Water Cooled Power Resistors

[defaced]

So, had a thought the other day. Anyone think I can cheat a 150w power resistor to 200w if I mount it to a water cooled copper base? I've got chunks of copper available to me at work, and making a water cooled power resistor would be kinda nerdy/cool.

[J M Fahey]

Just *submerge* the power resistor in clean tap water (use distilled, car-battery-grade if you wish) .
Use an insulated , heat resistant vessel, such as some Pyrex cookware.
You'll easily 4x the power rating.
After 15/30 minutes (more than ample testing time) you'll also be able to make some tea.
Just kidding on the tea water, it may have some lead in it, not kidding at all on the dissipation increase.
Make sure that you have *at least* 1/2 inch over it, preferably 1 inch or whatever your vessel height allows.

[defaced]

Never thought about that. Hmm... going play around with it. Really any non-conductive fluid with semi-good heat transfer properties would work.

[J M Fahey]

Really the classic fluid is oil , with the great advantage of perfect rust protection and long life.
There are some commercial examples, specially non inductive RF transmitter loads.
Anyway, for home use, nothing beats plain water, not only for the price, but because of its higher specific heat value, the fact that by evaporating it can absorb huge surges (although fot short time) and it's definitely less messy; not to forget you can dump it in the drain afterwards..

[defaced]

Excellent! Thank you. Now I can work on buying a scope and load testing my amp. Building a high powered amp presents some interesting challenges that you don't necessarily think about when you start the project.

[Steve Conner]

I've seen a few people stick those big wirewound resistors in a bucket of water, and overload them grossly beyond their ratings. Done it myself on a smaller scale with a jar full of water.

I don't know about the aluminium housed type that are intended for screwing to a heatsink. Seems to me that no matter how well you cooled the casing, there would be a heat transfer bottleneck between it and the element. I have shot clouds of smoke out the ends of those resistors with really heavy surges, and I've heard stories of the end plugs blowing clean out, trailing taser-like coils of electrified resistance wire. But 150W to 200W is by no means a big overload like that. It should handle it fine.

The really big versions of them have a water cooling tube embedded in the middle, and they need both the heatsink and the water to meet their rating.

[fyl]

Check Project 124

[Chuck H]

On another thread here someone wanted a 1500 watt dummy load. He was indeed going to be testing a 1500 watt PA. In the end he used a pair of stove burners in parallel (roughly 8 ohms each) suspended in a tub of water. The burners probably could have handled the watts without the water cooling but there was some concearn about a resistance change with too much heat. A virtually indestructable 4 ohm load.

I wonder what the ohmic value is on one of those nifty foot long 1500 watt stainless steel water heater elements.

[Steve Conner]

120 squared, divided by 1500: about 9.6 ohms.

Well assuming they're not 240 volt, they would be 40 ohms. Our appliances are pretty useless as dummy loads over this side of the pond.

[Enzo]

Don't be silly, just parallel a bunch of them to get the impedance down. And as a bonus they will handle many kilowatts.

[Steve Conner]

I tried using my toaster, but the amp doesn't generate enough voltage to run the magnet that holds the bread in. The toast just pops instantly and my power tubes explode. What do I do?

[redelephant]

I tried using my toaster, but the amp doesn't generate enough voltage to run the magnet that holds the bread in. The toast just pops instantly and my power tubes explode. What do I do?

Hold the toast down with the butter knife normally used to pry them up.

[cbarrow7625]

I have a 5 gallon bucket with a bunch of water heater elements bolted to the bucket lid. The elements are already rated for a few hundred Watts each ( in water). The bucket holds enough to keep things cool for quite a while. Need more dissapation? Put each element in its own bucket.

[R.G.]

I have a 5 gallon bucket with a bunch of water heater elements bolted to the bucket lid. The elements are already rated for a few hundred Watts each ( in water). The bucket holds enough to keep things cool for quite a while. Need more dissapation? Put each element in its own bucket.

That's a good one all right. Actually, water heater elements are usually 1-2KW rated in water. I just did a survey of water heater elements on line. A 120V/1500W element computes out to 9.6 ohms. A 120V/2kW element is 7.2 ohms. Notice that this rating is only when submerged in water. The 120V elements are about $10.00 each.

On a more practical note, the problem with resistance power is not the power rating, it's the power concentration. 100W in a single tungsten filament glows yellow-white, but 600W in a toaster element barely glows at all. The difference is power density.

Want a high power resistor? Spread it out. One 8 ohm 5W resistor will dissipate 5W. Four 8 ohm 5W resistors will dissipate 20W and can still be connected to be 8 ohms. Ten 80 ohm resistors in parallel is 8 ohms, but it's ten times the power dissipation of a single resistor.

From Mouser Electronics, a 24 ohm 25W Xicon wirewound in a cement package is $1.09, and three of these in parallel gives you a 75W 8 ohm resistor for $3.27. A dozen gives you 8 ohms, 300W, and the prices drop to 99cents each, or $11.88 for the whole thing. You could also get the same 300W for $11.88 in 2 ohm with the same 24 ohm resistors, or 4 ohms in 12 ohm resistors, still in the 25W for a dollar package.

[Chuck H]

Yupper. Those cement Xicon resistors are what I used to build my dummy load. I have a bunch of them in two rows, resistors spaced 1/2" apart on aluminum flat bars with aluminum U channels bolted on top of each row. 4/8/16 ohms at over 300 Watts for any setting. And that's just the rated value not taking the heat sinking into account. On the amps I build it just gets warm. Mouser will sometimes even honor the bulk price break when you mix values. Weber has been using the cement block resistors in their attenuators for many years. They're good performers with a very acceptible and predictable derate spec.