Relay logic for a current sensor relay not overriding user controls? - Page 1
Forum section: electronics / projects

author: ddbear
author_rank: Contributor
timestamp: May 06, 2022, 11:19:59 pm
content: This is a branch topic from my previous question at this link,https://www.eevblog.com/forum/projects/adapt-120v-switching-between-4-different-wires-to-0-10vdc-signals/msg4159498/..but different enough for its own topic.I am trying to visualize the relay logic when adding a current sensor relay for this desired behavior:When the user presses a button that activates one of four 120V relays (button press outputs 120V on one of four wires), each closing the circuit to a different DC converter (to choose from four fan speeds to the 0-10V EC motor speed control), the kitchen vent fan operates as usual, with the button interface.But here's the twist: I'd like to add a current sensor, such as one of the Eaton CurrentWatch relays on the 240V cooktop wires so the range hood vent can turn on automatically when someone turns on the induction cooktop, and figure out the best way to handle the relay logic of how the vent hood buttons vs. cooktop automatic control will override each other.Eaton current relays:https://www.farnell.com/datasheets/1674660.pdfIn my curerent design there are 4 relays for activating AC power to the motor plus another 4 relays for switching between DC speed signals. Figuring out the 120V power to the fan motor is easy - it's a logical OR operation so if any of the relays (including the cooktop current sensor relay) are On, the fan motor gets power.But how should I design the logic of relay switching between the fan speeds with these other four 120V relays that switch between the DC converters? If a button is pressed let's say for fan speed 3, it would probably be best for the control interface to leave the setting on fan speed 3 when the cooktop current sensor relay turns on - rather than to override the chef's choice with its own fan speed. My initial thought was to give the current sensor relay its own 2nd relay for its own DC speed signal setting, however the circuit should not activate two different DC converters (fan speeds) at the same time.What would be the simplest relay configuration/logic to make intuitive fan control behavior in this scenario?

author: ddbear
author_rank: Contributor
timestamp: May 07, 2022, 12:29:51 am
content: Perhaps I can have two of the DC converters on at the same time?The DC converters I'm planning to use, the Drok adjustable DC converter, has "output anti-backflow protection":https://www.droking.com/dc-power-supply/dc-boost-buck/Adjustable-Buck-Boost-Converter-Board-DC-5.5-30V-to-0.5-30V-4A-High-Power-Supply-Regulator-with-Case-LCD-DisplayThen the fan speed would be the one with the highest voltage? (so turning the cooktop On and the current sensor activating another relay would not reduce the user-selected fan speed, only possibly increase it)?If I could do it this way, it would make for a simple relay arrangement (5 relays and DC converters instead of 4, all working the same way).Maybe add a diode to use them together?

author: Terry Bites
author_rank: Super Contributor
timestamp: May 07, 2022, 01:34:07 pm
content: Current relays are serious cash.How about HCPL-3760 with an appropriate shunt and input protection?The outputs are easily ored or what ever you need. You can use a glue logic to get any combination you need.Before you go any futher create a truth table that makes sense to you.