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12v Automotive Cooling Fan Indicator Light

Discussion in 'LEDs and Optoelectronics' started by Paul92, Dec 29, 2016.

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  1. Paul92

    Paul92

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    Dec 29, 2016
    Hello,

    I don’t know much about electronic circuits and need some help with a project.

    I have a ’66 MG Midget and have replaced the belt driven cooling fan with an electric fan. The relay which turns the fan on and off has two separate output wires which are energized when the coolant reaches a preset temperature. The relay is designed to operate a single or twin cooling fan, which is why it has two separate outputs.

    I am only using a single fan and would like to use the unused output wire to run an indicator light on my dash so that I can see when the fan is running. Normally, at highway speeds the radiator would receive enough airflow to cool the engine without the fan being energized. I cannot hear the fan run so I would like to monitor its operation via the indicator light.

    I hooked up a 12V LED to the unused output wire. In the garage the light works well. When the fan turns on the indicator light is illuminated. When the fan turns off the light goes out. But when driving the car it does not work that way. The light does not go out or it operates sporadically, sometimes not fully illuminated.

    I am thinking that when traveling at highway speeds the wind through the radiator is blowing across the fan blades causing them to turn. With no power to the fan motor it begins to act as a generator when it is being turned. My guess is that this creates enough electrical feedback to energize the LED bulb.

    Could this be possible, and if so, what can be done to only allow the LED to receive power then the fan is energized? Would a resistor in-line work, or is there some device that would prevent the flow of current until it receives a full 12 volts from the rely?

    I will appreciate any advice you can give.

    Thanks,
    Paul
     
  2. Bluejets

    Bluejets

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    Oct 5, 2014
    A hand drawn circuit of what you have will get more help.
    Instead of telling us what you think, show us what you have.
    Must remember many of the automotive systems are earth return and could present problems to the uninitiated.
     
  3. duke37

    duke37

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    Jan 9, 2011
    If the relay has two sets of contacts, use one for the fan and the other for the indicator. Alternatively, connect the led across the relay coil with a resistance.
     
  4. Paul92

    Paul92

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    Dec 29, 2016
    Bluejets, sorry I didn't think to send a drawing. This should make it easier to understand, although this is not exactly what I have. If you look at the blue wire it does not go to a second motor. It goes to a LED bulb. 1-fan control.jpg
     
  5. duke37

    duke37

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    Jan 9, 2011
    Air conditioning in a 1966 MG Midget seems unlikely:)

    There is no information as to what is going on inside the control box to find out what is back feeding.
    Assuming the car has a 12V system, you could put a Zener diode in series with the led/resistor to drop 6V or so. Only when going fast, will there be sufficient voltage to light the led and you will be looking at the road then (I hope).
     
  6. Bluejets

    Bluejets

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    Oct 5, 2014
    Use another small auxiliary relay on the existing fan motor to drive the LED.
     
  7. Alec_t

    Alec_t

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    Jul 7, 2015
    Agreed. The simplest solution.
     
  8. Paul92

    Paul92

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    Dec 29, 2016
    Pardon my lack of understanding on these things. Please re-read my first sentence in the post.

    So I think you are saying I can add another relay or add a component or components to the circuit.

    I get how the relay would work, the feedback, or whatever it is, will not be strong enough to operate the relay. After the sensor reaches the set temperature, the circuit will be energized with the full 12 volts and the relay will close, completing the LED circuit. If I understand that, I see the downside of this approach is adding another relay, socket and associated wiring.

    My preference would be to add a component(s) to the existing circuit but I don't understand the suggestions. My understanding of a diode is a device which will allow current to pass through in only one direction. <<Zener diode in series with the led/resistor to drop 6V or so.>> I have no idea what a "Zener" diode is. Nor do I understand what you mean by "led/resistor". Are you saying I should add both a diode and a resistor into the circuit? Is the diode in series with the blue wire on the + side or the black wire on the - side? Which way does the diode point? What size resistor should I use?

    <<Alternatively, connect the led across the relay coil with a resistance.>> Not sure this is possible. The relay is in a socket and the "Control Box" is encapsulated and cannot be opened. I had figured the unused blue wire was connected across the relay.

