Reverse Polarity Protection

What is reverse Polarity Protection?

If a circuit is operated with a dc current it is mandatory to ensure reverse polarity protection through constructive measures. This prevents mix up of minus and plus pole to avoid damage. If the circuit is supplied with an alternating current external conductor as well as neutral conductor have to be connected right. Reverse polarity protection can be accomplished through electric and mechanical measures or combinations out of the two.

Mechanical Reverse Polarity Protection

A mechanical reverse polarity protection can be accomplished through a standardized plug in connection. The design of the plug case, coding of pins and geometrical design og the pins offers a number of possibilities to ensure reverse polarity protection of power and signal transmitting lines and prevent damages. An example is the RJ45 plug which can only be connected to the socket in one particular way and therefore offers reverse polarity protection through the design of its housing. An M12 connector is standardized and coded in accordance to multiple standards. It is typically coded in a way that only one way to position the plug and it prevents the possibility of connecting plugs and sockets with different codings.

Reverse Polarity Protection with Electric Circuits

Many components in automation technology such as switching power supplies, relays and engine suppressor modules are intended for connection to open cable ends. Examples are connectors with spring clamps, cut clamps and screw clamps. An integrated reverse polarity protection can not always be required. The connection technologies therefore require to be diligent and concentrated during installation. Disturbances of the process may lead to false connections and damages or malfunctions of machines and plants. Reverse polarity protection can be accomplished by electric measures.

Electric components for reverse polarity protection which can be used to set up a protective circuit are available. This includes the diode, the MOSFET and the bridge rectifier.

Serial Connection of a Diode for Reverse Polarity Protection

A simple circuit variant for reverse polarity protection is a serial connection of a diode with a voltage supply. If the polarity is correct the current is flowing. If it is a reverse polarity the diode blocks. A disadvantage of this type of circuit is the voltage drop due to the diode which lowers the operating current on further parts of the circuits which are also serially connected. It has to be taken into account that the diode has a maximum reverse voltage. If this is exceeded the diode conducts in either direction and the reverse polarity protection is non existent.

Parallel Connection of a Diode for Reverse Polarity Protection

If a diode is parallel wired to the voltage source an additional circuit vor reverse polarity protection can be set up. If the power supply voltage is correctly wired the diode blocks and does not let a current flow. If the polarity is incorrect the diode conducts the current and ensures the reverse polarity protection. The shortage of the voltage source caused by the diode has to be intercepted by a fuse. A conventional fuse has to be replaced after a reverse polarity to allow the circuit to be able to operate again. The advantage of this type of circuit in addition to its set up is that there is no voltage loss caused by the diode.

Reverse Polarity Protection with a Field Effect Transistor

A circuit using a field effect transistor variant called MOSFET (Metal-Oxid-Semi-Conductor-Field-Effect-Transistor) can be set up for reverse polarity protection to avoid disadvantages of a diode circuit. There are p-type (p-channel) and n-type (n-channel) MOSFETs. An advantage of this version for reverse polarity protection is the low amount of voltage loss. A disadvantage is the higher price and the use of multiple electric components for reverse polarity protection in the circuit.

Additional Circuit Variants for Reverse Polarity Protection

By use of a bridge rectifier in the supplier line a version of reverse polarity protection can be set up. A combination of a relay and a diode can also be used for reverse polarity protection. The inertia of the relay may cause pulses in the circuit. It may be required to apply further measures for reverse polarity protection.

Reverse Polarity Protection for 230 V Circuits

A mechanical measure for reverse polarity protection of 230 V is the use of a CEE plug if there is enough space. CEE plugs are standardized in accordance to IEC 60309 and depending on the voltage level are available in standardized colors. Location and diameter of the protective conductor contact in the plug create the mechanical reverse polarity protection 230 V.