Car Relay CC v'age

Discussion in 'General Electronics Chat' started by Kedishen, Nov 5, 2013.

  1. Kedishen

    Thread Starter New Member

    Aug 28, 2012
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    Hi all

    Will the control circuit of a 4pin car relay still function at more than 12v range ie 21v? Load will be 12v range.


    Currently

    I have fit 3 low consumption fans on the 12v leisure circuit of my camper.

    These fans will help cool the solar regulator and the fridge during hot weather.

    At the moment they are controlled by inline switches for manual control.

    Goal

    To only power fan circuit when solar panels are producing more than 12v.

    At night, the voltage drops below 12v and during day, voltage is up to 21v (and later I will join 2 panels in series for up to 42v).

    Can I achieve this by wiring the control circuit of a bog standard normally open 4pin car relay to the solar wires before solar regulator?



    Thanks

    Scott
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Maybe: The Pull-in voltage of a standard automotive relay is about 10V. The Drop-out voltage will be much lower, about 4V. If you put an appropriate value of resistor in series with the coil, it might be possible to make the pull-in ~20V, but then the drop-out would still be 8V, which would happen after the sun went down.

    Can you measure or tell us the coil resistance of the relay you intend to use?

    One problem I see is that the relay coil current will take a portion of your solar panel's output. An electronic switch circuit could be made much more efficient.
     
    Last edited: Nov 5, 2013
  3. Kedishen

    Thread Starter New Member

    Aug 28, 2012
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    Thank you Mike. I see no further reason to go down the relay route.

    Have you an example of an electronic switch circuit that would work or be similar?
     
  4. Kedishen

    Thread Starter New Member

    Aug 28, 2012
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  5. MikeML

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    Oct 2, 2009
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    So the fan(s) should be powered from the +12V battery post, but only when the solar panel voltage is >16V?
     
  6. Kedishen

    Thread Starter New Member

    Aug 28, 2012
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    Yes thats about it.

    I only want power to the circuit when solar panel sees sunlight, which will produce up to 21v.

    On days when I want the fans on, chances are panel will be at full voltage all the time (21v), unless weather is cloudy too but will most of time see at least 13-14v even in British winter.

    The purpose for all this is I don't want fans to run at night or during heavy overcast even if I have them switched on. I may not always be there to shut them off and don't want unnecessary battery draw.

    Some sort of adjustable thermistor would be ok, but just because its warm in the van doesn't mean its warm outside.
     
  7. Kedishen

    Thread Starter New Member

    Aug 28, 2012
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    If i need to keep it in the 12v range, then when solar panel is seeing sun, batteries are at 14.5ish volts and drop to about 13.1 when no sun and fully charged.

    In the end I don't care if fans are left on, long as the batteries are being charged at the time.
     
  8. eyesee

    Member

    Oct 19, 2013
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  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here you go.

    Turns on at 15.9V; turns off at 15V. Snap action. Hysteresis. Draws less than 1mA from the panel. Drives the PFet with 9V gate signal. Switches 3A.

    The PFET is here.
     
  10. cornishlad

    Member

    Jul 31, 2013
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    The actual voltage present at the solar panel output may be a lot lower than is being assumed because they have a fairly high internal resistance. AFAIK 21v is only seen when they are open circuit. You would need to measure the solar voltage when under load - ie charging the battery at a reasonable rate - to be sure of getting the switching point right.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Cornish is right. A panel that has ~21Voc will likely have a mppt of ~13V, so the trip point for the solar voltage present detector could be set lower by using a 10, 11, or 12V Zener in lieu of the 15V one in the simulation, above.
     
  12. Kedishen

    Thread Starter New Member

    Aug 28, 2012
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    Some interesting ideas. Thanks for the input everybody.
     
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