1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

New DIY 8 ohm load bank

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

  1. maxhifi

    maxhifi AK Subscriber Subscriber

    Messages:
    3,094
    Location:
    Edmonton Alberta Canada
    Over the years, I've always used a pair of ceramic wire wound 25W rheostats as load banks. I set the resistance I want to test, then just ramp up the power, and back down. I've connected them together and put 70W into them in parallel with no ill effects, many times.

    Lately, I've been repairing increasingly powerful amplifiers, to the point where I have no means of checking full power operation. I looked into load banks, and large power resistors, and have found a few options, but nothing which suits my needs, and desire for a low price point. Basically what I want is something which can take at least 200W per channel, has no heat sink or cooling requirement, and is as cheap as possible. Here's what I arrived at.

    20 x 160 ohm 10W ceramic wire-wound resistors in parallel per channel. I'm going to make a plywood base, and put two 14AWG copper bus wires about 3" apart, elevated about 1" off the surface of the wood by wrapping around and soldering to finishing nails every few inches. Then, just solder 20 resistors between them in a line, with about an inch between each resistor. Then, make a second one for the other channel. The total cost of the resistors was about $26CDN at Mouser for both channels, the rest can be made from scraps I have laying around. That's $13 for a mono 200W load bank which requires no external cooling or heat sinking.

    I'll post a pic when it's done. I think the first use will be bench testing a Sansui 9090DB properly at full power. Then I have a NAD amp which apparently goes into protection mode too early.. will be great to have the correct tool to figure out why!
     
    Bratwurst7s and Hyperion like this.

     

    Please register to disable this ad.

  2. BillWojo

    BillWojo AK Subscriber Subscriber

    Messages:
    2,786
    Location:
    Burlington, NJ
    Sounds perfect for roasting hot dogs on. Can't wait to see the picks.

    BillWojo
     
    RuffzGuts likes this.
  3. maxhifi

    maxhifi AK Subscriber Subscriber

    Messages:
    3,094
    Location:
    Edmonton Alberta Canada
    Makes me wonder if the hot dogs will taste better, if I'm testing a tube amp. If anyone can think of a cheaper way to make a useable load bank, I'm all ears.. I need to be able to drive reasonably big amps to clipping into 8 ohms, I'm not terribly concerned that cheap resistors have some inductance, I won't be measuring low level distortion, just loading up the amps to see how they handle it.
     
  4. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,611
    Location:
    Hertfordshire, UK
    Your post is giving me the stimulus to make one for myself, I have always needed a proper high power load - every time I have priced it up - it comes out REALLY expensive - AND I can't roast hot dogs on the ones I came up with. :D
     
  5. stoN_Cold

    stoN_Cold Super Member

    Messages:
    3,326
    Location:
    Belgrade, Serbia
    Me 2. :rockon:
     
  6. maxhifi

    maxhifi AK Subscriber Subscriber

    Messages:
    3,094
    Location:
    Edmonton Alberta Canada
    That's just it, I looked into those fancy new resistors which need a heat sink, then what the heat sink costs, then making a chassis for it... I also looked into the huge old school tubular ceramic resistors... any way you look at it, it's money I don't want to spend. I thought about this until I arrived at the cheapest possible way I could think of, and even then I did a few mouser orders before throwing in the resistors. What pushed me over the edge, is this pesky NAD which supposedly goes into protect mode prematurely. There's simply no way for me to bench test this without a proper dummy load.
     
    Hyperion likes this.

     

    Please register to disable this ad.

  7. avionic

    avionic " Black Knights " Subscriber

    Messages:
    44,613
    Location:
    Fort Dodge, Iowa
    Last edited: Jul 12, 2018
    Bratwurst7s, lifer70 and sdw54 like this.
  8. Ray Gianelli

    Ray Gianelli AK Subscriber Subscriber

    Messages:
    1,112
    Location:
    Lake Worth, FL
    I've been using 2 Parts Express 100 watt dummy loads placed in heat sinks like this:
    20161216_071042.jpg

    I've run them up to 400 watts for a brief time,although the resistance does climb rather quickly with heat. So I bought a pair of these from the auction site:

    20180711_052802-1.jpg

    Supposed to be 1,000 watts at 8 ohms. Need to screw them to a large aluminum plate, or maybe an old engine block. :)
     
  9. stoN_Cold

    stoN_Cold Super Member

    Messages:
    3,326
    Location:
    Belgrade, Serbia
  10. sregor

    sregor AK Subscriber Subscriber

    Messages:
    3,583
    Location:
    Mass
    Test loads get hot - plywood base is not recommended. We had one blackened and starting too smell like burnt wood in our shop.....
     
