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I'm using relays to drive a circuit of solenoids. Do I really need flyback diodes on the controlled circuit if there are only solenoids on it? The driving circuit is already protected by flybacks.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The solenoid does not care much, but the switch might not appreciate switching an indictive load. Check whether it is rated for this, and for what current. The fact that AC is switched makes using a simple diode impossible. \$\endgroup\$ – Wouter van Ooijen Feb 8 '15 at 19:26
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A MOV or snubber or bipolar TVS will usually extend the life of the relay contact. At 24VAC it may not be that important. Personally, I would consider a bipolar TVS.

It won't affect the relay directly, but if the controlling circuitry is poorly designed you may see effects from switching such an inductive load (microcontroller resets, flakey behavior increasing in frequency as the contacts age and so on).

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  • \$\begingroup\$ if voltage increase to help current jump the newly opened switch, does that mean voltage increases at the transformer too? Which would harm the transformer. \$\endgroup\$ – Alex Feb 8 '15 at 20:04
  • \$\begingroup\$ No, the high voltage appears across the contacts not across the transformer. If there was contamination and moisture on the solenoid or wiring it could eventually cause damage, but usually the insulation is good enough that the solenoid coil itself is not at risk (the voltage does not increase to infinity- it's limited by the distributed capacitance in the coil and wiring). \$\endgroup\$ – Spehro Pefhany Feb 8 '15 at 20:10
  • \$\begingroup\$ By the way, I would wire the relay common to the transformer and the N.O. contact to the solenoid if not using a snubber/TVS/MOV. \$\endgroup\$ – Spehro Pefhany Feb 8 '15 at 20:57
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It depends on many factors, including AC or DC operation, voltage, current, solenoid inductance, relay contact construction and relay contact material.

First: your show your solenoids having only 1 uH inductance. I don't believe that is anywhere near close to their actual value - several hundred milli-Henrys on through several Henrys would be more like it.

My company manufactures a lot of circuit boards used to build HVAC equipment where we have relays driving water circulating pumps that run at AC Mains voltage (both 120 Vac & 230 Vac). Those relays are NAIS / Aromat JS1 series and we have shipped boards containing several hundred thousand relays over the past 20 years or so. There is NO transient suppression on the contacts. I am not aware of any relay contact failures that weren't caused by installer wiring errors.

A different class of board uses either 1 or 3 American Zettler AZ2150 & AZ2150A relays directly controlling 3/4 HP induction motors (HVAC Blowers). Same deal there: no relay failures not caused by installer wiring errors.

In general, I worry about requiring arc suppression on relays contacts driving DC inductive loads. It seems to be much less of an issue when driving inductive loads from an AC source.

That said: I examine each specific application carefully at the design stage and determine if relay contact transient suppression is warranted.

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The relay contacts should be snubbed with a (bipolar) TVS device or R/C snubber network; this will protect the contacts from arcing damage/wear (pitting, etal) and extend their life dramatically in this service. See Chapter 7 of Electromagnetic Compatibility Engineering for more snubbing advice.

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  • \$\begingroup\$ Not sure if everybody is familiar with "Chapter 7 of Electromagnetic Compatibility Engineering" \$\endgroup\$ – jippie Feb 8 '15 at 20:59
  • \$\begingroup\$ I'll add a link to the book on Amazon :) \$\endgroup\$ – ThreePhaseEel Feb 8 '15 at 21:06

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