8 ohm dummy load

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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
 
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. :devily:

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. :D

Tim
 
I am using 5 - 40 ohm, 3%, 50 watt resistors in parallel. These are mounted on a sheet of PCB material -- and it really cooks. Over time the copper sheet has become "heat treated". I connected a piece of #12 wire onto the PCB material -- on which various clips can be attached (for the scope etc.). This also attaches to the amplifier ground.

Opening up the thread, however, I note that when you do the PDMax calculation for an amplifier it presupposes that you are using a resistive load, not the case in real life -- the transistors or opamps seem to bear more of the power dissipation than the load -- a little bit of trig comes into play for real life examples.

anyone up for some brainstorming a reactive load -- I think that Gary Galo has one in his book on testing loudspeakers, and seem to recall one on Marshall Leach's website ...
 
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pinkmouse said:
I'm wondering if the answer might just be to use a big old voice coil from a pro woofer. You could put fan on it, or maybe dunk it in a can of oil for cooling purposes.


My flat mate suggested something similar to that the other day, He said why don't you get some old speakers and rip out the cones, and then you will have a more realistic load ......... I wondered whether without the cone and surround, it would just shoot straight out of the gap ;)

Tony.
 
Apex Jr said:
For those of you who do need dummy loads I have in stock:

Dale RH 50 50 Watt 8 ohm alum housed resistors at $2.00ea

Ohmite 15 ohm 225W Ceramic 10 1/2" long resistors at $3.95ea

Series and Parallel will give you variations

Steve @ Apex Jr.
I bough a few of those ohmite power resistors from the original Apex about 10 years ago and they are still used as dummy loads today. They also make great cheese graters!


:D
 
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