Mechanical Relay Quality

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  • evilkitty
    Badcaps Veteran
    • Nov 2017
    • 299
    • USA

    #1

    Mechanical Relay Quality

    Yesterday i had to replace a relay i installed about 2.5 years ago
    This relay i used in my thermostat to use 5VDC logic to control 27VAC logic
    I had used the cheap relays you can get on ebay for about $0.50 each (Songle SRD-05VDC-SL-C)
    It failed cause the contact plate on the load side became dirty from electrical arcing
    here is a video showing how they are designed: https://www.youtube.com/watch?v=KzR7EtPM15E

    Would a 2.5 year life span be reasonable for a relay in this type of use? (defective?)
    would one of these two last significantly longer (enough to justify 2 to 2.5 times the price)?
    https://www.mouser.com/productdetail...wh-sh105d1f000
    https://www.mouser.com/productdetail/JS1-5V-F
  • stj
    Great Sage 齊天大聖
    • Dec 2009
    • 30910
    • Albion

    #2
    Re: Mechanical Relay Quality

    there should be no arcing, you need to add a snubber circuit across the contacts.

    Comment

    • PopcornMobo
      Senior Member
      • Dec 2017
      • 53
      • Lithuania

      #3
      Re: Mechanical Relay Quality

      There's a good video showing what the cheap relays are capable of.
      https://www.youtube.com/watch?v=ftJ17Cp6itw

      Comment

      • evilkitty
        Badcaps Veteran
        • Nov 2017
        • 299
        • USA

        #4
        Re: Mechanical Relay Quality

        Originally posted by stj
        there should be no arcing, you need to add a snubber circuit across the contacts.
        So i need to do this (see attachment)?
        What kind of cap would i need to use here? This is an AC circuit (meter says ~26.995V)
        about how many ohms should i use? (5, 10, 50, 100, 1000, 50k, 1m)
        * my on hand resistors only good for 0.25W, so going below 3k is probably asking it to burn
        AC caps are not polarized right?

        In this case Load B would be used when the unit is set to heat and Load A would be used if the unit is set to cool


        When i designed this board i used the relay to replace mercury tubes, i think i have seen a small spark when they switch, this is why i assume it was arc damage
        Attached Files
        Last edited by evilkitty; 01-01-2018, 12:24 PM.

        Comment

        • stj
          Great Sage 齊天大聖
          • Dec 2009
          • 30910
          • Albion

          #5
          Re: Mechanical Relay Quality

          that circuit is nice.
          caps should be film type,
          analog is not my thing, but the current drawn by the load is usefull to know.

          you can get snubbers containing the cap and resistor in a single package but most designers use seperate parts.
          probably for cost reasons.

          Comment

          • evilkitty
            Badcaps Veteran
            • Nov 2017
            • 299
            • USA

            #6
            Re: Mechanical Relay Quality

            I know the current is small, 26awg wires do not get warm
            wonder what my clamp on meter says...
            0.67A up to 1A then back down

            Comment

            • Curious.George
              Badcaps Legend
              • Nov 2011
              • 2305
              • Unknown

              #7
              Re: Mechanical Relay Quality

              Originally posted by evilkitty
              Yesterday i had to replace a relay i installed about 2.5 years ago
              This relay i used in my thermostat to use 5VDC logic to control 27VAC logic
              I had used the cheap relays you can get on ebay for about $0.50 each (Songle SRD-05VDC-SL-C)
              It failed cause the contact plate on the load side became dirty from electrical arcing
              Do any of:
              • Add a snubber across the contacts
              • Use Hg-wetted relay
              • Exert more control over when the armature is moved wrt the AC waveform

              Comment

              • evilkitty
                Badcaps Veteran
                • Nov 2017
                • 299
                • USA

                #8
                Re: Mechanical Relay Quality

                On the note of a snubber
                Assuming the power is 1A @ 27VAC (60Hrz i assume)
                what size/wattage resistor and film cap do i need?

                The spark i saw when it used the tubes was like you would get from a little static (petting cat and touch it's ear near the fire place)

                Comment

                • sam_sam_sam
                  Badcaps Legend
                  • Jul 2011
                  • 6018
                  • USA

                  #9
                  Re: Mechanical Relay Quality

                  Originally posted by evilkitty
                  Yesterday i had to replace a relay i installed about 2.5 years ago
                  This relay i used in my thermostat to use 5VDC logic to control 27VAC
                  What is the amperage of the device that you are controlling

                  Comment

                  • stj
                    Great Sage 齊天大聖
                    • Dec 2009
                    • 30910
                    • Albion

                    #10
                    Re: Mechanical Relay Quality

                    have a read.
                    Attached Files

                    Comment

                    • evilkitty
                      Badcaps Veteran
                      • Nov 2017
                      • 299
                      • USA

                      #11
                      Re: Mechanical Relay Quality

                      Originally posted by sam_sam_sam
                      What is the amperage of the device that you are controlling
                      during the switching process my clamp-on meter measured 0.67, with a peak of 1A (slowly climbed and slowly dropped off)
                      *I need to get fuses for my other meter (in my mouser cart)

                      Comment

                      • evilkitty
                        Badcaps Veteran
                        • Nov 2017
                        • 299
                        • USA

                        #12
                        Re: Mechanical Relay Quality

                        Originally posted by stj
                        have a read.



