How to use a relay to turn a fan off and on when a 12v battery is plugged in

Discussion in 'Digital Circuit Design' started by JaredNathaniel, Mar 8, 2018.

  1. JaredNathaniel

    Thread Starter New Member

    Mar 7, 2018
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    Backstory: I have a large table that i discharge multiple batteries on, when they are discharging they emit heat so I am looking for a way to push the hot air out from under the table.

    What I am doing: I am looking to turn a cooling fan on that uses a 120v wall socket, but in the interest of saving energy I only want it to turn on whenever a battery is plugged in. The batteries are 12v, so I would like to use a relay that is triggered by that 12v to then turn the fan on. I need help in choosing what relay would be best for this and the circuit design for this. Sorry I am quite new to this so thank you in advance for any help.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I've used a comparator (such as LM339, a quad comparator IC) to detect the presence of a connected battery in a charger. There are many other ways but in my circuit the comparator was already there anyway. One advantage is that the comparator is very sensitive and will not consume hardly any current. If you use the quad LM339, you could even detect 4 different voltage levels and light up indicator LEDs.

    If you go this route, the chip needs DC power. It can't control anything more than an LED without an external transistor, so you'll need one such as a MOSFET switch to control your relay. If you want to get fancy, it's not hard to build in a delay-off feature that will keep the fan on for a while after the battery is disconnected.
     
  3. JaredNathaniel

    Thread Starter New Member

    Mar 7, 2018
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    So when the battery is plugged in, it is discharging and giving off 12v. Would what you are saying work for this application? Sorry if that sounds dumb.
     
  4. LesJones

    Well-Known Member

    Jan 8, 2017
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    Why not just use a thermostat that switches the fan on when a preset temperature is exceeded ?

    Les.
     
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  5. wayneh

    Expert

    Sep 9, 2010
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    A comparator compares two voltages that must both fall between its power supply rails. You would set the comparator to watch a low "reference" voltage on one pin and the battery voltage on the other. There are many ways to create the low reference voltage, such as a voltage divider from your power supply. You might set it to 1V, say. When you connect a battery, the test pin will see whether the battery voltage exceeds the reference. I'll dig up a drawing if you want. There are many ways to set it up.
     
  6. JaredNathaniel

    Thread Starter New Member

    Mar 7, 2018
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    If you could find a drawing that would be so helpful thank you so much!
     
  7. JaredNathaniel

    Thread Starter New Member

    Mar 7, 2018
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    That was the original thought. The table is in a shop like setting that is not temperature controlled, so in the summer when it gets hot it may be running constantly. Which would not be an issue, but energy consumption was an issue. Do you think that the ambient temperature underneath the table would be greatly effected by the heat in the summer? Because this would obviously be the easier solution.
     
  8. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    What about a 12 V relay? SPST or SPDT. When the battery voltage drops below 6 V -ish the fan turns off.

    ak
     
  9. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    You can use a Tl431 as a comparator , in the circuit below the relay is on until the voltage drops below the set limit, you can omit the zener if you want to use a 12V relay. You can add hysteresis with an extra resistor .

    TL431.jpg
     
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  10. LesJones

    Well-Known Member

    Jan 8, 2017
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    You do not explain the reason for discharging the batteries. (I am assuming thete these are 12 volt lead acid.) Do you not allready have a voltage detector on each battery that disconnects it from the load before it becomes over discharged ? If so could you not use a signal from this circuit to activate the fan ?

    Les.
     
  11. Reloadron

    Distinguished Member

    Jan 15, 2015
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    Since the object seems to be heat removal is there any reason you can't just use a thermostat or temperature system to turn your cooling fan On/Off? I can buy a cheap non programmable bi-metallic type thermostat for under $20 USD at about any home improvement store. Most have heating and cooling so set it for cooling and when the temperature in an area exceeds the set point a fan comes on. Just seems a simple turn key solution unless you are concerned with battery voltages and such.

    Ron
     
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