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If you're going to control a relay with a transistor and will not include a flywheel diode, you're going to have a bad time, it's a known fact. But what about relay-relay circuits? I have mixed feelings about this:

  • When "driving" relay A is switching off another relay B, the inductive kickback of coil B would produce a spark in relay A contacts, thus, potentially reducing the overall life of relay A. So, we just include a flywheel diode on relay B and none on relay A. Well...what if A is also driven by another relay? (i.e. in a relay-based CPU? :))

  • So, if all relays have flywheel diodes, then we have a problem on "driving" or "controlling" relay A again: according to this article: http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3264_AppNote&DocType=CS&DocLang=EN adding a flywheel diode would modify de-energizing profile of the coil, which modifies how the contact spring reacts to a switch off, which damages spring and reduces life of a relay.

Any comments? Drop them, use them, or use diode+zener? I think I may have to just make a pair of inverter loops (with and without diodes) and see which one dies first :)

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  • \$\begingroup\$ There is no one size that fits all. It all depends on what voltage rise you can/want to tolerate and how fast you want to dissipate the energy. For changing this you can chose different diodes and resistor combinations. \$\endgroup\$ – PlasmaHH Apr 5 '18 at 10:49
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I would say diodes are virtually always better, ignoring the slight complexity increase and minor cost (pennies for parts or fractions thereof), unless there is some reason to get every fraction of a millisecond speed (and then you shouldn't be using relays at all in the current year).

Contact life reduction due to slower drop-out is not a serious concern because relays of normal size do not draw significant current for most power relays. i.e. you are using a 10A relay contact to switch 60mA. So if your PLC is switching the coil of a DC power relay, I say put the 1N400x in there and it will last much longer. Or use a solid-state output.

Yes, you can use the zener + diode or resistor + diode and it will give you a bit of a compromise (more sparking but faster drop-out), but it's not all that often necessary, and it could cause more issues with field repairs because it will be unfamiliar to service people.

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Last time I have seen modern relay-based logic circuits (in a railroad application), there were no flyback diodes. You don't want to include an extra point of failure into a system with a 40-years service life. Additionally, many relay circuits rely on being able to power a relay with either polarity, something a flyback diode would prevent.

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  • \$\begingroup\$ Was there any alternative, like R-C snubbers? Or was there anything special about the switching contacts, to protect against spark erosion? \$\endgroup\$ – Brian Drummond Apr 5 '18 at 11:43
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    \$\begingroup\$ @BrianDrummond Unfortunately I don't have access to schematics anymore, and cannot remember all details. I remember those were quite expensive sealed relays with bifurcated contacts which had very little sparking/oxidation in the first place. \$\endgroup\$ – Dmitry Grigoryev Apr 5 '18 at 12:39
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Choosing an adequate circuit also depends on how many operations will the relay be doing during its expected life. Will you flip it once a day in a garden light style? or will you switch it a high frequency in an antenna tuner style? Unless you are abusing it's current capability the number of operations will probably mark how long will your relay last. Use a circuit that is adequate for it. If once a day use the diode and forget it.

For intensive applications, an RC snubber will probably be better and cheaper than the diode + Zener approach. Choose R based on current coil I so that $$V_{peak} = V_{cc} + IR$$ is reasonably below your control circuit limits. Then choose C as big as possible within reason (acceptable board layout and C's price). I'd try 100 or 220 nF to start with and experiment a bit while recording the results on a digital scope. Remember C has to support a moderately high voltage. Make it at least twice IR to go safe.

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    \$\begingroup\$ yep, 20-30 Hz relay cpu :) but you need a snubber across each switch, right? not across each coil... \$\endgroup\$ – artemonster Apr 5 '18 at 14:37
  • \$\begingroup\$ @artemonster You are right, but that's a different question. In fact I would use two (different) snubbers, one at the coil an another at the load. \$\endgroup\$ – Toni Homedes i Saun Apr 6 '18 at 6:53

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