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Automotive Question

Discussion in 'General Electronics Discussion' started by Wibble, Mar 19, 2012.

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  1. Wibble


    Mar 19, 2012
    Hello, new here with a basic question.

    I have a thing called an FIA approved Cut-off Switch to fit to a competition car.

    Something intrigued me.

    It is supplied with a 3.3 ohm 11 watt resistor.

    This I am told is to prevent 'voltage spikes' when the main switch is operated and isolates the car battery. (The resistor protects the Alternator Diode pack apparently by providing a load to ground).

    What interested me though is the rating of the resistor. Is 3.3 ohm 11 watts the right rating for this application for a 12 volt system?

    Lots of people state that these resistors get very hot in use and are unreliable, it made me wonder if they are actually rated for 6 volt systems?

    What do people think, is it fine to use or what rating would you recommend?
  2. timothy48342


    Nov 28, 2011
    A bit of quick math yeilds closer to a requirement of 50 watts for a 3.3 ohm resister handling 12 volts, but that would be under constant load. This resister only conducts for one brief second after the engine has been cut off and the engine and alternator are spinning down. I think the designers were able to get away with a lower wattage because of the brief use.

    In this discussion here:
    they are talking about that exact issue and someone said that it is normal for them to get hot. Also, the designers chose 11 watts, so it ought to be fine.

    Now you said, that people are saying that they not only get hot, but sometimes "fail"? So, here is the other side of the coin. I can't think of any harm in any situation to replace a resistor with a higher wattage one of the same ohms. If the 11 watt verison does in fact sometimes fail then maybe an upgrade is in order.

    You'll have to make a cost vs risk judgement. The resister is not is use at all while the engine is running, so it won't affect performance. Even if the resister fails at some point, performance would be unaffected, but you would be at risk of damaging the doide pack on the alternator. (You probably wouldn't be at risk of damaging the doide pack until the NEXT TIME you cut off the engine.)

    So what is the cost of a doide upgrade? What is the cost of the alternator repair? Would you have to replace the whole alternator? I think a bigger resistor would be under 10 bucks. (10 quid lol)

    If the cost of anternator repair is ten times more than the cost of a higher wattage resistor, upgrade it. If nothing else you'll be able to brag about how yours doesn't even get hot like everyone elses. :)

    That's my thoughts anyway.
  3. Wibble


    Mar 19, 2012
    Thanks Tim,

    I'm glad my maths is still OK because that was what was bugging me, I figured the same around 50 Watts assuming battery (Alternator) voltage was a little higher than a nominal 12 volts.

    I had not considered the momentary use which is a good point, but I think I'll spend the 10 quid (bucks) on higher rated resistor (and feel smug that I thought to ask) :D

    Many thanks :)
  4. Wibble


    Mar 19, 2012
    It won't even cost me 10 bucks, found this on Farnell

    £3.84 which is less than most suppliers charge to replace the 11 Watt resistors supplied with the switches. Looks good too and has mounting brackets which is a vast improvement on the resistor supplied with the switch.

    It made me think though (always a problem) - is there a way to indicate the resistor has worked?

    The switch will be mounted on a panel where the radio would usually be in a car. A wire will run down from here to floor level where I will mount the resistor, the other terminal will then go to earth.

    I think I would like some sort of indicator to show the resistor is doing its job.

    Trouble is I won't have any electrical supply from the car at all once the switch is operated (that is its purpose!)

    Any thoughts on a system that might work from the momentary current flowing into the resistor, that would indicate the resistor is functioning?
    Last edited: Mar 20, 2012
  5. Rleo6965


    Jan 22, 2012
    Designers have reason of putting only 11 watts 3.3 Ohm resistor. Maybe it was designed to open if prolonged excessive current in the circuit.
  6. timothy48342


    Nov 28, 2011
    To Wibble:
    On a permanent basis I can't think of a was.

    I can't think of any way to test the resistor while it's installed either. Any testing method that is placed around it would only test the voltage around it. The current through it can be infered, but only under the assumption that it is working. (ie. A 12 volt drop across a functioning 3.3ohm resistor means a current of about 3.6 amps, but a 12 volt drop across a broken resistor would look the same. It tells us nothing.)

    On a temporary basis you could measure the current through it you by disconnecting one end and putting an ammeter in between there. You would want an ammeter that has less than 1/4 ohm or 1/2 ohm internal resistance because your adding that to the 3.3 and it will throw off the measurement. Also it needs to be able to handle the current. (4 amps or so)

    You wouldn't want to leave it connected up that way, though.

    One thing I was thinking that might be kinda cool... A light bulb. Replace the whole thing with a 40 watt 12 volt bulb mounted somewhere visible, so when you hit the cutoff, it glows. (You'ld know its working, It's designed to work at that level all the time. Some of the energy is disipated as light instead of heat. I think it'd be kinda cool.)

    To Rleao6965:
    Open like a fuse? Hadn't thought of that.

  7. Wibble


    Mar 19, 2012
    Thanks guys.

    Rleao6965, that is an interesting thought. I think though it should not be an issue if the rest of the switch works to its design function.

    The main switch removes battery current from the circuit as it is open when the switch is operated. The switch to the resistor is now closed which dissipates current left in the circuit (the alternator is still turning). The alternator should not run on for long though, as a third switch (all in the same unit) interrupts the supply to the fuel pump and ignition coil. So all should be well. They market the unit for 6V, 12V and 24V application. By upgrading my resistor as far as I can see (and I could be wrong!) I will have a situation nearer the intended design specification for 6V but with a 12V system?

    Tim, that could be a solution, my only reservation would be the reliability of an ordinary light bulb filament in a competition environment, be a shame if it cost me the alternator diode pack. I guess I was thinking more along the lines of a capacitor and LED which glowed for a second or two after the switch operation, but I've not got the knowledge to figure out how to do it.
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