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OT relay coil snubber

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  • #16
    Going down a slightly different path.... The whole idea here is to reduce transient energy discharge caused by the relays magnetic field collapsing. How about starting by reducing the stored energy? I would look at very low current (coil current) reed relays. As microphone signals are virtually 0 energy, the reed relays will help in several ways. One that I have had to fight is simple contact contamination over time. Most relays are designed to clean the contacts by arcing very slightly with normal operation. Switching microphone signals will NOT allow contact cleaning and WILL lead to signal degradationover time. Reed relays are much better and mercury relays are even better yet. Just a thought for you to consider....
    Robin

    Happily working on my second million Gave up on the first

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    • #17
      Ypu could rube goldberg the **** out of this by using a resistor to heat up the bimetalic strip of a mercury thermostat.

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      • #18
        Reed relays do indeed have those advantages. There are types specifically made for low energy dry contact use.

        The one disadvantage is that the coil is even closer to the wires than with a standard relay, and most are SPST, so cancellation of noise is not good.

        But it is a good solution, and has been used in equipment before.

        I took it that he has the parts and wants to solve the problem as well as possible with what he has. The "slow drive" (integrator) is a good way to do that.
        2801 3147 6749 8779 4900 4900 4900

        Keep eye on ball.
        Hashim Khan


        It's just a box of rain, I don't know who put it there.

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        • #19
          Couple of points- I want to use these relays because I have them, for one. They have gold plated contacts, and should last a while even with no current to speak of flowing through the contacts. I have three different switching needs, and I have one power supply that will operate all the relays. One is for switching mics, one is for switching line level signals, two are for switching 110 vac, and the rest are for switching speaker level signals. The power supply has the two relays for switching ac inside the box, the mic relay is in its own remote box, and the speaker relays are in a separate box away from the mic lines. For ease of access all the switches will be together in a slim line box on the desktop. It just makes sense in this case to do it this way.

          I haven't had the chance to follow up on this project since my plumbing fiasco, but I have been considering using fets to operate the relay coils. It will be easy to get a slow turn on and slow turn off for the fets- basically allowing the coil voltage to ramp up slowly and ramp down slowly. The alternative is what I suggested first- a capacitor across the coil and a series resistor to limit the rise and fall speed of the coil voltage.

          With this last method, the switches will be passing 40 ma when on, but about 200 ma when first switched on. The value of the capacitors will be high enough that the coil voltage will take perhaps 100 milliseconds to rise and fall, which should swamp any inductive spikes that might otherwise be developed.

          Two things concern me- the sudden current through the switch, and the sudden interruption of that current when the switch is turned off will generate a pulse in the switch wires. Where those wires come to the relay, I'll have to dress them away from the mic wires as best as possible. The other thing that I'm unsure about is what happens when the relay armature makes its sudden move to the pole piece- will there be a magnetic pulse generated, and will it be of a large enough magnitude to induce a thump in the mic wiring. The mics are wired as balanced, and in theory if both wires receive the same thump, it should not result in a net signal being amplified by the mixer.
          I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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          • #20
            just do a quick test wire-up of the mic circuit.

            i will bet that with just a diode, you don't get any switching noise on the mic line.

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            • #21
              The larger chance of a thump comes from the contacts closing if they are not all at the same voltage.

              The wires to the relay coils do not need to have a sudden change of voltage/current. Make that change slowly and there is no issue aside from a slight delay in switching.
              2801 3147 6749 8779 4900 4900 4900

              Keep eye on ball.
              Hashim Khan


              It's just a box of rain, I don't know who put it there.

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              • #22
                Because there will be two mics into one mixer input, one of them will always be connected. That SHOULD mean no switching transient from the input circuit when the relay switches to the second mic. It would be equivalent to pulling one mic out of the input jack and putting another one in. Normally you would bring the level control down before doing this, and we certainly have the option of doing this anyway. We won't have that option when switching the line level circuit- but here the problem would be much lessened. For the speaker level relays and the ac power switching relays, I'll probably just use the diode across the coil. I don't want to slow those relays down.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #23
                  A 12v relay for example will need around 9 to 10v to switch on with the correct "contact pressure" bur will hold this pressure if you reduce the voltage way down to 3v or less, depending on the relay. Used this way, you the stored energy will be much less. The switching circuit achieve this is also very simple and all it takes is a resistor and capacitor together with your existing circuit. The resistor is calculated by the relay's minimum holding current/voltage and is used to charge a big enough capacitor that will energize the relay with the nominal voltage.
                  Helder Ferreira
                  Setubal, Portugal

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                  • #24
                    I think that will work fine- I've just never tried it. These 24 volt relays will cut in at 12 volts and drop out at about 8 volts. My power supply makes 20 volts, so a bit low but will work fine.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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