How to choose a diode to mitigate back EMF in industrial control circuits

Why this article is important

• It explains the phenomenon of back EMF, which can damage PLC outputs and relay contacts, crucial for ensuring reliable machine operation.
• It details practical solutions like using flyback diodes for dc circuits and MOVs for ac circuits.
• It emphasizes the importance of properly sizing protection components, such as diodes and MOVs, based on current and voltage ratings.
• It highlights the need to understand whether relays or solenoid valves have built-in back EMF protection, ensuring appropriate protection measures are implemented during system design.

A starship moves through the neutral zone on a routine survey mission. The captain and the crew are busy performing their normal duties when, suddenly, sensors detect an enemy vessel approaching with their shields up and weapons locked.

The captain quickly reacts with an order to fire an energy pulse at the ship. The lieutenant commander locks on and fires.

Lieutenant commander: “Captain, we have fired an energy pulse at the target. Their shields absorbed the pulse, and that has created a large electromagnetic field. This field has collapsed and has sent the energy pulse back at us.”

Captain: “Shields up! Prepare for impact!”

The captain is, of course, correct to raise shields. A returning energy pulse can be very damaging. This is a bit of fiction, but it is also a description of what happens whenever a solenoid on a relay or valve is turned on and off by a controller output. It generates a pheonomenon called a back electromotive force (EMF).

What is a back EMF?

A solenoid is a coil of wire that creates a magnetic field whenever current passes through it. The magnetic field then triggers relay contacts or a valve to change state. This field is maintained as long as current flows through the coil.

When the current is switched off, the magnetic field collapses. This collapse induces a reverse polarity voltage in the coil, which is the back EMF. This voltage travels back to the source, which in many cases is the output of a programmable logic controller (PLC) and can damage an unprotected output circuit.

How does this cause damage?

The voltage spike from a back EMF can cause damage in several ways. If the output circuit is a set of relay contacts, it can cause these to arc. Arcs create small pits in the surface of the contacts. The pits make the arcing worse and the effect amplifies over many cycles until the contacts no longer make a good connection.

A voltage spike to a transistor output can damage the circuit components by overloading them and causing them to fail.

Even if the components are not damaged, the spikes can act as transient noise forced into the circuit and can cause unpredictable behavior.