Relay Box for Connecting 850LM to 28V AC Garage Door
Description
2-part 3D printed box to protect a relay soldered to perf board. The internal space is 30 x 20.5 x 22.5 mm tall; there are holes at both ends for wires. The screws have a 2.5 mm root, 3 mm max tooth OD, 6mm head, and 11.5 mm length An external relay is used because the relays inside the Liftmaster 850LM have a contact rating of 14V DC. The 850LM manual incorrectly states the contact voltage is 28V AC, but looking up the relay datasheet shows they're rated for switching up to 14V. The contacts only experience 20mA to trigger the door, but I'm choosing to wire it correctly. The 850LM also might not be able to be powered by 28V AC, despite its claims. The first one I bought burnt out in a few minutes and had to be returned. The wiring diagram shows the 12V DC brick I used to power both the 850LM and the external relay. The new relay has a contact rating of 125V, so I used it to trigger the 28V garage door. The 12V supply should be rated 0.1 Amps or more (50 mA receiver + 30 mA relay coil).
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