DIY Dummy Load on the cheap

premiumplus

premiumplus

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Stuck in the 70's
Hey guys...I've been doing some research and it looks like I can use an electric water heater element for a cheap dummy load. A 3500 watt, 240 volt element works out to be 16.46 ohms, and they're cheap too...around $8.00 at Home Depot. Two of 'em give 8.23 ohms, and I'm going to be needing a basic load for current sharing testing on my upcoming PL-700B White Oak project.
Is there any reason this shouldn't work? They're purely resistive, according to this report:

http://www.synaudcon.com/site/author/pat-brown/big-dummy-load/

I'm thinking this would be a good option to spending $400 on wirewound resistors...what am I missing here? There's gotta be a catch or we'd all be doing this.

c06b8ba3-ea19-467c-a82f-0cf19fb2281e_300.jpg
 
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The resistance is going to change with temperature...
 
laatsch55 said:
The resistance is going to change with temperature...
Click to expand...
Yes, it'll probably rise with temperature...but what I am looking for is just a simple load I can use for transistor current sharing tests.
I have some wirewounds that I used for the 400 but moving up to the 700 makes me pause. Maybe I can just submerge these in water or oil for the short term?
It seems like even an inductive load would be ok for current share testing as long as the amp doesn't decide to oscillate.
Some guys recommend taking apart old toasters and using the nichrome wire but I think those might be way too inductive...
 
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If you are around where they use VFD's the dynamic brakes are in the 4-25 ohm range andRated from 1000 to 5000 watts. If not 4-100 ohm ohmites worked for me for years. At times I had to submerge them in ice water. I now use a 2,000 watt BPI load that was used in the engineering dept at the Phase Linear factory.
 
I hacked this together when I needed a bigger dummy load. I don't claim it to be perfect, but I had the parts around the house, and the need, so this worked.

I have a pair of these.
http://www.parts-express.com/8-ohm-100w-non-inductive-dummy-load-resistor--019-020

sandwiched between two of these
http://www.apexjr.com/images/APEXJRHEATSINK6IN.jpg

with some heatsink grease on both sides of the dummies, and bolts at the corners to gently squeeze it all together.

Works surprisingly well. I can full power test my 700B for reasonable periods of time without the loads heating up too much.

Also, there's a guy who did/does prosound amp testing with a bank of water heater elements in a 5 gallon bucket full of water. Can't recall where that was... Somewhere on the internet.

Zach
 
Jumping in to say Nav's avatar solves the gun debate ... :happy2:
 
No sacred threads around here Dave, we resurrect all the time!!
 
With the heater in free air (not water) it's going to get extremely hot when used as a load resistor. In that case I'm guessing it's more like a 50-100 watt resistor. The old rule still applies. You don't get something for nothing

premiumplus said:
Hey guys...I've been doing some research and it looks like I can use an electric water heater element for a cheap dummy load. A 3500 watt, 240 volt element works out to be 16.46 ohms, and they're cheap too...around $8.00 at Home Depot. Two of 'em give 8.23 ohms, and I'm going to be needing a basic load for current sharing testing on my upcoming PL-700B White Oak project.
Is there any reason this shouldn't work? They're purely resistive, according to this report:

http://www.synaudcon.com/site/author/pat-brown/big-dummy-load/

I'm thinking this would be a good option to spending $400 on wirewound resistors...what am I missing here? There's gotta be a catch or we'd all be doing this.

c06b8ba3-ea19-467c-a82f-0cf19fb2281e_300.jpg
Click to expand...
 
wattsabundant said:
With the heater in free air (not water) it's going to get extremely hot when used as a load resistor. In that case I'm guessing it's more like a 50-100 watt resistor. The old rule still applies. You don't get something for nothing
Click to expand...

I must wonder how hot such a device would really get when powered with a typical audio amp. It's rated at 3500 watts, after all.
 
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