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Switch activated by Amps

Discussion in 'General Electronics Discussion' started by Farticus, Jun 25, 2016.

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  1. Farticus

    Farticus

    36
    1
    Jun 21, 2015
    I have a 240vac to 24 vdc 20A power supply which has an internal fan which is continually ON.
    95% of the time the power supply draws only 5 A and could easily run without the fan.
    Is there some kind of switch or device I could insert into the circuit which would activate the cooling fan only when the current draw exceeds, say 7 Amps, under that Amps the fan would not operate.

    Thanks in advance
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
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    Jan 21, 2010
    Even better, the fan would only come on if the temperature exceeds some temperature.

    What voltage does the fan run at? If it's 12V then I'd go looking for devices designed for use in PCs.

    Here is a simple option that would run the fan slower when cooler. This can be simply redesigned for 24V. It won't stop the fan when it's cool, but it will slow it down. The only issue is what value thermistor and resistor to use -- it may require experimentation.
     
  3. Farticus

    Farticus

    36
    1
    Jun 21, 2015
    Thanks Steve. That seems an elegant solution .Appreciated
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
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    Jan 21, 2010
    I later thought of a low temperature thermal switch after uncovering some 85C devices as I was cleaning up.

    Here is an example:

    https://www.rockby.com.au/catresultssQL_12.cfm?Searchkey=thermal+switch&stock_no=26833

    This isn't a good price, and I might try to get something with a lower temperature unless you can safely connect it to an appropriate heat sink.

    I'm not sure though, the fan cycling on and off may be more annoying than one which is constantly on at a low speed.
     
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