    Bill Ferris likes this.
  11. maxhifi

    maxhifi AK Subscriber Subscriber

    Messages:
    3,094
    Location:
    Edmonton Alberta Canada
    Ah, I was trying to figure out how hot the wood would get, I'm glad you brought this up. I was thinking to stand the resistors 2" off the base, and maybe throw in a metal heat shield under the resistors to protect the wood. Too bad asbestos is so bad for you, it would be ideal here!

    Another idea is to somehow orient the resistors vertically. I will come up with something.

    I do like the other ideas presented here,
     

     

    Please register to disable this ad.

  12. gslikker

    gslikker Super Member

    Messages:
    2,160
    Location:
    Close to Alkmaar, Netherlands
    I made this one at work, 10 x 0.1 ohms (they are both sides of the cooling tunnel) , to be able to test some low voltage high current smps power supplies we still use.

    It uses some old deflection amplifier cooling device, 10 millimeter alu plating bolted to the cooling tunnel outside, and copper bars from old equipment, and 24 very nice Japanese TO3 transistors came off. I added a mains socket for the 230V fan. I have some short old thick cables from old equipment fed by the type of power supplies, for high currents cables get troublesome especially when not having proper tools in the house.

    The resistors look the very same as the ones @avionic uses.


    20170608_062430 - kopie.jpg 20170608_062449 - kopie.jpg 20170608_062522 - kopie.jpg 20170608_062733 - kopie.jpg 20170518_1541823 - kopie.jpg 20170518_1541837.jpg
     
    Last edited: Jul 11, 2018
  13. stoN_Cold

    stoN_Cold Super Member

    Messages:
    3,326
    Location:
    Belgrade, Serbia
  14. blfttieq

    blfttieq New Member

    Messages:
    39
    20180711_105259.jpg 20180711_105312.jpg I'm in this process right now. My solution is a 2000 watt water heater element screwed into an old oxygen tank. This represents a 9.6.... something ohm load at very high wattage. I'll fill the tank with oil or water with some anti corrosive agent. Working on the bore and tap now. I intend to strip all from the tank outside and stencil the words "BIG DUMMY" on it. I have 2 of these tanks so in parallel I should be able to test most anything I would come across.
    The heater element was 10 bucks, the tanks free & I have machine shop access locally to any machine I dare to use.
     
    Last edited: Jul 11, 2018
  15. maxhifi

    maxhifi AK Subscriber Subscriber

    Messages:
    3,094
    Location:
    Edmonton Alberta Canada
    All you need to do is put a 50 ohm 20W ceramic resistor in parallel with that to get a perfect 8 ohm load.

    Mine is sure less elegant and uglier than what you guys are coming up with!
     
  16. blfttieq

    blfttieq New Member

    Messages:
    39
    Thanks maxhifi but that would kill the wattage capbility. Weakest link in the chain.
     
    Bill Ferris likes this.

     

    Please register to disable this ad.

  17. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

    Messages:
    4,212
    Location:
    Munich, Germany
    It's only 1-channel, but, 6x ATE 12ohm 150w resistors and 8 kilos of heat sink. It barely got warm to the touch at 200w testing my M-70...

    DSC05211.jpg DSC05214.jpg DSC05215.jpg DSC05216.jpg

    ...I had 2 12v fans ready to use but they weren't needed.

    DSC05526.jpg DSC05527.jpg

    Cheers,
    James
     
    Hyperion and Beau Geste like this.
  18. maxhifi

    maxhifi AK Subscriber Subscriber

    Messages:
    3,094
    Location:
    Edmonton Alberta Canada
    This looks great! I have to say, when I noticed your location, absolutely no surprise - I can't imagine a more German way to build a load bank :)
    Mine is going to be much less impressive, an extreme exercise in "value engineering"
     
    Bratwurst7s likes this.
  19. Bratwurst7s

    Bratwurst7s In The Frying Pan Subscriber

    Messages:
    4,212
    Location:
    Munich, Germany
    Well, your concept looks very effective, actually a quite elegant solution imho. I'm sure that it will work quite well. All of mine came from parts that were destined for the recycling bin so it didn't cost me a cent. If I had to pay for it I would have been happy to have gone your route. :)

    Heh heh, I'm actually a transplanted Pacific Northwesterner. One could say that it was the ex USAF C-130 mechanic's way of building a load bank. ;)

    Cheers,
    James
     
    Beau Geste likes this.
  20. maxhifi

    maxhifi AK Subscriber Subscriber

    Messages:
    3,094
    Location:
    Edmonton Alberta Canada
    I was looking into the thermodynamic equations to try and figure out how close I can be to the wood, to start to burn it, and I realized that it's probably way easier to just build and try, than account for all the variables. I won't run it for very long anyhow, I don't want to risk damage to customer's amps.. it's just gotta work for testing.

    A decent electronic surplus store is something we don't have around here, it's been years since we had anything close. It would have been very useful for this project if we did!
     

Share This Page