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Old 20th June 2014, 09:02 PM   #1
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Question Power Soaks and Dummy Loads

Thread split off from Speaker Voltage Test
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Originally Posted by Pano View Post
... But most people here dont have a scope and dummy load, so cant test directly as you can...
About the scope I can't help, but as for the dummy load I can show how I built mine, since this is a DIY forum. Forgive the somewhat off topic, though.

Dummy Load made with a little help of a computer ATX Power supply and P2 CPU heatsink.

Click the image to open in full size.

The board was built actually for another project, since it has an unused 5 volt output. This internal power supply is dedicated to the 12v FAN, and could be built only with the 9v transformer, the small bridge rectifier (or even single diode will do), and the smoothing capacitor (could be something like 470uF 25V).
The resistors are 8 Ohm 50W, screwed to the heatsink, and using thermal compound.

Click the image to open in full size.

This Load does not behave as a speaker, obviously, but will save your hears from excruciating frequencies playing loud wile testing amplifiers.
This is just an idea. One can use other resistor values, combinations of resistors to be able to simulate, 4 and 16 Ohm loads, and of course higher power resistors than the 50w I used, provided the adequate cooling of those.

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Old 21st June 2014, 12:41 AM   #2
Pano is offline Pano  United States
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Clever idea, I like it!

I have some big 4R and 8R dummy loads that came from Parts Express. They DO get hot. That's a clever way to cool them.
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Old 21st June 2014, 06:32 PM   #3
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Originally Posted by Pano View Post
I have some big 4R and 8R dummy loads that came from Parts Express. They DO get hot. That's a clever way to cool them.
I don't worry about my 1500 watt load resistors getting hot, since I use space heaters. The 1500 watt setting (900 + 600 watt) is 9.6 ohms, two heaters on the 900 watt setting comes out to 8 ohms. After burning up a power amp from shorted clip leads connecting the heaters , I made a proper adapter from a pair of 3 way AC cords to a 5 way binding post, so can plug in as many as 6 heaters at once, using the combinations of 900 and 600 watt elements can get pretty much any resistance desired.

Toasters, toaster ovens, 1000 watt par lamps and engine block or radiator heaters (in a water bucket) all make good high power load resistors.

Art
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Old 22nd June 2014, 05:22 PM   #4
Pano is offline Pano  United States
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Art, we must have the same heaters, or close. I think mine are 9 ohm for each element- each heater has two elements. They make for an excellent high power load.
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Old 22nd June 2014, 05:32 PM   #5
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Do you really test that much RMS power into a dummy load ... ?
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Old 22nd June 2014, 05:46 PM   #6
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Art, we must have the same heaters, or close. I think mine are 9 ohm for each element- each heater has two elements. They make for an excellent high power load.
Two 9 ohm loads in parallel would be only 4.5 ohms, 3200 watts at 120 volts (12,800 watts at 240 volts ), you would need a 30 amp 120 V circuit to operate that heater .

All my larger oil filled space heaters (Patton, Duracraft, Delongi) use 900 watt (around 16 ohm) and 600 watt (around 24 ohm) elements so they can run on a typical (old school) 15 amp 120 volt circuit.
Some of the smaller heaters are only 500 watts, about 29 ohms.
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Old 22nd June 2014, 05:51 PM   #7
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Originally Posted by a.wayne View Post
Do you really test that much RMS power into a dummy load ... ?
I did when doing PA amp comparisons. Many "high powered" amps can't deliver their rated power for more than a fraction of a second without current limiting or popping a breaker, for high powered sub use they don't cut it.

Art
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Old 22nd June 2014, 07:48 PM   #8
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Quote:
Originally Posted by weltersys View Post
Two 9 ohm loads in parallel would be only 4.5 ohms, 3200 watts at 120 volts
Been awhile since I checked. You're right, probably 9 ohms with both elements in.
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Old 23rd June 2014, 02:35 AM   #9
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If you are using heaters or light bulbs as dummy loads, bear in mind that the resistance varies a lot with temperature. The calculated resistance only applies at their rated power.
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Old 23rd June 2014, 02:50 AM   #10
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Light bulbs yes, for sure. Do heaters really change much?
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