My resistive dummy load

[rockman627]

Is this workable? It's a 4 ohm 100 watt wirewound resistor and a fender speaker cable. It seems too easy.

[Helmholtz]

That resistor will need a massive heatsink for 100W continuous.
Look up the datasheet.
Without heatsink max. dissipation might be around 25W.
Power derating often starts at 75°C and ends at 150°C case temperature.
Means that at 150°C the resistor can't take any more power without risking damage.

[rockman627]

I bought three resistors, 4,8,&16 ohms. I guess I'll need to buy 3 heatsinks. I was hoping Mouser would recommend an appropriate heatsink but no. How do I even determine a correct one?

[The Dude]

I used a large aluminum plate to mount mine. There are certainly other ways to go.

[nosaj]

I bought three resistors, 4,8,&16 ohms. I guess I'll need to buy 3 heatsinks. I was hoping Mouser would recommend an appropriate heatsink but no. How do I even determine a correct one?

Reuse a car audio amplifier heatsink?
nosaj

[glebert]

I think any chunk of angle iron or steel tubing or whatever would probably work, especially for guitar amps. Bass amps you need to be able to move more heat out. I use a couple of converted space heaters that should be able to handle a couple thousand watts, but that is perhaps overkill.

[Chuck H]

My dummy load uses the same aluminum housed resistors in the fifty watt version (they're A LOT cheaper per watt than the 100W units) run in parallel with multiple series/parallel switching to accomodate 4/8/16 loads. I think my minimum rated selection is 150W . And I have them mounted with generous spacing on the inside, top surface of a larg-ish aluminum chassis with an extra 1/4" aluminum plate right on that surface. Heat conductive paste was used for the assembly. I haven't had to run anything over 120W through it. And that particular project had me using it hard at that wattage for several minutes at a time. It hasn't given me any trouble. Mine also has a speaker connection that saps a little off the load with a volume control so I can listen at will while testing. That's actually come in real handy a few times.

[Chuck H]

We had a dummy load thread here a while back and it was mentioned that electric water heater elements present a useful resistance for our purposes. They remain relatively consistent in value in this application and, as long as they're in a large water bath, can handle some fifteen hundred watts. I've considered building a such a thing for the cool factor but the truth is I really don't have a use for it.

[soundmasterg]

Hunt down some old electronics surplus stores if there are still some in your area and find the huge resistors that look like a big tube. You can likely get them really cheap and they will handle huge amounts of power in comparison to that little one you have there. Or if you want to buy a new one, get one of these.

https://www.mouser.com/ProductDetail...QaAi8lEALw_wcB

I have a metal chassis that I mounted some of these Ohmite resistors like that one in, all 16 ohms and around the same size as that one, and then I have switching to combine them together to get 16 ohms, 8 ohms, 4 ohms, or 2 ohms, and all 500W or better minimum, so a 4 ohm load is 2000W I guess. Anyway, the resistors never got even minimally hot with a SVT running full tilt into it for awhile to burn in some tubes. I think I got them all at a surplus place for maybe $1 each.

Greg

[x-pro]

It is not necessary to buy 3 radiators. You can have one common one.
The heatsink does not have to be large. You can use, for example, a heatsink from an old computer processor. It can even be equipped with a fan. Such a fan in operating mode allows you to significantly reduce the size of the cooler.
But the fan will need a power supply.

[Jon Snell]

I power the fan I use on a heatsink by rectifying the AC across the load. The more power = the more voltage = the more cooling due to the speed of the DC motor driving the fan.

[Helmholtz]

The reference or standard heatsink for the Ohmite/Arcol type HS 100 is an aluminum plate having a thickness of 3mm and an area of 995 cm² (corresponding to around 12" by 12").

https://eu.mouser.com/datasheet/2/30...et3-779273.pdf

[Chuck H]

The reference or standard heatsink for the Ohmite/Arcol type HS 100 is an aluminum plate having a thickness of 3mm and an area of 995 cm² (corresponding to around 12" by 12").

https://eu.mouser.com/datasheet/2/30...et3-779273.pdf

Well I'm not using that much footprint per resistor for my unit. I haven't had a problem, but I've never pushed it THAT hard. If I ever thought I needed a really high power dummy load I'd build something bullet proof from heater elements or a motor/fan like Jon.

Most tests don't take that long. This could be something to consider. But then a lot of techs use their dummy load for burn testing repairs. In that case you really do need to be sure your load is up to the continuous amplifier power.

[loudthud]

See if you can find a CPU heatsink with fan for a CPU that isn't made anymore.

[Chuck H]

Perusing Home Depot you might find aluminum channel. My store here does carry it in many sizes. When I fisrt saw it "heatsink" was the first thing I thought of.