BYO Dummy Load Resources?

MasterEvan07

Silver Supporting Member
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4,285
After the resounding victory of successfully building an extremely complex, time-consuming bleed resistor, I've decided to take aim at a dummy load so I can test builds under load while on the workbench.

Anyone have some good direction for building one, and also how to understand what I'm building and it's use in application?
 

Jeff Gehring

Silver Supporting Member
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5,801
Some basic info, if you are building it as a bench tool:
  • Try to accomodate all (or as many as you deem necessary) of the 'usual suspects' for amp output impedances.
  • Ensure the power handling of the total load is at least 2x the rated power of the amp you are testing.
  • Ensure each resistor in the load will not have it's power rating exceeded in any impedance setting.
  • Know that it will get HOT! so construct it accordingly. If you are using aluminum chassis mount resistors, you must provide them with an adequate heat sink.
  • Provide a means for connecting other test equipment to the load (besides the amp under test) like scopes, DMMs etc.
  • Did I mention that it will get HOT!
 
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Tony Bones

Member
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1,214
Part of the joy of making things like this is being at least a little bit creative in scrounging parts, packaging, heat sink, etc. At least that's what I think. So, I'll give you the simplest possible instructions. You can embellish to suit your own sensibilities.

1. Buy a 4, 8, or 16 ohm power resistor. If you have multiple amps that need different load resistances, then buy more than one resistor. You can combine them in series and parallel to obtain different resistances. or just buy one of each and be done with it. It's up to you.

2. Get a 1/4" phone plug. You can buy one from Mouser or wherever you like to buy parts from. Or you can take one off of a guitar cord that doesn't work anymore. (Don't use the guitar cord, just the plug from the end.)

3. Find a couple feet of lamp cord somewhere. Wire the thing up. @Jeff Gehring offered a good suggestion to provide a way to attach DMM leads and scope probes.

The resistor will get hot. Its only job is to convert power into heat, so the more power it needs to convert the hotter it will get. If you're plugging it into a 15W amp run at low volume settings then you might get away without attaching the resistor to something to dissipate the heat. Otherwise you'll need a heat sink. Maybe find an unused metal enclosure to screw it to, but don't hide the resistor inside a box without airflow. The idea is to transfer the heat to the air.
 

Brokenbones

Member
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60
Look up Mark @ Blueglow Electronics on Youtube and search for his "Dummy Load" video. His only uses 8 ohm resistors but his design is excellent for incorporating test gear and should give you some ideas.

Use non-inductive resistors. They're expensive once you get above 50-100 watts but good deals do pop up on eBay. The resistors themselves do not need to be exactly 4,8,16 ohms but relatively close. You can use cheap, eBay, low watt, chinese inductive resistors for quicker measurements that don't require accuracy. These can be soldered to a simple 1/4" input jack.

There is another guy on youtube, i believe his name xraytonyb, who built a similar dummy load to Marks. His videos can be kind of hard to watch because he's very thorough. I'd say those two gentlemen plus the schematic from aikenamps should get you what you want.
 

PushedGlass

Member
Messages
599
I have a heating element assembly out of a floor-standing electric convection space heater. It is about 18" long, 3" wide, and 4" tall. It has two resistive legs (meant to be 2x750W at 120VAC) off a common connection. Each leg is 18 ohms cold, so you can run them in parallel to get 9. The elements consist of either steel or nichrome ribbon and they run in a zigzag so they're not going to show much inductance.

As is, it's good for spitballing 8- or 16-ohm loads using the existing spade connectors. If you want to nail ~6 or ~12 ohms to better match real speakers' DC resistance, you'd have to use an alligator clip to find the right spot along one element or another; this material can be spot-welded or crimped but it can't be soldered. Power handling is not going to be a problem unless just maybe you find some insane Sound City head and want to run it at 4 ohms.
 

Bandit127

Member
Messages
110
Never test it with your fingers to see how hot it is. Especially if you are investigating the smell of something getting hot...
 

AL30

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3,000

reaiken

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1,873
I used those style of resistors when I built a heat sink a few years ago. No issues with them. The heat sink is a good idea.
You *must* use a large heat sink with those metal-bodied resistors. They will not dissipate anywhere near their rated power without one. In fact, they will get so hot the inner element adhesive will melt and it will slide out of the body of the resistor.

The large vitreous enamel ceramic wirewound resistors from Ohmite, Vishay, and others are also good, they are normally rated for free-air dissipation, which is why they are so large, so you don't need a heatsink, but you do need to provide ventilation and mount them up in the air on the mounting brackets you can purchase for them.
 
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MasterEvan07

Silver Supporting Member
Messages
4,285
You *must* use a large heat sink with those metal-bodied resistors. They will not dissipate anywhere near their rated power without one. In fact, they will get so hot the inner element adhesive will melt and it will slide out of the body of the resistor.

The large vitreous enamel ceramic wirewound resistors from Ohmite, Vishay, and others are also good, they are normally rated for free-air dissipation, which is why they are so large, so you don't need a heatsink, but you do need to provide ventilation and mount them up in the air on the mounting brackets you can purchase for them.
Thanks, I’ll look into those!
 




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