Resistor getting hot

Discussion in 'The Garage' started by opinion914, Dec 6, 2017.

  1. opinion914

    opinion914 Adventurer

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    I'm not as knowledgeable w/ electronics as I'd like to be, (my middle name is actually Lucas) and I have a resistor question. I currently have 3watt 16ohm Ohmite resistors installed in the bike (to bypass the electronic suspension). These resistors get hot, to the point where you can't touch them. The ohms required is set in stone; 16. Would bumping up to a higher wattage help w/ the heat though? Does running 2 resistors in parallel work as well?
    Spank you.

    Kevin
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  2. manstache

    manstache Jed

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    Higher wattage resistors will be built bigger and be less likely to get as hot, as a general rule. You'll change the resistance by putting two 16 ohm resistors in parallel, so don't do that if you want to stick to 16 ohms.
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  3. RomaDakota

    RomaDakota Nixie Me

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    Don't fully know your application. However about your direct questions -
    Yes to both. A higher watt resistor will work and a number in parallel will work. The parallel scenario will require math.
    1/R = 1/R1+ 1/R2 +... In your case R will be 16 ohm.

    edit: if you want 2 resistors the same value, each would need to be 32 ohm.
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  4. opinion914

    opinion914 Adventurer

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    Very much appreciated. This application is for a Ducati Panigale. I've removed the electronic suspension from the forks. The ECU throws a CEL if it doesn't see the forks in the loop. Why this small harness gets so hot, I don't know. the harness goes to small motors at the top of the forks that alter the comp and rebound valving. I'd have guess there would be zero juice running thru this harness unless I was actually adjusting the settings.

    K
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  5. RomaDakota

    RomaDakota Nixie Me

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  6. victor441

    victor441 Long timer

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    Is the resistor seeing 12 volts across it? If so it would be conducting about 0.75 amps and dissipating about 9 watts. Anyway the wheel below shows all the equations derived from Ohms law you need to do calculations. "Watts is watts" but as was said a physically larger power resistor with a bigger surface area and/or a heat sink would not get as hot to the touch.
    [​IMG]

    [​IMG]
    #6
  7. opinion914

    opinion914 Adventurer

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    I have 16 ohm 50watt resistors already, but they're bloody huge and I don't have room for these sausages on the racebike.

    There are 2 harnesses. 1 for the left fork and one for the right fork. Each harness has 4 pins. I'm jumping pin 1 and pin 2 with a 16 ohm resistor and pins 3 and 4 with a 16 ohm resistor and this eliminates the Check Engine Light. I do this for both harnesses. This requires 4 resistors total

    A though has occured to me; can I use one resistor (8ohm ?) To jump multiple pins? Jump pins 1 and 2 from BOTH forks with a single 8ohm resistor and jump pins 3 and 4 from both with another 8 ohm. ?
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  8. shovelstrokeed

    shovelstrokeed Long timer

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    Touch is a bad way to determine the temperature anyway. 140° F is about all you can hold your finger on and that isn't very hot in the resistor world. 50° C rise is usually OK and that is over ambient.
    As stated above by others, just get a higher wattage rated resistor and handle the heat by increasing both surface area and air flow over the unit.
    I do find it odd that the current flow is that high, it probably isn't with the motors but the electronics that control the motors are looking for some low voltage, hence the 16 ohm value.
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  9. shovelstrokeed

    shovelstrokeed Long timer

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    I doubt it but would have to see the motor control circuit to know and that may be problematic to obtain.
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  10. opinion914

    opinion914 Adventurer

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    That's what I'll do then. There are some 10 watt cerment resistors that may fit.
    I did create a sketch illustrating the other thought about using only 2 resistors but I think I'll stick w/ the current scheme. I'll still post the pic amd you let me know if i was on to something or if I'd end up Chernoble-ing my bike.
    20171206_183802.jpg
    #10
  11. broncobowsher

    broncobowsher Long timer

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    Without getting into the full wiring schematic it is hard to say what would happen. The circuits are probably separate so the computer can test each fork. Combining wires will likely let the magic smoke out.
    #11
    victor441 likes this.
  12. GreaseMonkey

    GreaseMonkey Preshrunk & Cottony

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    As mentioned, what is "hot" to most resistors is "way too hot" for most fingers to hold. If you are concerned, I'd just figure out a heat sink to attach to what you have. It could be simple as smearing some heat sink compound, or if you don't have any handy just use some zinc oxide diaper rash cream on one side and zip tying them to your frame or other piece of exposed to air metal, or what I'd do if I wanted to be cheap is just find some heat sink material and sandwich the resistors between 2 pieces. Remember, even if you get a "bigger" resistor, it will still have to dissipate the same amount of heat. Good luck!
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  13. RVDan

    RVDan Long timer

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    Sounds like you're wasting electricity, and therefore horsepower by having the system heat up the resistors. For some reason the electronics are trying to make the motors run.

    Is the ohm rating really set in stone? I'd try a light bulb instead of a conventional resistor. We know a light bulb won't overheat.

    How big are the motors that used to be hooked up? Can you just got the suspension and hook up just the motor?
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  14. Salsa

    Salsa Been here awhile

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    First measure the resistance of your resistor. Then measure the voltage when it is hot.
    Watts = V * V / R

    What is the Voltage, Resistance and Watts ?

    Now we know what we are talking about.

    Don
    #14
  15. opinion914

    opinion914 Adventurer

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    And that's the part that rubs me the wrong way. In addition to the CEL, It's the potential for this resistor heat to be stealing even a milliamp of juice from my sparkplugs that concerns me. Maybe I'll just ride with an open loop on the suspension harness and a CEL and ditch this resistor idea. Plan B may be to find rashed forks and harvest the motor from them.
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  16. opinion914

    opinion914 Adventurer

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    Don, the resistance I can figure out, but how do I measure the voltage? Tap ground somewhere with the NEG lead and tap the resistor (before or after) w/ the POS lead?
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  17. Grinnin

    Grinnin Forever N00b

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    A bigger 16-ohm will create as much heat as the smaller 16-ohm resistor. It'll just dissipate the heat better and be less likely to fail.
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  18. Salsa

    Salsa Been here awhile

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    Measure it (put your meter leads) on each end of the resistor.

    Don
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  19. Beezer

    Beezer Long timer

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    what you have already works. if you want something more efficient then somebody has to look at the diagrams and understand the system in order to rig it.

    the Spenardo method (with no diagram) would be to read the volt drop on each resistor (or read current through each resistor and calculate). measure volts as described above. if the total ships voltage is dropped on the resistor, then thats the end of that avenue because the system apparently needs the current flow to operate. if on the other hand, lets say the resistor only drops 6v, then the sensors may only need to 6v to keep from going into panic mode. hook in a regulator chip like an LM7806 (the 06 means 6v) & see if that will satisfy the brain box. 12v in, a regulated 6v out.... current would be minimal and the chip is the size of your thumbnail. the LM78xx series chips can handle an amp or two

    also, if the resistor drops 4 volts, then you want a 7808.... to give the remaining 8v to the brain box.

    if the resistor drops 10v, you want 7802.

    if the resistor drops the same as the battery voltage this won't work
    #19
  20. opinion914

    opinion914 Adventurer

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    Placing the + lead of my multimeter on one side of the resistor and the - lead on the other, when powered up and hot I'm getting between 4.6 and 5.3 volts on the 4 resistors.
    They're 16.2 ohm resistors, rated at 3 watts.
    What's this tell me? I'm intrigued by the regulator chip route.
    #20