I need to control a 2-way 3-wire 230VAC synchronous motor (6W) driven HVAC valve with SSR. (This is a custom thermostat with heating-cooling valve controller).

The valve internal circuitry is extremely simple with 2 SPDT switches and a camshaft, and breaks the contact if it's on desired position. The motor always rotating in one direction, the valve can rotate 360degs open-close-open-close-etc. Works perfectly with relays.

synch valve internal

Now the motor is driven by two SSR with RC-snubber in it. When the valve is driven to open position, and reach it, the "to open" switch is turned off in the valve, and breaks the "To open" line, and a motor stops (another switch still in ON position).

ssr wiring

The problem: The motor does not stop on desired position with SSR, because the inactive SSR + internal RC snubber + another closed switch in valve still leaking some current to motor even if the another (recently used) switch is turned off (because reached the desired position). It's in momentum and the motor rotates over the desired position with the little leaking current on other SSR.

My investigations: The SSR is working properly, not SSR failure. If the RC-snubber is removed, the valve working properly. With standard relays also working properly.

My solution tip: one SSR for on-off functionality and standard relay to select the changing direction (and changing only if the SSR is turned off - no load), but I don't want to use relays in this application.

The market has many thermostats with SSR-output 2/3-way valve control option, i dont know why these does not have this issue.

Application is similar to this (check the Valve output wiring): https://becaenergy.com/wp-content/uploads/2020/08/BAC-3000-1.jpg


1 Answer 1


You need this arrangement,

Snubber load referenced configuration

Cx shall be a X1/X2 rated type, larger than the C in the snubbers. Probably 0.22 to 1µF will do, and the snubbers can be 100Ω + 47-100nF or something like that (exact values depend on motor winding inductance).

When the valve is the dominant inductance in the circuit, Cx can be omitted, but the line will normally be some 100s µH, and having the local Cx bypass accounts for that.

Alternately, there's the "don't worry about it" option. If the SSRs are "snubberless" type, or AC/DC type with adequate avalanche rating (and switching is infrequent enough not to mind the avalanche wear mechanism, i.e. you might get only so many thousands or millions of operations before failing shorted), then the snubber can be omitted.

It's generally better to keep the snubber (more reliable / more operating range / more consistent operation / etc.), but there are cases where it might be omitted.

  • \$\begingroup\$ Many thanks for your suggestion. The modified arrangement works well, I used 0.1uF X2 and 100R for snubber. Waveforms also checked with scope, and all's ok. The valve still skipping sometimes (only sometimes) the end position but it's not the circuit problem. \$\endgroup\$ Commented Aug 22, 2023 at 11:46

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