Advice on building a dummy resistor load

Discussion in 'DIY' started by SaSi, Sep 29, 2015.

  1. SaSi

    SaSi Seriously Illogical Subscriber

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    If I wire several high power (say 10W) resistors in parallel, I can easily figure out what the total resistance value will be (1/R = 1/R1 + 1/R2 + ... + 1/Rn) with the total power "capacity" being the sum of the individual ratings, so 10x 10W resistors will give me 100W capacity, right?

    Now, what happens if I wire two such blocks in series? Total resistance will be the sum of the two but what about power capacity? Will that double as well?
     

     

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  2. DRecovery

    DRecovery Pants-wearer Subscriber

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    The power will only share evenly if the resistors are identical, otherwise you are right.

    The second block of parallel resistors will double the power capacity.

    The situation can be a bit more complicated as power ratings often have caveats. All assume they are allowed to reach obnoxiously high temperatures in the 150-250C range, the high end of which is usually unacceptable for non-cooking related applications. Finned resistors may assume some amount of forced air flow to reach their power rating, and most assume a certain amount of dead space around them. Pack therm in too closely and they will reach peak temps before their max power rating and start scorching the board, wires, caps, etc., basically you then have a Pioneer receiver power supply, really.
     
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  3. gadget73

    gadget73 junk junkie Subscriber

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    You can also cheat and dunk them in liquid to increase capacity quite a bit.
     
  4. SaSi

    SaSi Seriously Illogical Subscriber

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    Thanks, I need to burn about 150W of power at 4Ohms.
    Will look for high powered resistors to use as few as possible.
     
  5. whoaru99

    whoaru99 Epic Member

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    I've seen people make high power dummy loads by using water heater elements mounted to the pail cover and submerged in 5gal pails of water.
     
  6. avionic

    avionic " Black Knights " Subscriber

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  7. blhagstrom

    blhagstrom Mad Scientist, fixer. Subscriber

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    ?!?!?!? water heater elements are 4-8 ohm????
     
  8. whoaru99

    whoaru99 Epic Member

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    Series-parallel arrangement. In this example primary use was 4 ohms testing pro audio amps. 4 was not hit exactly, but 4.32 ohms is pretty close and knowing that it's easy enough to normalize the results.


    [​IMG]

    It's switchable to 8 ohms and 2 ohms as well. Again, neither were hit dead on but like 8.4 and 2.2 actual.
     
    Last edited: Oct 2, 2015
  9. omishra

    omishra Active Member

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    I use room heater filament coil encircled on wood and with 2 crocodile clips on ends. Measure the resistance with DMM and use this resistance load upto 2KW power.
     

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  10. gadget73

    gadget73 junk junkie Subscriber

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    Just have to watch using inductive loads like that. It will screw up what the scope tells you if that happens to be important to you.
     
  11. goat67

    goat67 AK Subscriber Subscriber

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  12. stoN_Cold

    stoN_Cold Super Member

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    I made these:

    [​IMG]

    [​IMG]

    - 4 x 8Ω
    - 2 x 4Ω
    - 1 x 2Ω
     
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  13. gadget73

    gadget73 junk junkie Subscriber

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    i was thinking of building something like that ^^^. I've got a carcass of a car amplifier that would make the perfect heat sink. I have a couple of 5 ohm 100w wirewound at the moment but its an odd impedance and they are inductive. Good enough for a dummy load, but not what I really want to get measurements with.
     
  14. ConradH

    ConradH Addicted Member

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    I scored a bunch of 40 ohm power resistors at a hamfest, so my load is 5 x 40 in parallel, good for about 250 W continuous and a whole bunch intermittent. Most low value wire wound resistors aren't inductive enough to worry about unless you do a lot of high power testing at 20 kHz, and then I'd expect to do more fixing than testing. I try to keep my high power high frequency testing to a minimum.
     
  15. stoN_Cold

    stoN_Cold Super Member

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    Dale is the best.

    [​IMG]
     
  16. sKiZo

    sKiZo Hates received: 8642 Subscriber

     

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  17. shelly_d

    shelly_d Not An Audiophool

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    I have 4 of the 8 ohm 100 watt parts express resistors:

    http://www.parts-express.com/8-ohm-100w-non-inductive-dummy-load-resistor--019-020

    That can combine into 4 ohms / 200 watts times 2 or 16 ohms / 200 watts times 2, 8 ohms / 100 wats times 4 or 8 ohms / 400 watts times 1.

    Even on a 200 watt per channel monster amp I was safely able to use the 100 watt resistors by careful monitoring the voltage applied to the resistors and keeping it under 20 V rms (50 watts into 8 ohms)
     
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  18. JURB

    JURB Super Member

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    Ha, you can say that again. Just long enough for the distortion meter to get in tune. Very rarely is an amp expected to do full power at 20 KHz.
     
  19. Ray Gianelli

    Ray Gianelli AK Subscriber Subscriber

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    Very Nice!
    The cooling fans are a nice touch.
     
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