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Old 30th May 2004, 12:16 PM   #1
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Default 8 ohm dummy load

Hi everyone,
just wondering if anyone out there knows how to build a simple dummy load for testing out amps?
I accured a valve test tone generator recently and wish to complete my testing equipment range, without resorting to blowing up amps in the testing process.
The dummy load must be able to take 8 ohms.
Thanks heaps
andrewe1
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Old 30th May 2004, 12:46 PM   #2
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For low power, a 20W 8Ohm wirewound resistor is fine, if you need more than that, just make up a network until you get the power dissipation you need.
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Old 30th May 2004, 05:05 PM   #3
SY is offline SY  United States
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For low power, a 20W 8Ohm wirewound resistor is fine
Non-inductive is best.
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Old 30th May 2004, 05:08 PM   #4
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nichrome wire... and water... lol or maybe oil... as it boils at a higher temperature...
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Old 30th May 2004, 05:46 PM   #5
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Originally posted by SY


Non-inductive is best.
Probably, yes.

I just have a deep down feeling that a bit of induction more closely approaches a real load, ( even though it's nothing like the real thing)....
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Old 30th May 2004, 06:44 PM   #6
Nisbeth is offline Nisbeth  Denmark
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7 pcs. 56 Ohm power resistors in parallel. Available up to 15-20 Watts each. Bolt to a heat sink for extra dissipation


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Old 30th May 2004, 10:10 PM   #7
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I have a monster 200 or 300W wire, or actually corrugated strip, wound vireous enameled resistor with sliding contact. I set it around 8 ohms (ohmmeter is a necessity) and blast away. Note that power capacity divides along the length, since if you're using half of it, only half is able to dissipate heat... so I've only got like 8 ohms 120W. Still more than I'll ever need. Until I use those 4CX250Rs.

As far as inductance goes, I don't know if that's what's causing the slight rise around 500kHz I usually observe while testing, but dammed if I care about a rise of 2dB when it's already down 40dB and at a frequency higher than the amp can respond to anyway.

Tim
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Old 30th May 2004, 10:25 PM   #8
Magura is offline Magura  Denmark
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Ive bought a bunch of 100W thick film resistors in TO-247 housing...mounted on a heatsink from the junk box it works very well. Its close to 100% non inductive. I think Dhaen bought a few of them from me a while ago to use them as dummy loads as well.

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Old 31st May 2004, 10:59 AM   #9
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WES Components in Ashfield, Sydney have 50W and 100W non-inductive resistors for just this purpose. Web : http://www.wescomponents.com/

I have 8x 100W units in a series-parallel network for high powered amps (got to fan cool them, though!).

Cheers
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Old 31st May 2004, 11:00 AM   #10
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Default Thanks all

Thanks all who replied to my question. This has given me some good ideas. I found that you can buy an off the shelf dummy load for about $100 au which I think is a rip off when you can make one for a lot cheaper than that.
Thanks again
andrewe1
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