DUMMYLOAD

[TIMBO]

Hi guys, With a new purchase of an oscilloscope brings new things to build
I have drawn a fairly simple circuit that should meet my needs.............
A local parts store has the aluminium cased resistors for high wattage.
I have chosen 150w as these will be in parallel and series to give me 300w coverage.
Unfortunately they don't come in 8ohm values.
So by the circuit I can get close to desired values.
I have a quite a lot of vintage amps that have some of the older value speaker loads.
When using these dummyloads for testing is it critical that the loads are exact to the OT's output.
Also these resistors are +/-5%. Thanks

[Moonage]

Can you convert drawing to PDF?
Not all of us have Altium...

[sluckey]

When using these dummyloads for testing is it critical that the loads are exact to the OT's output.

The amp will be fine. Just remember what the actual resistance is if you intend to do accurate power measurements/calculations.

I'd be nervous about relying on a phone jack switch contact to provide the 16Ω load.

[Willabe]

Can you convert drawing to PDF?

Here's a link to download the SCH program, many of us here use this to make drawings;

http://el34world.com/Forum/index.php?topic=590.0

[TIMBO]

Thanks Sluckey, I was applying the KISS content.
Any ideas for a better solution would be great. Thanks

[sluckey]

My concern about using the phone jack switch contacts was about having a reliable connection through the switch. For my dummy load I just used a barrier terminal strip with screw terminals. This gives a worry free positive connection.

I wired two 8Ω resistors in series. Then connected all three wires to the terminal strip. By using a short spade lug jumper wire, I have available choices of 4Ω (parallel), 8Ω (single resistor), or 16Ω (series). Simply change the jumper. No worries about a poor connection.

You can get high power 8Ω resistors on eBay for cheap.

[TIMBO]

What power rating are the resistors.

[sluckey]

I used two 8Ω 50 watt. But you can get any wattage you want on eBay. Search for "8 ohm resistor". Here are a couple pics of my cheap D/L...

[pompeiisneaks]

I've been thinking of this lately too and found this link that has a schematic as well. 

http://aikenamps.com/index.php/dummy-loads

He's got various switchable types there. 

[jjasilli]

+1 to sluckey's eBay suggestion.  That's where I got my power resistors.


FWIW: Here's my stereo dummy load box, with switchable Ohm ratings:  http://el34world.com/Forum/index.php?topic=9925.msg104987#msg104987

[pompeiisneaks]

Excellent info jjasilli, thanks.

Phil

[TIMBO]

Thanks guys, I've got it sorted and like Sluckey's idea using the barrier strip to make the required ohms.

[Paul1453]

These are very cheap if you are still sourcing your resistors.   

http://www.ebay.com/itm/Golden-Mini-8-Ohm-100W-Watt-Power-Wirewound-Resistor-Metal-Aluminum-Shell-Case/291670336157

[Willabe]

These are very cheap if you are still sourcing your resistors.   

http://www.ebay.com/itm/Golden-Mini-8-Ohm-100W-Watt-Power-Wirewound-Resistor-Metal-Aluminum-Shell-Case/291670336157

Yeah I've been looking at those on ebay. 

The price is great, + free shipping, but do you guys think we can trust the products ratings/build quality?

Mouser has the same type in Dale@50w for ~$5 plus shipping.

At the ebay price I could get 2 each for ~$16 @ 2/4/8/16 ohm for 200w total for each ohm load and be done with it.

[p2pAmps]

Here is one I put together...

http://www.p2pamps.com/dummy-load/

[Willabe]

Thanks Mike, I saw that on your web site.

I did order 2 x 8 ohm x 100w and 2 x 4 ohm x 100w off ebay. Their on a slow boat from the Philippines.

Do you have a schematic draw up for your dl? 

[TIMBO]

Very nice,
I still have to get some resistors ordered.

[Mike_J]

I used two 8Ω 50 watt. But you can get any wattage you want on eBay. Search for "8 ohm resistor". Here are a couple pics of my cheap D/L...

I am looking for a dummyload for 2,4,8 and 16 ohms for up to a 100 watt output amp. Read enough about the chassis mounted resistors to get confused. One thread mentioned that it is not automatic that 100 watt dissipation will be obtained with a 100 watt chassis mounted resistor. From what I read factors like using an aluminum plate to install the resistor on, thermal paste on both the resistor and finned looking heatsink and ventilation to help dissipate the heat contribute to the wattage dissipated. These methods are all present in your design.


What I was wondering was whether you think a single 100 watt resistor would do the job or if larger resistors should be used or two 100 watt resistors in series (for example two one ohm 100 watt resistors in series to give two ohms). Would these two resistors be able when combined dissipate the 100 watts?


Hopefully this isn't outside Timbo's topic. If it is then my apologies.


Thanks
Mike

[sluckey]

...or two 100 watt resistors in series (for example two one ohm 100 watt resistors in series to give two ohms). Would these two resistors be able when combined dissipate the 100 watts?

