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Thermostatic switch work for AC or DC?

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  • Thermostatic switch work for AC or DC?

    The thermostatic switch in the 30 year old solar space heater that came with the house is working fine. Just wondering if a DC blower motor was used will that thermostatic switch also activate and work with a DC blower motor. Thermostatic switch is connected to blower and blower connected to thermostat switch on the wall. Kept the house at 60 some degrees today with a high in single digits. I also have thermostatic switches that came with some DC fans. But the thermostatic switch that's installed is deep in the collector, probably hard to get to.

  • #2
    Originally posted by Brian53713 View Post
    The thermostatic switch in the 30 year old solar space heater that came with the house is working fine. Just wondering if a DC blower motor was used will that thermostatic switch also activate and work with a DC blower motor. Thermostatic switch is connected to blower and blower connected to thermostat switch on the wall. Kept the house at 60 some degrees today with a high in single digits. I also have thermostatic switches that came with some DC fans. But the thermostatic switch that's installed is deep in the collector, probably hard to get to.
    A lot depends on the current and voltage that is being switched.
    An AC snap switch or thermostat contacts only need to open fast enough that they are far enough apart the next few times the current goes through zero (every half cycle) that the arc between the contacts will not restart as the voltage rises.
    A DC switch has to move the contacts apart far enough fast enough to quench the arc by distance, since the current will never, by itself, go to zero.

    The result is that switches that are rated for both AC and DC will typically have a higher current and voltage rating for AC than for DC.
    But if the current and voltage are low enough the switch or thermostat should work just fine.
    A switch designed specifically for DC may have different contact metal alloys, greater separation, etc.

    Are the thermostats switching line voltage or some lower control voltage? What will the current through the fan motor be?
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      AC, probably 110v. I don't know the wattage . The ones that came with my DC fans, claimed to be 80 degrees Fahrenheit turn on, 65 degrees Fahrenheit turn off. Are the ones that are installed with this AC blower motor probably that same temperature range? Because they seem to be activating at about that temperature range. And is that standard for these types of thermostatic switches?

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      • #4
        Originally posted by Brian53713 View Post
        AC, probably 110v. I don't know the wattage . The ones that came with my DC fans, claimed to be 80 degrees Fahrenheit turn on, 65 degrees Fahrenheit turn off. Are the ones that are installed with this AC blower motor probably that same temperature range? Because they seem to be activating at about that temperature range. And is that standard for these types of thermostatic switches?
        Switches can be easily made with that sort of temperature differential (hysteresis) built in.
        My guess is that you would be OK with a lower voltage (5-12V) DC blower, but the current might be too high for the contacts if it was specified for a 120V AC fan.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

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        • #5
          Thank you for all the help. Is thermostatic switch the right terminology ?

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          • #6
            The general rule of thumb is once you get over 30V switching DC can be a problem with arcing in mechanical switches. You can now buy FET solid state relays to switch those dc voltages. But, another concern is contact current. If switching current and/or voltage is too low, mechanical contacts can oxidize over time and not conduct. A problem for a remote switch.

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            • #7
              Originally posted by PNPmacnab View Post
              The general rule of thumb is once you get over 30V switching DC can be a problem with arcing in mechanical switches. You can now buy FET solid state relays to switch those dc voltages. But, another concern is contact current. If switching current and/or voltage is too low, mechanical contacts can oxidize over time and not conduct. A problem for a remote switch.
              It is a problem; solved in info industry by gold plating "dry" contacts. I have tried using a big shunt resistor to raise
              the switched current maybe 2 orders of magnitude. But a better solution might be a solid state temp sensor, which
              also allows setting the hysteresis. Bruce Roe

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