacitors are sensitive in regard to reverse voltages. Applying a reverse voltage, even by accident and for a short time, can easily destroy devices. In this article, we will look at simple reverse polarity protection circuits using just a diode.
There are two different ways how to place the diode in such a protection circuit:
- Putting a diode in series to the load
- Using a diode as a shunt in parallel to the load
Let's have a deeper look at these two options and their advantages and disadvantages.
Diode in Series
The most common reverse polarity protection circuit is to connect a diode in series to the circuit. If reverse biased, the diode blocks the current flow and by that protects the attached circuit.
An example for this is the protection circuit for the Arduino Uno's barrel jack connector. As shown in the picture below there is a diode right next to the connector. This diode provides protection, in case a reverse voltage is supplied via the barrel jack.
Link: Arduino Schematic
Diode as Shunt
Alternatively, a diode can be used as shunt. A shunt is an alternative low-ohmic current path. On this path, the current can pass around the protected circuit in case of reverse polarity.
For diodes and LEDs a typical example for this is a so-called antiparallel diode. As you can see in the circuit diagram below, the diode is added in the reverse direction of the LED.
Under normal conditions the protection diode is reverse biased and the current flows through the LED. In case of a reverse voltage the diode becomes conductive and the current flows through it instead. The LED is protected.
From Theory to Practice
In theory, the in-series diode is the optimal reverse voltage protection. It seems like a simple, but effective measure to block any reverse current flow. However, it is the practice that matters. In practice, there is no ideal solution. Why, you ask? Because in practice diodes are far from perfect:
- Diodes are no perfect conductors in forward direction
The forward voltage drop is the measure that quantifies the diode's deficiency in regard to being a perfect conductor in forward direction. For standard silicon diodes it is around 0.7 V and for schottky diodes around 0.3 V. - Diodes are no perfect isolators in reverse direction
The reverse leakage current characterizes how well the diode can suppress any reverse current flow. A standard diode has a leakage current of a few microamperes. Schottky diodes have significantly more leakage current in the area of several milliamperes.
Consequences for an in-series diode
Whenever you put a diode in series to a circuit, the supply voltage will be lowered by the diodes forward voltage drop. The lost energy will be dissipated as heat produced within the diode. In consequence an in-series is diode is a bad choice for both low voltage circuits powered by a single battery or coin cell and high current circuits.
The reverse leakage current over the diode is negligible for most circuits. However, there is one big exception: LEDs and similar semiconductor devices with a low breakdown voltage. Even though the leakage current through the protection diode is small, this is not automatically true for the voltage. Since LEDs typically have a smaller leakage current than standard diodes, the majority of the applied voltage drops over the LED and not over the protection diode. The LED's breakdown voltage could be exceeded causing it to degrade over time and eventually fail. To safeguard against this, it is possible to add resistor in parallel to the LED. However, for LEDs, using an antiparallel diode instead of an in-series one usually makes more sense.
Consequences for a shunt diode
For a diode used as shunt these issues do not apply. As opposed to the in-series diode, the voltage drop will have no negative effect on the circuit during normal operation. This makes it possible to use this protection circuit for both low voltage and high current circuits. Only when the reverse voltage is applied the forward voltage drop becomes relevant. The diode will limit the reverse voltage for the protected circuit to this value. The remaining 0.3 - 0.7 V of reverse voltage are tolerable for most circuits.
Is a shunt diode the better solution? In regard to the deficiencies of diodes, yes, however, the circuit has a major drawback. A shunt does not stop the current flow. In case a reverse voltage is connected, the diode will short out the voltage source which likely causes damage. At this point you'll either need a current limiting resistor like in the LED circuit above, or you will need some sort of short circuit protection like a fuse:
To avoid the need to replace the fuse after applying a reverse voltage, a self-resettable PTC fuse can be valid alternative. For the circuit to work, the diode needs to be properly dimensioned to withstand the current flow until the fuse trips. As a PTC fuse reacts slower, using one will increase the toll on the diode. Luckily, if a diodes fails, it produces a short. This means the main circuit will be protected even if the diode fails. However, the diode needs to be replaced, if this should happen.
Conclusion
In conclusion an in-series diode remains the simplest solution for circuits, that do not require lots of power and do not operate on a tight voltage margin. For LED circuits using an antiparallel diode is the preferred option. For other circuits there is no simple answer. If you are willing to sacrifice the diode, willing to invest in a fuse or have some sort of short circuit protection anyway, then using a shunt diode may be an option.
If both options don't work, it makes sense to look into MOSFET reverse voltage protection circuits instead. An in-series MOSFET is able to block the current flow in reverse direction and allows for much higher currents and less power loss during normal operation. With that it gets much closer to an actual ideal diode. As such there are even read-made ICs marketed as ideal diodes that will relieve you of the job to design a circuit for correctly driving the protection MOSFET.
The best and simplest option, however, will always remain using a connector that does not allow for plugging it in the wrong-way around in the first place.