    Duke37, my MG has the same kind of air conditioning that your tractor has. ;) The yellow wire is not connected. The drawing above comes from the fan manufacturer and is not specific to any particular car.

    The question in my mind still remains, where is the current coming from? My guess is, and it is just a guess, is that when the fan is being turned by the wind, electricity is being generated which travels back to the control box along the orange wire where it connects with the blue wire. The electricity then travels down the blue wire, across the LED, and finds it way to ground. I also assume the erratic nature of the LED operation is due to insufficient voltage/current being generated. Does that sound right?
     
  9. Bluejets

    Bluejets

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    Oct 5, 2014
    Leds being small current devices are susceptible to any small feedback giving spurious results if you connect across tha actual fan as you originally explained.
    What you need to do is as explained, remove the LED from the actual fan load circuit and place in the "control" circuit. This isolates it from the problems you described.
     

    Attached Files:

  10. Paul92

    Paul92

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    Dec 29, 2016
    Bluejets, I am not sure that I am able to connect the LED to the control circuit as in your drawing. See my photo of control box with relay. The control box is totally encapsulated.
    Control box.jpg

    You show F1 and F2 in your drawing. I assume those are the fans. I only have one fan motor. So substitute the LED for F2 and the drawing will then be correct. That would show that the LED is on a different wire than the fan motor, but since we don't know how the control box is wired, we don't actually know if the LED is on a separate circuit. Correct? If the control box were wired as in your drawing, there would be no connection between F1 and LED because both circuits are open at the relay, and therefore no path for the electrical feedback. Is that correct?

    My assumption is that both the blue wire and the orange wire have a common connection inside of the control box. If that is the case, I can see how current generated at F1 could be backfed into LED. The only thing that I know for sure is that when the sensor (in the first drawing) reaches the set temperature and the relay is activated, both the blue wire and the orange wire measure approximately +13.5V with a fully charged battery.

    Back to your drawing, I see what I think is a resistor and a LED in parallel with the coil that energizes the relay. To do that I would have to have access to the wiring inside of the control box, which I do not.

    If both the orange wire and the blue wire share a common connection inside of the control box then I could see how feedback could go to the LED. If that is the case, would it make sense to put a diode in series with the orange wire allowing the current to flow towards F1 when the relay is activated but not allowing any current to flow back towards the LED when the fan motor is spinning freely?

    Again, I don't have an electrical background. If I am being a total dunce, I apologize. :(
     
  11. duke37

    duke37

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    Jan 9, 2011
    A diode allows current to pass in one direction and not the other. A diode in series with the motor will stop back feeding but will drop about 1V. However, 11V should be enough for you. The diode will need to be fat enough to pass the motor current, 5A should do. A convenient place would be the fuse holder in the orange wire.

    A zener diode is similar to an ordinary diode but will allow current to pass when their blocking voltage is exceeded, they are available in voltages from about 3V up. They are usually rated for small currents but plenty for an led. The idea was to drop about 6V of the back feed so the zener would only pass current to the led when the back feed was over 6V.

    I have connected wires to a relay coil by soldering. The relay will then be limited on how far it will plug in, so lose about 1mm. This was to check whether my petrol pump was being energised. (The trouble was a faulty MCU).
     
  12. Alec_t

    Alec_t

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    Jul 7, 2015
    You don't need a second relay. What has been suggested is that you connect the LED (which if rated for 12V already has a series current-limiting resistor) to the switched positive terminal of the coil of the relay which turns the fan on and off.
     
  13. Bluejets

    Bluejets

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    Oct 5, 2014
    Op says he cannot access the relay ........
    Therefore an aux relay on the fan motor.
     

    Attached Files:

  14. f_richey

    f_richey

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    Sep 20, 2019
    Paul, what did you end up doing? putting a diode on the relay coil or did you run the LED from the switched positive of the relay coil?
     
  15. Paul92

    Paul92

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    Dec 29, 2016
    f_richey, ultimately I did nothing. I have about 10k miles on the fan install and the temp setting seems to be properly adjusted. When I pull off the highway the fan is normally not running. When I come to a stop light in hot city driving I normally hear the fan running.
    This was a ground up restoration and the fan installation is well hidden. I did not want to add additional components/wiring that would call attention to themselves and I wasn't completely sure what to do, so I did nothing. :)
     
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