DC Timers, interesting phenomenon

Discussion in 'Electrical Systems' started by missinginaction, Aug 26, 2013.

  1. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    I'm doing my wiring and I'm just curious about this.....

    I've wired in some computer fans to move air around the v berth and the electrical cabinet. They work well. I'm curious about something I noticed with the DC timer that I used.

    http://www.amazon.com/gp/product/B008999RYY/ref=oh_details_o03_s00_i02?ie=UTF8&psc=1

    I feed power to the timer from a 5 amp, 24 hour breaker such as would be used for an automatic bilge pump. In series with the timer, between it and the fans I wired in a couple of SPST switches from Carling. The reason for the switches is so I can set the timer to "manual" when I'm using the boat and activate the fans myself as I please. Everything functions as it should and tests out fine.

    One thing puzzles me. With the timer off and the SPST switches in the on position the fans correctly do not function. However when I happened to check for voltage at the fans I got a reading of about 0.80 volts. Switch the timer on and I get about 13.8 volts and the fans run. Switch the timer off and set the switches to off and I get 0.00 volts.

    Why do I get that small fraction of a volt in the circuit when the timer is off but the switches are on?

    I'm not concerned about it, just curious. BTW I'm using a good multimeter (Fluke) so I'm not concerned about that aspect of things.

    Thanks in advance for any responses,

    MIA
     
  2. bcervelo
    Joined: Sep 2005
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    Location: Somerset West, SA

    bcervelo Junior Member

    Is it possible you are reading the voltage of the internal battery in the timer?
     
  3. jonr
    Joined: Sep 2008
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    Location: Great Lakes

    jonr Senior Member

    I expect that it is using a mosfet, not a relay. So you get a tiny bit of leakage.
     
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  4. missinginaction
    Joined: Aug 2007
    Posts: 781
    Likes: 76, Points: 28, Legacy Rep: 512
    Location: New York

    missinginaction Senior Member

    Thanks for that jonr, I did a little research on field effect transistors and believe you are right.
     
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