Dissapating heat.

Discussion in 'Electronics & Electrics' started by Back Door Shen., Jul 11, 2010.

  1. Back Door Shen.

    Back Door Shen. Member

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    Not sure as to where this should go but i'll place it here as its semi related.

    Fixing a problem on my car requires a aluminium plate with two 1ohm 100w resistors. Apparently this gets up to around 100degreesC in england. Since our summers are hotter i would like to give it some additional cooling. Unfortunatly the mounting space is covered with not much wind or air flow from what i can gather so was thinking of adding some fans or fins to help it. Does anyone have any suggestions to remove the heat or know where i can get some weather proof small fans?

    Picture of heatsink and resistors
    [​IMG]

    Cheers
     
  2. Menthu_Rae

    Menthu_Rae Member

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    What temperature do you want to keep the resistors at above ambient?

    Also, what orientation are the resistors in? Is that picture from above - looking down? If they are mounted vertically rather than horizontally you can make better use of natural convection.

    EDIT: BTW are they running in series or parallel (looks like parallel)? If parallel then you are pumping ~144W through them... for a nominal voltage of 12V or 196W for a nominal voltage of 14V =\
     
    Last edited: Jul 11, 2010
  3. mittons

    mittons Member

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    what about changing the style of heat sink ?
    ie making it more enclosed, then adding fans that may not need to be ip rated.
     
  4. mittons

    mittons Member

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    Looking at the pic, they look like they're parallel. Assuming from the 4 wires running back.

    Also, what is the purpose of the resistor ? Are you running led lights or is it to do with an ECU or something
     
  5. OP
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    Back Door Shen.

    Back Door Shen. Member

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    Not fussed at what tempurature just trying to keep it low as possible. The resistors fix internal resistors in the radiator for slow speed that allready burnt out. Therefore the cooler it can be the better it should run and less chance of also burning out.

    The picture is taken looking up underneath the car so the plate is sitting horizontally with the heatsinks closer to the ground. The reason they are mounted like that is for convenience. No hole drilling and the space is allready avaible.

    They are in series with roughly 7.5A running through each one. They have been rated by other people so i'm not to fussed about that.

    No in series with 2 fans so one for each fan. As above.

    Yeah thats an option but would require a fair bit of fabrication to make it fairly weatherproof. Other option was to reroute some air from a vent in the bumper.
     
  6. dephilile

    dephilile Member

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    Just some ideas,
    -Using some black anodised aluminium would work better than silver
    -Cutting grooves in it to work as fins would increase the surface area
    -Putting silicon between the car and the aluminium would act as a crude thermal paste (but work better than nothing) and help transfer heat to the car body.

    Out of interest, what are the resistors for?
     
  7. YoungFarting

    YoungFarting (Banned or Deleted)

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    Bolt them directly to the cars metal work.
    Biggest heatsink there is.
    Providing you can get a nice flat mating surface.
    See if you cant place them in the air flow from the fans.
     
  8. LINUX

    LINUX Member

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    Except steel isn't the best thermal conductor in the world. It might work better or it might insulate them. Ever seen a blade in a furnace? The end not sticking into the furnace isn't exacltly red hot.

    I'd suggest going to Bunnings and buying the biggest aluminium strip you can get (~50x1000x3mm should do it ~$10 rough guess) then fabricate a simple heat sink from them. You can drill holes in the correct spots to mount the strips between the resistors and the existing plate, along with thermal grease.

    The easiest way to get the strips to dissipate heat would be to cut slits in each end with a hacksaw (mouning the resistor about midway) and bending the resultant "fins" up/down alternately. Look at a few PSU heatsinks, they do it this way because it's trivial to just stamp them out rather than machine them.
     
  9. mittons

    mittons Member

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    ^ Good advice here ^
     
  10. paulvk

    paulvk Member

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    If this is to change the speed of DC fan motors I have designed and built a unit which varies the speed of my radiator fans depending on the temp of the motor it does not produce the heat you will and controls 45 amps.
     