Two 1Ω/100W resistors in series would be equal to one 2Ω/200W resistor.

There are other style power resistors that don't need a heatsink. But they need plenty of air space around them (not talking about forced air).

And there are resistors specifically made for dummy loads that are integrated into an aluminum heatsink.

[DummyLoad]

http://www.parts-express.com/8-ohm-100w-non-inductive-dummy-load-resistor--019-020?utm_source=google&utm_medium=cpc&utm_campaign=pla


--pete

[sluckey]

That's it!

[shooter]

I use mine this way,
I just have *the standard ones* jacked up, my 8 is 10 250W, but the math still fits in my room

[Willabe]

 I'm starting this here then will move to Amp Tools/Tech Tips, Dummy Load thread;

Can I use 2 X 1/4" switchcraft or Cliff jacks to switch between 2 aluminum housed power R's for parallel/series instead of a bat type switch?

I built a dummy load using 2 X 4ohm @ 100w's each for a 2ohm and 8ohm load and 2 X 8ohm @ 100w's each for a 4ohm and 16ohm load.

Now I need to be able to switch the 2 R's between parallel/series for the correct load. It will be easier, cheaper and less space to just plug into either jack for correct load.

I paid $1.99 for each power R (free shipping), 4 x $1.99 = $7.96, finned (2mm thick) heat sink, 11-1/2" x 5-1/2" x 3/4" for $15.17 + $3.15 shipping = $18.46 and silicon heat pad, 100mm x 100mm x 1m, so $28.15 total for 2/4/8/16 ohm dummy load.

[PRR]

May be possible with a Cliff. Won't be trivial to prove the answer. A screw-up in concept or wiring may short or un-load the amp.

You got tons of space there.

A switch isn't much bigger than a second jack.

Me, I'm lazy. Switch gets my vote *IF* it was used enough to make clip-leads annoying. Testing 100 amps a day, switch it is. One amp today and another one on the weekend, clips.

[pompeiisneaks]

I really should post some of mine I did last week.  I followed the schematic from Aikenamps.com and used a switch, I have it with 2, 4, 8, and 16 ohms on a 4 way switch, I tested it with the DMM after and its all good to go.  I did a short video of it too, so I may just post a link to that when I upload it. (tonight maybe?)

~Phil

[Willabe]

I found this chart from Arcol resistor company on their aluminum housed power resistors. Notice the power
de-rating when not using a heat sink.

Here are some pics of an Arcol power R, 100w and the 100w power R's I bought on ebay (China) right next to each other, Arcol is ~ ~ double the mass in size.

Arcol; 2-9/16" x 1-7/8" x 15/16" with 1/8" thick base plate, ebay/China; 2-3/8" x 1-1/8" x 5/8" with 1/16" base plate.



 

[pompeiisneaks]

Here's that video I mentioned, my build: https://www.youtube.com/watch?v=ZRs6vkqq5E8

~Phil

[Willabe]

How much does adding a cooling fan add to the dissipation wattage rating of heat sink, will it double it?

I have 2 @ 2.5" 12vdc brush less fans I will rig up for larger amps.
 
Looking at the dissipation chart in reply #25, for a 100w's they list for a 'proper' heat sink, a 3mm x 995sq/cm = 154.23sq/inches. My heat sink works out to be ~~ 161.56sq/inches but only at 2mm, so ~~1/3 less overall mass.

Since I was worried that the power R's I was buying from ebay might not be up to their power dissipation wattage rating being ~~ 1/2 the mass in size (see pics in reply #25) I bought 2 of each @100w hoping that would get me much closer in power dissipation, but I'm still worried.     

[pompeiisneaks]

I was recently watching a video of EEVBlog and he talked about heatsinks and they seem to have two ratings, one static and one with airflow. It's pretty significant if you add a fan.  He did a demo on a airplane boost converter, and it was pretty surprising how much better the performance was with airflow.

Here's his video: ! No longer available 

[sluckey]

Willabe, here's a series to parallel switch that may be used for your application. Personally, I'd just use a barrier terminal block. More reliable connections. Besides, how often will you use this? I've used mine once, twice, three times a...   

[Willabe]

Personally, I'd just use a barrier terminal block. More reliable connections.

Thanks Sluckey, I went back to the beginning of this thread and saw your DL with the barrier terminal block, I like that idea. I'm gonna go with that. 

What are the measurements and wattage of your DL power R?
 

[Willabe]

From reply #25;

I found this chart from Arcol resistor company on their aluminum housed power resistors. Notice the power
de-rating when not using a heat sink.

Here are some pics of an Arcol power R, 100w and the 100w power R's I bought on ebay (China) right next to each other, Arcol is ~ ~ double the mass in size.

Arcol; 2-9/16" x 1-7/8" x 15/16" with 1/8" thick base plate, ebay/China; 2-3/8" x 1-1/8" x 5/8" with 1/16" base plate.

