Dummy load project, 800w hybrid ...

[Klondike Mike]

I picked up one of these from Henry Radio with the intention of building an inexpensive, quality dummy load. With no intention of bolting this to a railroad rail for maximum dissipation, I have been pondering what kind of home to build for this without getting too elaborate. All ideas welcome.

New 800 Watt Hybrid 50 ohm Load Resistor to 500 MHz


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[Robb]

Screw it down on a fine-finned heat sink and put it in a 1 qt paint can filled with mineral oil. Put a SO-239 on the top/center of the can's lid. When it is in the oil, it will dissipate a lot more watts that it would in the air with a heatsink alone. If you can get some real xformer oil instead of mineral oil, it will dissipate ~2X as much heat as the mineral oil will. That should do it . . .

 

[Beetle]

Railroad ties are wooden, as in "they wooden dissipate much heat!".

Seriously. Wood is a poor heat sink. Even a section of rail itself wouldn't be all that good, and really inefficient. You need lots of surface area (a heat sink, as Robb suggested) and lots of air - or oil - or some other non-electrically-conductive medium to conduct the heat away from the heat sink.

Visit your local electrical utility. You can usually get a gallon or two of never-been-used transformer oil for the cost of a dozen donuts.

 

[Captain Kilowatt]

If you want a dry load then start off with two of these Large Flat Aluminum Heatsink Heat Sink 11 5" x 4 5" x 1 5" | eBay
bolted to a single piece of copper plate. The copper plate is called a spreader and will transfer heat much better. You MUST use a heatsink compound between the device and the copper plate and between the copper and aluminum plates. Only a very tiny amount is needed. Too much is as bad or worse than not enough.You will need a fan on it as well. If you want to put it in an oil bath then omit the copper plate and use one of the heatsinks. Above all do not push it too hard.

I have been looking at those myself to make a high power load. Getting tired of my present load even though it will handle a couple kilowatts easily but it is oil filled. I want something cleaner.
http://www.ebay.com/itm/large-flat-...176?pt=LH_DefaultDomain_0&hash=item2ec996b698

 

[Klondike Mike]

Railroad ties are wooden, as in "they wooden dissipate much heat!". Seriously. Wood is a poor heat sink ...Visit your local electrical utility. You can usually get a gallon or two of never-been-used transformer oil for the cost of a dozen donuts.

I'm working in my yard today on landscaping ties and obviously was front in mind. I meant railroad rail. So much for trying to be witty.

Screw it down on a fine-finned heat sink and put it in a 1 qt paint can filled with mineral oil.

Classic idea with the can. My initial thoughts were to mount to a chunk of heat-sink and case, something along the lines of your standard mobile amp style. I will consider your idea and research you've done on the mineral/transformer oil heat dissipation properties.

If you want a dry load then start off with two of these Large Flat Aluminum Heatsink Heat Sink 11 5" x 4 5" x 1 5" | eBay bolted to a single piece of copper plate. The copper plate is called a spreader and will transfer heat much better. You MUST use a heatsink compound between the device and the copper plate and between the copper and aluminum plates. Only a very tiny amount is needed. Too much is as bad or worse than not enough.You will need a fan on it as well. If you want to put it in an oil bath then omit the copper plate and use one of the heatsinks. I have been looking at those myself to make a high power load. Getting tired of my present load even though it will handle a couple kilowatts easily but it is oil filled. I want something cleaner.

CK, I'm glad you were thinking about this already and shared your thoughts. I really wanted to avoid the potential, leaky oil container.

 

[Shockwave]

The size of the heatsink and oil cooling will determine the duty cycle and if the part can reach its full power rating. Transistors that dissipate anything close to this power must be mounted on a copper spreader as CK described regardless of how big the heatsink may be. The reason is 800 watts of heat coming out of a postage stamp sized package cannot be dissipated directly into aluminum fast enough. The part will overheat before its maximum rating. If you don't like oil, consider adding a fan if you don't mind the power cord.

 

[Klondike Mike]

Thanks for the tip on the copper spreader, I had not considered this.

 

[RatsoW8]

I think your construction will depend on how much power you want it to handle and for how long. Go small for low power and short duration or full blown heat sink in the paint can of oil fro high power.

I rarely run more that 100 watts so I'd go the route you were thinking about the size of a small mobile amp.

 

[KE0ZU]

As has been mentioned by many already, the difficulty is getting the heat OUT of that miniscule device. Loads require good thermal conductivity, and a lot of surface area to dissipate the heat, and if the vendor has any applications/design info for actually making a working load, that would be the place to start.

Here, is what the size of an 800 Watt conventional, Carbon element, oil bath, continuous duty rated, load looks like. I guess your load is not much bigger than the size of the "N" connector on the end of this load. You have some serious thermal transfer problems to overcome, especially if you want to achieve an 800 Watt continuous dissipation rating. By no means am I trying to discourage you from going for the gold here, just trying to give you a sense of scale.

 

[Bwiliam]

Doing something similar, using 2 x 100 ohm 250watt hybrid resistors to make a dummy load for working on a ft-901dm Yaesu hf transceiver. This is going to be a wet unit. Using one on each of these HS and they just fit into the gal paint can. DO NOT use heat sink compound in oil or the compound will flow out, and contamiate the oil. Most HS compounds are solids mixed with silicone oil. The cooling oil after a few seconds will act as a heat sink compound between the resistor and the heat sink body, ie under the risistor, as it flows into the tiny nooks and ridges on the heat sink.
The sinks have room for 2 more 250 watter when i get a few more $$ to reconfigure then or replace them with 500s or 800s.
The heat sink i am using is here , http://stores.ebay.com/Eagle-Electronics/_i.html?_nkw=heat+sink&submit=Search&_sid=996244968
useing the silver ones 90x90x25 size.
You can use them dry to as you can put them in a tube with a fan on bottom and blow thru the tube helping make a good cooler for the resistors too.