  11. OP
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    Back Door Shen.

    Back Door Shen. Member

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    Haha never thought of that really good idea cheers mate :). Will be able to cover most of the area and get it easily custom made. Simple ideas always good.

    Bit late as allready have the Sheet made tho. I do have thermal paste at the moment and if i have spare left over will try and mate it to the car.

    Read above for their use.

    I think its just easier to have them on their own heat sink easier to remove and install. Was thinking of a small computer fan to move some air but unsure if there is current airflow overthem.

    The car controls the fans just the internal resistors have burnt themselfs out as a common problem and this is an diy fix.
     
  12. LINUX

    LINUX Member

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    How much airflow do they get already? If there's any minor flow at all when the car's in motion then a computer fan will do very little for their temperature. To avoid the fan you will require enough thermal mass in the heatsink that it won't overheat when you're, say, waiting at traffic lights.

    Basically going from no airflow to *any* airflow dramatically reduces the temperature. Going from low airflow to high airflow doesn't reduce the temp anywhere near as much.
     
  13. OP
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    Back Door Shen.

    Back Door Shen. Member

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    Unsure as they arn't in direct line of any opening in the bonnet. Only thing that could cause some air flow is that the other side is fairly open to the engine bay which has a large amount of airflow due to the radiator and grill opening which could blow some air over it. So unsure as to how much will go over it. You identified the problem to when waiting in traffic its not going to be able to dissapate it very well and could overheat but i will have to see.
     
  14. mittons

    mittons Member

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    not able to mount closer or inline with the fan or shroud ?
     
  15. RobRoySyd

    RobRoySyd Member

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    I've had some experience with a similar problem. I use PTC heating elements bolted to quite large heatsinks with fan forced airflow in drying ovens.

    Problem I quickly discovered I had was the heaters would get very hot and the heatsinks didn't. This was because the anodised finish on the heatsink is a quite good thermal insulator. Thermal paste does not help much if at all.

    So what I'd suggest you do is to diligently lap away that gold anodizing on the bottom of the resistors and do the same with the area of the heatsink where they make contact. Pretty much the same as lapping a CPU to its heatsink. Then use a THIN smear of thermal paste.

    If that is still not enough then try creating a sandwich of two sheets of Al with the resistors in the middle. Have both the bottom and the top of the resistors contacting a sheet of Al. Those resistors should withstand well over 100degC and that sized heatsink should dissapate quite a lot of heat at around that temperature but the trick is going to be to reduce the total thermal resistance between the actual resistance elements and the heatsink itself.
     
  16. paulvk

    paulvk Member

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    My fix was to replace the fan/clutch on the water pump driven by the belt (get rid of it!) and go to 2 electric fans, so very much diy but your car appears to have my temporary fix which was 2 speeds slow and flat out with a simple thermostat switch but when moving at speeds of 50k or more no fan was needed so for most of the time now my fans do not rotate at all, this reduced my fuel bill as well!
     
  17. breech

    breech Member

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    We use those at work and bolt them to aluminium heatsinks but if they are being run under their rating they will be fine. lapping and thermal compound helps. Make sure those circuits are fuse protected. IMHO fans are not needed.
     
  18. OP
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    Back Door Shen.

    Back Door Shen. Member

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    Huh? That really doesn't apply to my car. This is a well known issue and this is the best fix for it.

    They are running at 12v and 7.5A max which should be around 90W which is 10W under the rated so should be ok. Just worried about heat soak when stationary in traffic. But going to see how it goes at the moment and will see if it needs additional in the end. Their hopefully should be airflow due to the engine bay.
     
  19. paulvk

    paulvk Member

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    Can you tell us the make of car and point to site telling of this fix or a wiring diagram I am curious?
     
    Last edited: Jul 12, 2010
  20. OP
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    Back Door Shen.

    Back Door Shen. Member

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