                        IL = Amperage, given my meter peaked at 1A , guess that is 1
                        E = ???
                        k= (unknown value from 0.1 to 2)
                        this is the issue i have when i go to look a circuit formula, i have no idea what the variables stand for...
                        Last edited by evilkitty; 01-01-2018, 08:05 PM.

                        Comment

                        • Curious.George
                          Badcaps Legend
                          • Nov 2011
                          • 2305
                          • Unknown

                          #13
                          Re: Mechanical Relay Quality

                          Originally posted by evilkitty
                          On the note of a snubber
                          Assuming the power is 1A @ 27VAC (60Hrz i assume)
                          what size/wattage resistor and film cap do i need?

                          The spark i saw when it used the tubes was like you would get from a little static (petting cat and touch it's ear near the fire place)
                          It's not the amperage or the voltage but, rather, the rate of change of voltage/current that drives the design.

                          The capacitor (in series with the resistor) ensures there is no DC path through the snubber -- you wouldn't want current flowing AROUND your switch contacts all the time as that would defeat the purpose of the switch! The capacitor ensures that only the "transient" (spark) finds its way through the snubber, being dissipated in the resistor.

                          With the mercury switches, the mercury acts to "clean" the electrical contact. Hence the use of mercury-wetted relays if you want to avoid the snubber.

                          Comment

                          • sam_sam_sam
                            Badcaps Legend
                            • Jul 2011
                            • 6018
                            • USA

                            #14
                            Re: Mechanical Relay Quality

                            Or you could use this

                            Crydom S3013A S3 Series Solid State Relay, PCB Mount, 3.5-8 VDC, 3A, 20-140 VAC

                            Comment

                            • eccerr0r
                              Solder Sloth
                              • Nov 2012
                              • 8658
                              • USA

                              #15
                              Re: Mechanical Relay Quality

                              Had my microwave oven's relay go kaput recently, jury rigged a replacement until I can go order one (with other stuff).

                              It died when a capacitor died... Contact life exceeded/destroyed.

                              Comment

                              • redwire
                                Badcaps Legend
                                • Dec 2010
                                • 3900
                                • Canada

                                #16
                                Re: Mechanical Relay Quality

                                I just did a snubber design for 24VAC zone-valve controller.
                                The values are not critical, I found 0.1uF-0.22uF 160V and 10R 1/2W worked fine.
                                This is to keep the back EMF from arcing relay contacts or damaging a triac.

                                The cap ends up being large physical size, I used junkbox X or Y-caps.

                                Comment

                                • evilkitty
                                  Badcaps Veteran
                                  • Nov 2017
                                  • 299
                                  • USA

                                  #17
                                  Re: Mechanical Relay Quality

                                  Originally posted by redwire
                                  I just did a snubber design for 24VAC zone-valve controller.
                                  The values are not critical, I found 0.1uF-0.22uF 160V and 10R 1/2W worked fine.
                                  This is to keep the back EMF from arcing relay contacts or damaging a triac.

                                  The cap ends up being large physical size, I used junkbox X or Y-caps.
                                  so these are good then?
                                  https://www.mouser.com/productdetail...s2d031001a00ko
                                  https://www.mouser.com/productdetail...os1-2ct52r100j

                                  Comment

                                  • stj
                                    Great Sage 齊天大聖
                                    • Dec 2009
                                    • 30910
                                    • Albion

                                    #18
                                    Re: Mechanical Relay Quality

                                    Tesla once said that too much theory and not enough experimenting will get you nowhere.

                                    Comment

                                    • redwire
                                      Badcaps Legend
                                      • Dec 2010
                                      • 3900
                                      • Canada

                                      #19
                                      Re: Mechanical Relay Quality

                                      I would use a higher voltage capacitor as 63VAC/100VDC is close to the part's limit.

                                      The load R L values and math and what you can fit/have on hand- it's easier to just look at it with a scope. Zone valves have a lot of inductance 200mH. I can post SPICE sims.

                                      With no snubber I got 300-500V spikes, and down to about 100V with a 0.22uF/10R
                                      Last edited by redwire; 01-02-2018, 12:16 PM.

                                      Comment

                                      • evilkitty
                                        Badcaps Veteran
                                        • Nov 2017
                                        • 299
                                        • USA

                                        #20
                                        Re: Mechanical Relay Quality

                                        well then guess i will go with a 200v cap 505-mks2g031001k00ko or 505-mks2g031001k00jo which ever is in stock...

                                        In the interest of science would it be a problem if i used only one resistor with a double throw switch/relay
                                        I know it would only save $0.10 but it is 1 less part to solder, now that the logical reasons are out of the way... i am really just curious about it if you can do that

                                        EDIT:
                                        Would i need a snubber for a relay that controls a incontinent light bulb and a florescent bulb?
                                        Attached Files
                                        Last edited by evilkitty; 01-02-2018, 06:45 PM. Reason: 200V not 400V (mixed up AC /DC voltage specs)

                                        Comment

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