New DIY 8 ohm load bank

Discussion in 'DIY' started by maxhifi, Jul 10, 2018.

  1. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

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    Sure! If it gets too hot you can always use 2" standoffs instead of 1". Air is a good insulator.

    Cheers,
    James
     
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  2. triode17

    triode17 Well-Known Member

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    I looked at resistors and there are many easier ways to do this, but at $26 for both channels, you get the 'power load thrift award' of the year.
     
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  3. maxhifi

    maxhifi AK Subscriber Subscriber

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    I don't know what it is about test equipment, but I just hate spending money on it... I think it goes back to starting in this hobby as a kid with no money and lots of time, I just can't lose that mentality.
     
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  4. blfttieq

    blfttieq New Member

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    Got the tank bored and tapped. Got to purty the tank up now. These have a heavy clear coat that is plasticky. The element fits perfect. Now deciding what to fill it with. I'm thinking bout copper bbs and mineral oil.
     
  5. BillWojo

    BillWojo AK Subscriber Subscriber

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    Is that heating element going to be completely submerged? Is the tank going to be sealed or vented? If sealed, what about pressure build up? I'm just worried that under high load your not sitting on a bomb.

    BillWojo
     
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  6. blfttieq

    blfttieq New Member

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    It will be submerged. It will be sealed. I've considered some type of vent. I need to throw some thermal math to it. With copper bbs as the main volume the oil would have very little expansion. I would not be running it with so much power as to be dangerous. All good questions and already on my mind. I also can sit this in a cooler full of ice water if I needed to really push it. I plan to observe temp, but my gut feeling before any thermal math is this should dissipate heat well. I could point a fan at it also. I'm stripping the tank coating and will leave it uncoated.
    Your thoughts?
     

     

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  7. BillWojo

    BillWojo AK Subscriber Subscriber

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    I'm not a fan of heating liquid filled sealed vessels as the pressure can rise at an alarming rate as there is no place for the expansion of the liquid to go. At best it will bulge the cylinder or blow out the threaded plug.
    Are you familiar with hydro testing? Any air trapped in a vessel under testing that fails will result in an explosion.
    A vent tube or a pressure gage with a pressure relief valve to monitor the internal pressure would be a good safety feature. Also, keep the entire heating element submerged or the exposed part is likely to get much hotter, possibly igniting the oil if oxygen is present.
    Not trying to scare you but be mindful of the dangers involved.

    BillWojo
     
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  8. BinaryMike

    BinaryMike Pelagic EE Subscriber

    Be aware that cheap wirewound resistors can have significant nonlinearity issues. If you ever invest in a good distortion analyzer, this problem could conceivably cause out-of-spec distortion measurements on amplifiers that are actually free of defects.
     
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  9. I LIKE MUSIC

    I LIKE MUSIC Super Member

    I like your water heater dummy load project.

    Some general observations.

    The thermal performance of the lower half to two thirds of your tank will likely not be a major contributor to the overall performance of your dummy load.

    The heated oil will try to rise...

    The oil needs to circulate and it will be somewhat difficult for the oil to circulate between the BBs.

    The thermal resistance of the BBs, BB to BB will be rather high because of the relatively small contact area, BB to BB.

    One might expect some amount of stratification of the mineral oil.

    The picture below shows the internal construction of a Heath Kit "Cantenna" 1KW RF dummy load. The metal tube not only provides a high power ground path to the far end of the load resistor, it act as a chimney to aid the circulation of the oil.



    upload_2018-7-13_23-58-29.png
     
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  10. blfttieq

    blfttieq New Member

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    All good info and I respect your opinions. I just dont plan to use this for long periods. But yes I can easily add a gauge and vent. Could also lay it down. These tanks are when filled at 675 psi so the tank is very strong but the threaded element is the weakest link.
     
  11. blfttieq

    blfttieq New Member

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    So what would be best to fill the tank with?
     

     

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  12. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Mineral oil is what is commonly used, but...

    Mineral oil has a specific heat of 1.67 kj/kg°k and a thermal conductivity of .133 w/m°k. That's better than air, but it can't compete with water at 4.19 kj/kg°k and .58 w/m°k.

    Plain water does have its drawbacks, although your heating element is designed to be used in water.
     
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  13. triode17

    triode17 Well-Known Member

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    Same here. I used Radio Shack, 8 Ohm, 20w, Non-inductives, wired series -parallel to get 80 w, 8 Ohms /channel. I also added a small power supply and 1" fan to cool the enclosure.
     
  14. stoN_Cold

    stoN_Cold Super Member

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    Auuuuuuu, you made a science of very simple things. :rockon:
     
  15. WE6C

    WE6C Well-Known Member

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    For the Heathkit Cantenna mentioned above, the duty cycle using transformer oil is the best. I've heard you can get some from your electrical company, from the maintenance yard. Never tried myself.
     
  16. mike567

    mike567 Member

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    Get some heaters from old central air conditioning units. You may have to cut and modify the elements to get 8 ohms as most are about 13 ohms. These heaters come on a rack type setup. just stand it up and convection will take care of cooling. A modified heater will withstand about 1000 watts. Any more than that may require a fan.
     

     

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  17. avionic

    avionic " Black Knights " Subscriber

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    Not in aircraft..
    https://www.mobil.com/English-US/Aviation/pds/GLXXCOOLANOL?p=2
     
  18. avionic

    avionic " Black Knights " Subscriber

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    DSC03546.JPG DSC03545.JPG Left 2 each 8Ω , Right 2 each 16Ω in parallel and 2 each 4Ω in series for .. All non-inductive 300 watt Arcols.

    Right heatsink cannabalize from and 24VDC forklift battery charger.
     
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  19. I LIKE MUSIC

    I LIKE MUSIC Super Member

    If one was looking for the best, it would be nano-particle diamond.
     
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  20. sregor

    sregor AK Subscriber Subscriber

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    (Warning - I haven't tried but considered it) Your basic steam iron. Get one from the thrift store. Measure its resistance.Easy to mount on heat sink.... I would be curious of a lot of these solutions for thermal changes in resistance. Not as bad as light bulbs, but expect resistance to go up with temperature.
     

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