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    #21
    Re: Mechanical Relay Quality

    you cant do that, AC can pass through the caps

    you need a snubber for inductive loads.
    for an incontinent load a bucket or cloth is more usefull.
    Last edited by stj; 01-02-2018, 08:55 PM.

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      #22
      Re: Mechanical Relay Quality

      Redwire does 10R stand for 10 ohms or 10 thousand ohms?

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        #23
        Re: Mechanical Relay Quality

        10 ohms.

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          #24
          Re: Mechanical Relay Quality

          When using a snubber with a AC circuit the direction of current flow matter, if so should the cap be on the Line side or the neutral side of the switch/relay?

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            #25
            Re: Mechanical Relay Quality

            direction is not important because none of the parts are polarised.

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              #26
              Re: Mechanical Relay Quality

              thanks, hopefully i can start my projects next weekend

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                #27
                Re: Mechanical Relay Quality

                Originally posted by evilkitty View Post
                When using a snubber with a AC circuit the direction of current flow matter, if so should the cap be on the Line side or the neutral side of the switch/relay?
                Polarity wouldn't affect a devices location IN a circuit (line vs neutral) but, rather, the device's orientation as it is inserted. As this circuit will see both polarities (which is something more than just saying "AC"), you need an NPO device (which can be made from polarized devices placed "back-to-back")

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                  #28
                  Re: Mechanical Relay Quality

                  is that what the film cap does?
                  does that mean i could use 2 normal caps back to back to do the same thing?

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                    #29
                    Re: Mechanical Relay Quality

                    a film cap can hadle high current pulses well - that's why you use film.

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                      #30
                      Re: Mechanical Relay Quality

                      Originally posted by stj View Post
                      a film cap can hadle high current pulses well - that's why you use film.
                      Metallized film is also self-healing.

                      The drawback with film is usually size/cost as you look for higher voltages and values of capacitance as well as a much smaller range of options for SMT production.

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                        #31
                        Re: Mechanical Relay Quality

                        Lasted ~4 years this time
                        Here is a image of the relay contacts after failure
                        these are the parts i used:
                        https://www.mouser.com/ProductDetail...S2G031001K00KO
                        https://www.mouser.com/ProductDetail...OS1/2CT52R100J
                        Attached Files

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                          #32
                          Re: Mechanical Relay Quality

                          The relay contact's plating is gone on both N.O. and N.C. which is a bit strange. A thermostat uses one set only I thought. But no pitting or etching and not black. This is 24VAC HVAC? Looks like your snubber 0.1uF 400V and 100R is not doing great. The resistor is too high value I think.
                          I find 24VAC HVAC makes some pretty big arcs with zone valves due to their large inductance. A Honeywell V8043 measures 200mH and 21 ohms, but is only about 8VA load. Best I could fit is 0.22uF and 10R after using scope traces to calm down the spike. Snubber values for mains circuits have different values, there you can use a bigger resistor 33-100R and smaller cap 0.01-0.1uF

                          Before, I recommended 10R resistor and maybe double the cap size. I don't know what the load is, if it's a contactor or furnace gas valve etc. Why did you go 100R not 10R? I'd change the snubber or maybe a MOV is really what's needed.

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                            #33
                            Re: Mechanical Relay Quality

                            "0.1uF 400V and 100R is not doing great"
                            "Why did you go 100R not 10R?"
                            uhh the spec page says it is a 10 Ohm resistor, I'm not sure what you are talking about, i do not see 100 R on the spec page for either the cap or the resistor. does R not mean resistance?

                            as for why there is wear on both NC and NO

                            Image for reference: https://github.com/GM-Script-Writer-...0.JPG?raw=true

                            I have 2 relays, one simulates the top mercury tube and the other emulates the bottom tube
                            the top one acts like a double throw switch and the bottom is a single throw
                            both failures have been the top relay so far (it gets the most cycles)
                            the coil powered off state is what failed 1st on this relay
                            the bottom relay only gets switched when the temp goes down even though heat is on (this uses a lot of electricity) but when it gets really cold we have a wood stove for heat so that cuts the cycles the bottom relay gets alot

                            this is the 2 bit logic my relays have
                            Top relay coil = on; if cooling: cool=true; else if heat: heat=false
                            Top relay coil = off; if cooling: cool=false; else if heat: heat=true
                            Bottom relay coil = on; if heat: moreheat=true
                            Bottom relay coil = off; if heat: moreheat=false

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                              #34
                              Re: Mechanical Relay Quality

                              Sorry I clicked on the resistor Mouser link and saw "100J" and thought 100 ohms, but it's 10. I'd say the relay should be lasting much longer then.

                              The top relay seems to be a master, its coil is on whenever there is demand- for either heat or cooling. The bottom relay is for switching on aux. heat. is that right? How does it switch between heating or cooling? I'd expect one relay for demand and another for selecting heat/cooling. The Honeywell thermostats are brutal without a schematic showing the A/C or changeover valve etc.

