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Dummy dummy load question..

bryans12v

bryans12v

Marantz / Altec Junkie
Subscriber
Planning to build a dummy load setup. I have a few amps around the 70w and 250wpc. I believe it's possible to use a higher rate dummy load than the actual amp output such as a 120w load on a 70amp but how close should it be the other way around? Would a 240w load be sufficient for an amp rated at 256wpc?

IF so, this would make a simple project. I was thinking of grabbing 4 8ohm 120w non-inductive dummy load resistors. Using two for the smaller amps and running two in series then parelleling the two pair which would give me 2 240w 8ohm loads.

If I would need a rated load wattage for the big amp I may just make two seperate dummy load blocks.

Thanks for all the help guys!
 
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It just needs to be rated higher than the amp. Actually, all wirewound resistors can take brief overloads. My resistor bank is rated at 250 watts, on a heatsink and with a fan, but brief tests at 300+ watts are no problem.
 
Don't forget they generate a fair amount of heat. 250 watt loads can char a piece of plywood if doing some high wattage testing - recommend metal plate and standoffs (and a fan as Conrad does) if testing big amps. My 2 cents....
 
Like you say, you can use the higher rated dummy loads on a lower power amp, no problem. But exceeding the rating of the loads (256W amp with 240 Ohm loads) is a gray area. Technically you can't, but you might just get away with it for a very short time.
But if you use 120W, 8Ohm resistors in a series parallel combination, the power handling capability would be 480 Watts. The combination you suggested would be two 8 Ohms in series = 16 Ohms, and then two of those strings in parallel to get back to 8 Ohms. So it requires 4 resistors per channel, and the power would be 4 x 120W = 480W.

One thing to keep in mind is the derating of the power vs the heat. When manufacturers rate the resistor power, they rate it at a certain temperature, usually 25C. So the rated power only applies as long as you can keep the resistor at room temperature. Obviously that doesn't happen. The resistor gets hot. Very hot. And at an elevated temperature, the power handing capability diminishes toward zero. The manufacturer will provide a graph (usually) of the power handling capability vs. the temperature. It is up to you to provide enough heat sinking to ensure the temperature stays within the limits for the power you are applying. 256W is going to require a very large heat sink to maintain a reasonable temperature. Without a heat sink, even using the 120W resistor on the 70 WPC amp is going to get too hot very quickly. It would probably be OK for a few seconds (tops), but then you have to let it cool back down.

Terry
 
Those ratings also assume a certain area worth of heat sink to get the rating on the resistor. The bigger the heat sink, the longer you can overload them. If you really need to long term abuse a dummy load you can liquid cool it, though that gets messy. Distilled water has very low conductivity if you need to go that route.
 
Thanks for the helpful info.

Here's what I was thinking about making. Something similar to this with the big green coated wirewounds.
DummyLoad.jpg

With these a heatsink wouldn't help much so I was going to fan cool them when in use. Otherwise, I'll try to find aluminum housed packages that can be mounted to a heatsink.
 
I have used 2 x 250 watt 8 ohm, 1% Dale (Vishay) RH series resistors for years for all my amp load needs and have had absolutely no problems. I would recommend mounting them on a big heatsink, adding a fan. I used crimp terminals to the speaker cable paralleled with RG58 coaxial cable terminated into BNC connectors to be connected to ch1 & 2 of a scope.
 
Arrange the fan to blow through the holes of the round ones, not just from the side.

Somewhere I have an old Ohmite tech paper on the overload capability of their wirewound resistors, so they did anticipate these sorts of applications. I don't want to stress the gear by running for a long time at full power, so I tend to pay more attention to the temperature of that, rather than the load bank... hey, what's that burning smell coming from the lab?
 
ConradH said:
Arrange the fan to blow through the holes of the round ones, not just from the side.

Somewhere I have an old Ohmite tech paper on the overload capability of their wirewound resistors, so they did anticipate these sorts of applications. I don't want to stress the gear by running for a long time at full power, so I tend to pay more attention to the temperature of that, rather than the load bank... hey, what's that burning smell coming from the lab?
Click to expand...

Haha! Everytime I fire up the soldering iron my old lady asks what's on fire!

I'd like to do this once and right the first time as my work area is in my bedroom and wouldn't want to start any fires!
 
Those things get really hot when run near the maximum power levels. I'm not sure I'd mount them to a block of wood.

Terry
 
Reminds me of more than a few theads 'hooking up to a pair of junk speakers'.. and?

Q: ? how long do you guys run these tests. assuming a stable amp long term... but what with load connected how long?
 
A constant current load like that doesn't really simulate a load like a loudspeaker very well. A resistive load is better.

As far as how long, I often only run at full power for a few seconds at a time. Just long enough to make a distortion reading at full power. But sometimes it is good to run at a fairly high power for a long time to stress the output transistors and make sure everything is healthy.
If you are just checking that the power supply is healthy, a short blast should suffice.

Terry
 
I second not mounting to wood, or other flammables. It is not that hard to get a small amount of sheet metal, like flashing for a chimney which is thick enough to hold a shape if you bend it right and rivet it. Rivet guns are cheap, and for sheemetal get a small hole punch, don't bother trying to drill it. Cut and bend to shape and then just punch the hole through both pieces while holding it in the position you want.

More recommended would be to get the bid Dales (they been bought out I hear but are still available). you might be able to find an old carcass of a stereo and screw them to the heatsink.

Terry is right about derating when the things get how, but it is not like a transistor. Those things can go above 200C and be fine usually. That doesn't mean they like it, and also, 200C is quite hot. You simply do not want them to get that hot, 100C is boiling water. At 200C you are talking pizza baking temperature, hotter than most food cooking. Depending on conditions could get close to third degree burns instead of second.
 
One can buy rather large heat sinks for variable speed/frequency drives on the Bay. Usually not too expensive.

Often you can find them made in the shape of a tunnel, so to speak. Mount the resistors to the external sink plates, then affix an appropriately sized fan to blow across the fins in the tunnel.
 
Yes, similar, but inside out. :)

The heatsink I bought to do this (have not yet executed the plan) has the fins on the inside. The resistors would mount externally. Sorta like this one but more square; about the size to facilitate a 120x120mm fan blowing through the fins.

s-l1600.jpg
 
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whoaru99 said:
I've also contemplated a tunnel cooled Class A amp for my heatsink. Maybe a Pass DIY jobbie? Too many project ideas. :)
Click to expand...

You can make it as big as you want with that much surface area.

Like big big! I think a project like this is in my not so near future and would be very rewarding.
 
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