I am specifying a relay I/O card for a Beckhoff PLC.

(Link to an overview of Beckhoff digital output slices, scroll to see Relay section)

Some of the relays have a soft switching characteristic called out. For example, see the description for Beckhoff's EL2602-0010.

What does soft switching mean, in the context of specifying a relay?

Bonus points for explaining how soft switching physically works.

This article may be relevant, but I found the explanation hard to grasp.


3 Answers 3


They appear to be delaying the (mechanical) relay energization to attempt to have the relay contacts close near the zero crossing with a tolerance of +/-1ms. It's not really related to SMPS soft switching.

The function requires a reference AC voltage to be applied so that the relay driver can be timed. They also attempt to correct for variation of switching speed of the relay with temperature using a temperature sensor, which, of course, introduces possible issues when the temperature is not stable.

For a bit more information, see the manual section 5.5.


The EL2602-0010 output terminal has two relays each of which has a single contact. The relay terminal is designed for soft switching at the zero voltage crossing to protect the contacts and is particularly suitable for capacitive loads such as LED lamps. [From the linked catalogue page.]

The manual (thanks, Spehro) seems to indicate that only modules with a single NO contact per channel have the soft-switching feature. Changeover contacts don't. This makes me suspect that there is also a triac switch with zero-crossing capability.


simulate this circuit – Schematic created using CircuitLab

Figure 1. A relay contact with zero-cross triac to give a "soft start".

enter image description here

Figure 2. With zero-cross switching there is no sudden change in current as the turn on and turn off occur at close to zero volts. Source: Opto-triacs, solid-state relays (SSR), zero-cross and how they work.

The idea here is to provide the best features of a triac and the best of a relay contact simultaneously.

  • The triac can easily be controlled to switch at zero-cross thus minimising noise on the power lines. Their downside is that due to the voltage drop across them while on they dissipate some power as heat and for long term use heatsinks would be required. The triacs have no moving parts so should last forever if rated correctly.
  • The relay contacts are better in that the voltage drop across them is low but arcing during opening and closing wears the contacts and, as they are mechanical devices, millisecond accuracy is difficult so they make poor zero-crossing switches.
  • The combination works well because the triac just has to carry the current for a half-cycle or two while the relay contacts close and again when they open. The triac avoids all the normal pain for a relay.

This is all conjecture. If you ever get one then open it up - like a real engineer - and let us know if this answer is correct.

  • \$\begingroup\$ That would properly be called a "hybrid relay" and was my first suspicion, but the manual seems to indicate they're just using the mechanical relay. \$\endgroup\$ Dec 18, 2019 at 19:03
  • 2
    \$\begingroup\$ Thanks. I read your answer and was interested / intrigued to think that they might time the relay switching accurately enough. I didn't find the temperature monitoring bit so you may well be right. \$\endgroup\$
    – Transistor
    Dec 18, 2019 at 19:07

“soft switching” is just another way to describe Zero Crossing Switch ZCS for turning on a Triac, which is as old as Triac technology.

Opto isolation isn’t necessary but there are Opto ZCS Triac IC drivers for power Triacs.

what is just as important is the turn off Snubber design which depends on the load. Although Triacs will unlatch below holding current when Driver is OFF,with motors that generate EMF with inertial velocity, there also needs to be a brake or snubber for turn off.

(Anecdotal: I still have a relay timer that turns off a ceiling fan with enough EMI that it clicks my video signal in the net TV room from PC waiting for me to add a snubber)

For loads >10A smart soft switchers use ZCS triacs to turn on 1st then bypass with a 0 Ohm relay then shut off in the reverse order.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.