Thought I'd post these dimension charts, comparing my ebay/China 100w power R's measurements to the Arcol chart mine are much closer to the Arcol 50w R NOT the 100w R;

[sluckey]

I used two Dale RH-50 50W 8Ω 1%. They are about 2 inches long. Here's the data sheet...

     http://www.vishay.com/docs/50013/rh.pdf

I have them wired in series with three wires brought out to the terminal block, black to one end, white to the other end, green to the junction. So, I have 8Ω/50W between black and green. I have 16Ω/100W between black and white. And if the yellow jumper is connected between black and white, I have 4Ω/100W between black and green. Pretty simple.

[Willabe]

Thanks for the Dale link, the Dales are spot on to the Arcol dimensions.

So mine look to be realistically 50 watters not 100 watters but with 2x I'm back to 100w's.

They should be fine with the heat sink and 2x2.5" 12vdc fans.     

[sluckey]

OK Rube! 

[Willabe]

Here's that video I mentioned, my build:

It looks nice in that box but with the lid closed it will build up heat instead of letting the heat dissipate into free air space and de-rate the wattage value. 

[Willabe]

OK Rube! 

Oh, you know Mr. Goldberg too. 

(The young'ns won't know who were talkin' bout. )

[pompeiisneaks]

Here's that video I mentioned, my build:

It looks nice in that box but with the lid closed it will build up heat instead of letting the heat dissipate into free air space and de-rate the wattage value.

Yeah I had a few people comment the same on reddit.  I'll have to remember to open the lid, or I may just pop some of the conduit holes in it, and push a fan through it, something like that to help dissipate heat.  I also have a spare cpu cooling thing that I may be able to just stick on with paste and give better dissipation on the underside.

~Phil

[shooter]

Mr. Goldberg

I couldn't graduate college till I built one of His devices, mine started with a toilet brush and ended with a nerf ball threw a hoop

[Attila]

Heres a layout of a dummyload with a meter,


I have not built it yet  but seems to make sense:


http://www.ozvalveamps.org/techsite/dummyload/dummyload.htmWhen you work on amplifiers you need a dummy load to take the place of the loudspeakers.
If you doubt that just try it for a while, and if you don't agree now I'm sure that those around you (even if you live in the Gobi desert) will convince you.
One reason is that you want to be able to listen to the amp chassis itself for audible clues during testing.
There isn't a lot to be said about a dummy load.
It's a bunch of power resistors mounted somehow so that you can connect easily and they can get hot harmlessly.
This suggests a metal rather than plastic case, and good ventilation.
It needs typical connectors such as a 6.5mm socket and XLR male and/or any other connectors you encounter like “speak-on”.
I also have a set of binding posts which double for wire connections and as somewhere to sample the signal for the test instrument.
The output power can then be calculated using;

>P = E2 / RWhere;>P is power in watts E is voltage in volts R is resistance in ohms. As a result you find yourself stabbing “25.4 times times equals divide 8 equals” into your bench calculator a lot. So a very handy addition to a dummy load is a wattmeter.A full-on wattmeter is quite a device, measuring voltage and current and accounting for phase differences for accurate power measurement. There is also a device called a Bolometer that measures RF power by heating effect.[/size] But if you are dealing with a fixed, resistive, load as we are with an amplifer, then a square-law voltmeter can be calibrated to read directly in watts on a normal linear-scale meter. As it happens, for low voltages the turn-on characteristic of a germanium diode is a square-law. Initially, and up to a point, the current through the diode rises as the square of the applied voltage across it. As simple as it is, this makes a surprisingly accurate linear-scale wattmeter. Connect and bring out the mid-points for 4 ohm work[/size]Calibrated at 50 watts mine is still better than 5% at 10 and 100 watts. I don't take it too literally in the threshold below 10 watts, but it's still pretty good. The rectifier bridge is four OA81 germanium diodes but it shouldn't be critical and you might find that the e-b junction of germanium transistors salvaged from an old radio could be pressed into service. I was also lucky enough to score a 270-degree scale 1mA meter which makes accurate readings easy.“Are these supposed to glow red-hot?” asked Al as I got a bit carried away with a 400-watt rack amp. An inviting addition would be a cooling fan driven by a rectifier and regulator connected across just one of the load resistors. A possible alternative indicator is to use a logarithmic Line-of-Light LED display driver LM3915 powered like the fan. If I built it again I'd make these changes; more watts, say 400 self-powered fan high-low range on wattmeter 50 ohm wirewound pot and jack to drive monitor speaker

[BlackCrowe604]

Some nice setups here! Thanks for sharing

http://www.parts-express.com/8-ohm-200w-non-inductive-dummy-load-resistor--019-030

I am using the resistor above. Affordable, and does the job for my purposes.

[p2pAmps]

I kinda want to build one along these lines and stick it up under my bench...