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Thread: ssr replacement for 20A relay?

  1. #1
    *Flashaholic* Brock's Avatar
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    Default ssr replacement for 20A relay?

    Here is the situation. My father in law has on peak / off peak or time of use electrical rates so I installed a panel next to his main panel that has three 20A relays that control his base board electric heaters. The relays have a 120vac coil and are fed from a simple 7 day programmable timer. Everything works great. The problem is at night he can hear the contactors humming or buzzing, I told him to turn them off and back on and that usually fixes it for a while but then they start buzzing again. I went and listened to them and they sound normal to me, but I can understand being quiet at night the noise getting annoying and unfortunately the panel is in their bedroom.

    So my plan is to replace the good old contactors with SSR’s. The catch is I want basically an on or off situation and the control side to be 120vac. It's been a while since I took electronics class but aren't most ssr's variable and want something like 24 or 32vdc in? Do they make plain on or off units that could be a simple straight forward swap out for the existing contactors I have in place?
    Brock - Used to have some web sites

  2. #2

    Default Re: ssr replacement for 20A relay?

    I don't believe that the relays should be buzzing unless there is a load problem. The relays may be seeing lower than spec voltage causiing an oscillation effect on the latch coil. There can be carbonization of the relay contacts that create the voltage drop.

    Maybe the fix would be a newer/more efficient set of relays. They can age and sometimes failure can be preceded by buzzing.
    I got nothing else to say...

  3. #3

    Default Re: ssr replacement for 20A relay?

    If you can't find 120v input ssr's then a small transformer, a bridge rectifier, and a capacitor could easily make the necessary dc voltage to run the ssr's. Or maybe run conduit and cabling and a box somewhere else and move the existing contactors somewhere that the noise won't be a problem?

    I'm no electronics guru. Obviously.
    Got Biodiesel?

  4. #4
    Flashaholic* DUQ's Avatar
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    Default Re: ssr replacement for 20A relay?

    You can use isolators for the panel if the sound is traveling through the walls.
    LED's impress but Incans satisfy

  5. #5
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    tvodrd's Avatar
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    Default Re: ssr replacement for 20A relay?

    Hi Brock,

    You might take a look at cat# 8670K8 at McMaster Type the cat# in the search field. It's 3PST-N.O. and rated 35A @ 240VAC. Control voltage is 115-240VAC.

    Larry
    So much for not being able to find my happy a** with both hands and a flashlight! (Do not look into Tank Searchlight with remaining eye!)

  6. #6
    Flashaholic* mudman cj's Avatar
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    Default Re: ssr replacement for 20A relay?

    As tvodrd has pointed out, SSR relays are available with AC control. If you go that route, remember to heat sink them or you will get no where near the rated current before . A good rule of thumb is to stay below 80-85% of the rated current for best results.

    If the heater is cycling power often to hold temperature, then the contacts have probably been through their rated lifetime already. It is not uncommon for mechanical relays to be rated for only 100,000 cycles. A SSR will last much longer in this type of application.


  7. #7
    *Flashaholic* Brock's Avatar
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    Default Re: ssr replacement for 20A relay?

    Thanks those solid state one look like they will do the trick. In checking the load each contactor now is about 7 amps so 35 would more than cover it.

    The relays are new, well a year old now. I have never had a good sized contactor that didn't humm or buzz while powered on. Maybe I am not explaining it correctly, but it is the magnetic noise as the coil vibrates, nothing to do with the contactors themselves. The contacts look clean, I don't think anything is wrong with them, other than where I put them
    Brock - Used to have some web sites

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