8 ohm dummy load

[andrewe1]

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

 

[pinkmouse]

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.

 

[SY]

For low power, a 20W 8Ohm wirewound resistor is fine

Non-inductive is best.

 

[SkinnyBoy]

nichrome wire... 😀 and water... lol or maybe oil... as it boils at a higher temperature... 🙂

 

[pinkmouse]

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)....😉

 

[Nisbeth]

7 pcs. 56 Ohm power resistors in parallel. Available up to 15-20 Watts each. Bolt to a heat sink for extra dissipation 🙂


/U.

 

[Sch3mat1c]

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

 

[Magura]

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.

Magura🙂

 

[Centauri]

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

 

[andrewe1]

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

 

[jackinnj]

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 ...

 

[pinkmouse]

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.

 

[DigitalJunkie]

I use 4x 16ohm 75W wirewound resistors I got at a surplus place for $1 each. 2 per channel with a switch to parallel them,16 ohms 75W or 8ohms 150W per channel.Or I can parallel both channels for a 300W 4ohm load (could add a second switch for that too.)

 

[wintermute]

I bought two of the 100W ones Centauri is talking about, from WES a couple of weeks ago. They are non-inductive and I think cost less than $20 each. So would be a lot cheaper than your $100 one.

Tony.

edit: you might want to check this thread I started a while back too 🙂
http://www.diyaudio.com/forums/showthread.php?s=&threadid=27364&highlight=

 

[wintermute]

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.

 

[markp]

I use two 16 ohm wirewound 300watt monsters in parallel wired with the spirals opposite each other to lower inductance. Its a big one!

 

[Apex Jr]

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.

 

[Mr Teal]

I have two of these, they're pricey, but if you bolt them onto an old heatsink and get some air moving over it, they can take ALOT of power.
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=019-030

 

[markp]

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!


😀

 

[amptech]

dummy load idear

here's an high powered load for testing amps😀

I use two 10 ohm @200w each and three 1 ohms @200w wired for a 8 ohm load '1000w. or 10 ohm@300w each plus 1 ohm@300w each. total power 1200w nice

have fun😀