                              You might have an issue with the relatively fast transition from heat/cool as the top relay pole moves, as an arc between N.C. and N.O ends through the center pole. But that's kind of grasping at a straw.
                              Or maybe software glitching the relay because it looks like you first switch the relay then conditionally update flags, it should be the other way around. The decision (relay on/off) should be the last action taken. But I'm not sure what flags 'heating' or 'cooling' are in your program. I might use code like this knowing that some undefined states exist and need error handling. It's good to never assume things and have extra code to catch invalid flags. This isn't correct code but trying to show what I mean.
                              Code:
                              if (room_temperature < (setpoint - hysteresis)) {
                              	heat_demand = true;
                              	cooling_demand = false;
                              }
                              elseif (room_temperature > (setpoint + hysteresis)) {
                              	cooling_demand = true;
                              	heat_demand = false;
                              }
                              else { // we are at target setpoint in the middle
                              	cooling_demand = false;
                              	heat_demand = false;
                              }
                              //-------------- now switch outputs --------------------
                              if (heat_demand == true) {
                              	Top relay coil = on; 
                              }
                              elseif (cooling_demand == true) {
                              	Top relay coil = off; 
                              }
                              else { // we are at setpoint or an invalid i.e. both true heat +cool requested
                                  do nothing?
                              }

                              Comment


                                #35
                                Re: Mechanical Relay Quality

                                There is a slide switch on the thermostat to select heat/cool/ emergency heat/off, in my software you have to set what the physical switch is set to, though software has a option to just pass-though the readings from the mercury tubes (this is also a fallback mode if there is a fatal error)

                                all i did was use relays in place of the tubes, all other stock hardware is still in use

                                the 'code' if you can call it that i posted, was just a simple oversimplify logic meant to be easily readable

                                if you want to see the actual script: https://github.com/GM-Script-Writer-...bin/thermostat

                                is it possible to have the snubber circuit backwards? no idea if common is the high or low side, i just consider it a mechanical switch so it should not matter it may as well be a piece of wire as far as current flow is concerned
                                Last edited by evilkitty; 01-30-2022, 11:04 PM.

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                                  #36
                                  Re: Mechanical Relay Quality

                                  PWM your furnace instead of bang/banging it

                                  (and replace it every month...)

                                  You know what, the computerized thermostat I have now does not automatically go run the refrigerant compressor when it's in natural gas mode, and never uses the natural gas when it's in compressor mode. It's interlocked - you have to specifically choose cooling or heating and won't automatically do either. Do "smart" furnaces now automatically choose cool when it's hot and heat when it's cold? Seems easy enough to do but this old microcontrolled thermostat should be able to do it without an issue yet it's explicitly disabled...as I'd expect it to be...

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                                    #37
                                    Re: Mechanical Relay Quality

                                    "Do "smart" furnaces now automatically choose cool when it's hot and heat when it's cold?"
                                    i have no idea, my dad had bought a $50 thermostat (not even a smart one) and it did not support aux heat, i got him to return it and made made a hybrid thermostat using the ancient thermostat and a raspberry pi, he was saying the tubes act like they are getting 'stuck' (maybe a metal fatigue issue) i decided well if the only issue is the tubes why re-designed everything when i can just use a relays to in-place of tubes, they are just a STSP and STSP switch

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                                      #38
                                      Re: Mechanical Relay Quality

                                      Tubes? or just mercury switches?
                                      I still have the old mechanical heat/cool switch which likewise require to be forced in heat or cool mode but not automatically switch between the two. While it does have a mercury switch in it, I thought it was more of the bimetallic strip that senses temperature and the mercury switch was solely for electrical contacting. Having the switch in a hermetically sealed environment with liquid wetting gives long life... alas a TRIAC should also have a long life.

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                                        #39
                                        Re: Mechanical Relay Quality

                                        Just looking at reasons for the relay to have a short life, even with the snubbers.
                                        I am familiar with separate heating and cooling systems, which use a natural gas furnace or boiler, and separate A/C unit. So that just needs a heat and a cooling relay. What model #'s are the furnace or A/C, to see if the current or spike is much larger than we think, for the hardware.

                                        The Honeywell mercury thermostats have their own issues. If anything touches the coils they can jam a bit, if anything nearby makes heat they will also get fooled into thinking the room is warmer than it is. The mercury bounces, even as people walk by or drafts from doors etc. There is a need to sample then and average over many seconds so the sloshing isn't a problem.
                                        The amount of hysteresis (there's two types) is fixed and is just due to the weight of the mercury blob moving over. So at 70F the blob rolled to the side so it takes more force to make it roll back, say 68F.
                                        Your RPi and the relay coils make enough heat to make the mercury switches far off. Your heat sensor is away, so that is accurate.

                                        In the software, I don't see any averaging of temperature readings, or debouncing of the mercury switches. This is a problem because temperature moves up and down from people walking by, doors opening, cooking etc. I would be averaging over a long time like a minute or so.
                                        As well, the mercury switches don't just switch, they bounce and slosh so their readings need to settle for many seconds.

                                        The software seems to read the mercury switches many times in a subroutine which is a no-no because the reading can change while you are in that routine. This might cause oddball errors. A better way is to read tubes once, atomic read into a temporary variable and only use that var for all the consequent compares and conditionals because you know it won't change until next time you read the switches.
                                        The code to reconcile the tubes with the relays state might not work so great, it could cause chatter of the relays if they get turned on and off many times. But it still shouldn't cause that short a life that we are seeing.

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                                          #40
                                          Re: Mechanical Relay Quality

                                          I found my furnace/CAC actually sort of low pass filters switching events so bouncing by the mercury switch actually does not affect it. However, not sure about cooling/heating cycles, though I suspect it will do so too.

                                          Maybe the code is changing state too often. My current microcontroller thermostat also uses a (sealed) relay and so far so good, though I don't recall what snub network is in it. It's also a tiny relay like the size of a very tall DIP14 IIRC... It doesn't seem to switch that often maybe leading to longer life.

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