Dummy Load question

[Reader]

So if I parallel four 16 ohm 100 watt dales then I can clear up to 400 watts at 4 ohm load.

Are these the right ones?
https://www.mouser.com/ProductDetail...oSeteqMtbnA%3d

Yes. But see the resistor data sheet and note that the 100W rating only applies when the resistor is properly mounted on a suitable heat sink. Without the heat sink it is only a 30 W resistor.

[Enzo]

Add a fan too.

My bench loads came up to my patch panel, and using the jumpers I had for the panel, I could configure various load impedances. I agree that is a handy side effect of making one up from smaller units.

[Helmholtz]

Not that it should matter terribly if you're testing a 4 ohm output at 5 ohms or 3 ohms. But everybody seems to think it does.

For SS amps with close to zero output impedance it does matter. As output voltage essentially stays constant, the power delivered into 3 Ohm will be up to 66% higher than with 5 Ohm.

[Chuck H]

For SS amps with close to zero output impedance it does matter. As output voltage essentially stays constant, the power delivered into 3 Ohm will be up to 66% higher than with 5 Ohm.

Yes. And thank you for the distinction. I should have said that for many repairs it shouldn't matter terribly.

[nevetslab]

Add a fan too.

My bench loads came up to my patch panel, and using the jumpers I had for the panel, I could configure various load impedances. I agree that is a handy side effect of making one up from smaller units.

When faced with insufficient wattage in a dummy load for testing power amps (or output amps in a typical guitar or bass amp), I've often resorted to making the measurement under Tone Burst mode. The most common source for that is with function generators that have both a Triggered and Gated output function. Gated mode would be the one, where you can set the 'ON' time to be long enough to see the full power before it gates off. Such as 200mS ON time, 800mS Off time. You'd trigger the scope on the gate pulse, and use a 1 Sec/Div time base setting, and set your frequency as desired. If you need to use a low frequency signal, you'd set the on/off times longer, as well as your time base setting would be longer.

I also have an old GenRad 1396B Tone Burst Generator that does the same thing, while being able to use any signal source you want. (I regularly use the 1396B together with a GenRad 1382A Pink Noise Generator with a Krohn-Hite 3550A filter for setting the noise bandwidth as desired. Usually set for 2 Sec on, 3 sec off as the rep rates) Very few of us have Audio Precision analyzers or Sound Technology 3100B generators, both of which have dual level Tone Burst modes, allowing either counted cycles, on/off times, as well as setting the Off time as a dB level below the On time. Any of these allow the use of your limited wattage dummy load, which will normally handle short bursts of power into the load well above the steady state power rating.

[J M Fahey]

I make my own with precision and for peanuts out of cheap spiral wound nichrome wire intended for ultracheap food heaters (probably forbidden in lawyer happy First and Second World for safety reasons, since live and red hot wire is exposed).

You can not solder it so eyelets or screws are needed for contact.

Use them straight for low power tests (up to 100W), add a fan or drop (200W) them in a bucket of water or oil for scary high power dissipation (400W each resistor section) .





made for these:



Just stretch wire , measure 8 ohm sections, cut and mount each, combine as needed.

Remember mounting base must be insulating, resist red hot temperatures and , if used wet, stand water or oil.

I use Micanite, which is the sheet material normally used for such purposes .... wrongly called Mica which it is not.

Micanite is to Mica what chipboard or MDF is to wood.

You can get both out of a cheap hair dryer:



Notice the elements I mentioned: nichrome wire, micanite and eyelets.

You may even add an external power supply for the motor and fan and kludge an air cooled load resistor.

Motors are typically 6 to 12V or thereabouts and run in series with heater resistor, that´s why I suggest an external independent supply.

Here you can see the bridge rectifier so they get DC for motor.

[Chuck H]

I like the waffle iron thingy. You can test your amp while toasting a snack

[g1]

I think the nichrome is also used in clothes dryers?
One thing I do like to make sure of with non-conventional loads is that the resistance does not increase when under full power it will be used at. (check that current verifies with voltage for stated resistance)

[nosaj]

I like the waffle iron thingy. You can test your amp while toasting a snack

Some people use a hot water kettle with water in it.
nosaj

[Reader]

I remember that Heathkit used to sell a dummy load built into a 1 gallon paint can filled with oil. IIRC it was an RF load meant to take the place of an antenna. At the time I did not understand what it was for. I was just browsing my Dad's Heath catalog looking for HiFi kits.

[Enzo]

The Heathkit dummy load built into a gallon paint can was called the...

Cantenna

https://www.orcadxcc.org/content/cantenna_va7jw.pdf

[big_teee]

The Heathkit dummy load built into a gallon paint can was called the...

Cantenna

https://www.orcadxcc.org/content/cantenna_va7jw.pdf

I still have one of those..
I bought it in the early 80s.
You have to furnish your own oil.
I tuned my KW ham amp into it.
Worked great.
T

[TimmyP1955]

A pair of these: https://www.alliedelec.com/ohmite-c3...yABEgI8z_D_BwE

[DrGonz78]

I found a good deal on a Milwaukee ribbed wire wound 4 ohm 420 watt resistor. Would this be a good one to get? The price is like $25 or something.

https://www.vishay.com/milwaukee/ribwound/

[Reader]

I found a good deal on a Milwaukee ribbed wire wound 4 ohm 420 watt resistor. Would this be a good one to get? The price is like $25 or something.

https://www.vishay.com/milwaukee/ribwound/

One of those would work . You will need to mount it in a suitable enclosure that will allow for convection cooling. You can check if the supplier offers mounting clips. They could be a better solution than coming up with your own.