Test Loads for Dummies [N00bs]

Hey Guys,

Just after a bit of help re dummy loads for testing comps/power amps etc

What sort of power handling do the resistors need to have? Presumably greater than the power output of the device under test yeah?

So I thought of making a switchable dummy load with some 4 ohm 50W resistors on a heatsink, in a box to test some guitar amps... I had read about suspending them in clean oil for heat absorption but am not sure about this.

What about a 600ohm for testing La2a/STA etc?

Also, can anyone recommend some good books for learning about general audio circuit testing/troubleshooting?

Thanks for your time (and patience with the N00b)

Kind Regards,

Matt

600Ω load for line level stuff shouldn't be a problem, the output power of those is quite moderate and a few watts resistor will stay cool.

For testing amps using just a bucket of oil will extend the heat up time but it won't cut it for long test periods, then you'll need a radiator. Just do the math as any thermal design, if you get a 50W resistor it should be capable of dissipating 50W, probably it will come with a heatsink already mounted, in which case you could add a 12V pc cooler to help.

There are already a few designs out there which may help you, go to google and you find a few useful stuff.

JS

Hey JS,

Thanks for quick reply and the advice.

Have found some schematics online, will give one a go.

Thanks again.

Matt

I build the first schematic in this PDF for a guitar amp dummy load.  Works great.  Gets warm,  but not too hot.

http://www.aikenamps.com/images/Documents/DummyLoad.pdf

I have the mouser cart i put together saved still:

http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=fe8fa19779

For serious power amp testing you can use water heater elements (sitting in a bucket of water) to manage kW power levels.  8 ohms would be an 1,800W heater element.

JR

Woot said:

What sort of power handling do the resistors need to have? Presumably greater than the power output of the device under test yeah?

Click to expand...

The rated power of resistors is the power they can handle and not being destroyed. At rated power, resistors get very hot and as a consequence, their value increases, which in turn falsifies the measurement. This is not a major issue for DIY, but is a major concern for professional measurement.
I would recommend you apply a factor 4 or 5 between the estimated power you want to measure and the rating of the resistor(s).
High-power resistors tend to be very expensive.
In most cases I have found using multiple resistors of lower rating to be economically sensible. One has to be careful with the connections. Resistors get so hot that solder melts!

Hey Guys,

Thanks for the replies and the info.

Yeah the Vishay wire wound resistors are not cheap at 180USD each!

JR, presume that element would need to be different for 240 supply?

Thanks for your time.

Matt

Woot said:

Hey Guys,

Thanks for the replies and the info.

Yeah the Vishay wire wound resistors are not cheap at 180USD each!

Click to expand...

  Indeed. 50W resistors can be found at less than $5.

JR, presume that element would need to be different for 240 supply?

Click to expand...

At 240V, an 8ohm load is 7200W. Indeed you have to order a US kettle from amazon  ;D

At work in the R&D lab we sometimes used clothes dryer or oven heating elements as dummy loads (not hi-fi related)
Now the local surplus shop sell's 100W power resistors for $10 to 30 USD.

Speedskater said:

At work in the R&D lab we sometimes used clothes dryer or oven heating elements as dummy loads (not hi-fi related)
Now the local surplus shop sell's 100W power resistors for $10 to 30 USD.

Click to expand...

No need for surplus.
Digikey has 50W chassis types for less than $4 a piece.
http://www.digikey.com/product-search/en?pv1=1350&pv2=13&FV=ffe88a31&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

Woot said:

Hey Guys,

Thanks for the replies and the info.

Yeah the Vishay wire wound resistors are not cheap at 180USD each!

JR, presume that element would need to be different for 240 supply?

Thanks for your time.

Matt

Click to expand...

Yes sorry, i calculated for 120VAC mains...

A high power load will make a bunch of heat, so looking at heater elements is not crazy... Do the math for your mains voltage.

JR

No need for the surplus store, for sure!
They want $5.00 for most 50W chassis types.

Speedskater said:

No need for the surplus store, for sure!
They want $5.00 for most 50W chassis types.

Click to expand...

So you only need 20 of them to test a 1,000W amp.  ;D

I attached a picture of  a DIY cooker I made from 4x 50W resistors....  After a few months of use all the solder connections are dull (overheated) and some of those white wires turned brown.

JR

Heathkit had an oil cooled dummy load for Ham radio transmitters.

At the FM broadcast transmitter, they had a 25,000 Watt @ 100 MHz water cooled dummy load.  The load was a tiny thing, but the water cooling system was large and problematic.

https://www.google.com/search?q=cantenna+dummy+load&biw=1024&bih=725&tbm=isch&tbo=u&source=univ&sa=X&ei=c5c7VeuZLIGjsQWttoCgAw&sqi=2&ved=0CDIQsAQ

RF dummy loads present an additional challenge; the size must be smaller than quarter wavelength. That often means requiring forced cooling.

The FM dummy load was very small.

but that photo was bigger than I expected.

Tom Christiansen write about his dummy load:

Nearly a decade ago I picked up a bunch of Vishay-Dale RH-25 25 W non-inductive resistors. I found 20 of them on eBay for not that much money. 0.8 ohm each. Perfect for building an 8 ohm dummy load with a 4 ohm tap in the middle. Put the two halves in parallel for 2 ohm.
I have them mounted on a heat sink that came from a cell phone base station (also an ebay find). The heat sink has two fans on it, temperature controlled with an NTC resistor. Works great. I think the entire rig set me back, maybe, $50 plus a few hours of drilling and tapping holes.
I routinely dissipate 100-200 W into that thing. With the fan, the resistors stay lukewarm, but my lab sure gets toasty... Nothing beats a space heater with 0.00014 % THD.