I am planning to have relays controlling power to a linear actuator. It runs on 12V needing max 5A current.
What would be the best way to protect the contacts from the back emf?

 

You can consider a flyback diode or a snubber circuit.

 

As used in automotive points protection ~200nf to 470nf. as they still call a "Condenser" (for some reason?)

 

As used in automotive points protection ~200nf to 470nf. as they still call a "Condenser" (for some reason?)

Interesting question about Condenser.

Volta uses the concept of capacity. Moreover, he defines tension (T) as the tendency of electricity to escape from an electrified body. He also introduces a relation between these two quantities and the amount of charge (Q). What’s more he finds out experimentally that capacity is inversely proportional to the distance between the two armatures.

Alessandro Giuseppe Antonio Anastasio Volta first used it and it's still used as a root word for auto electric parts in languages like Japanese. The word 'battery' translates to 'Lead Accumulator'.
https://en.wikipedia.org/wiki/Denso

 

Adding a 3A diode across the contacts, cathode to positive, will absorb any inductive spike.

 

Adding a 3A diode across the contacts, cathode to positive, will absorb any inductive spike.

How did you come to the 3A current rating?

 

Another method I have used allow the use of smaller relays is to use arc blow-out relays, they have a magnet adjacent to the contacts.

"One of the earliest ideas was to use a magnet to extend the arc distance (or “blow out” the arc). The magnet could be a fixed magnet or an electro-magnet and is still in use today. This technique enables a smaller DC relay or contactor than otherwise would be possible."

 

If the motor must have it's polarity reversed to run the opposite way, then the shunt diode willlead to smoke.

 

How did you come to the 3A current rating?

The diode only has to carry to 5A for a short period until the motor stops, so the diode surge rating should easily handle that.
Even a 1A diode would likely have an adequate surge rating but I suggested a 3A for plenty of margin.

 

If the motor must have it's polarity reversed to run the opposite way, then the shunt diode willlead to smoke.

In that case you use an H-bridge configuration of diodes going from the supply voltage to ground with the motor connected to the center of the bridge.

 

The diode only has to carry to 5A for a short period until the motor stops, so the diode surge rating should easily handle that.
Even a 1A diode would likely have an adequate surge rating but I suggested a 3A for plenty of margin.

Yes, I see.

In that case you use an H-bridge configuration of diodes going from the supply voltage to ground with the motor connected to the center of the bridge.

That is how it will be arranged, so four diodes needed.

Thanks @crutschow and all.

 

How about a capacitor in series with a 27 ohm resistor across the contacts? The resistor will limit the current while the capacitor will serve as a load to drop the theoretical voltage spike.
OR you can select a control relay with contacts rated adequately so that "the spike" will not cause any damage. That is how it is done for motors used in production machinery. Seldom is the cheapest possible relay even considered.
Of course, this may not include CNC machines, which ARE a special case.