dumping a lot of heat

Yes. A toaster element: Nichrome wire woven onto a mica card in a perforated metal box.

Already packaged, one can get ballast resistors for high-power VFDs used to power machine tools. The ballast resistor absorbs the energy stored in a large chuck spinning fast, when it's time to come to a stop quickly without over-voltage tripping the VFD.

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Joe Gwinn

I once used a 2kW grill element as a surge current limit resistor, mounted in top of the 19" cabinet. A thermal cutout (clixon) above it interrupted mains power (and issued an alarm) in case the shorting relay failed to close after 5 seconds. The idea was to charge up all PSU input caps for a 10kW power system before enabling the outputs. Worked fine.

Arie

Cooker hob ring?

I once used three industrial fan heaters as the starting resistor for a large 3-phase motor with a big inertial load. They were wired in delta but connected to the star point of the motor - an arrangement which gave the required starting current and torque. After a predetermined time, a normal 3-phase contactor shorted them out and closed the star point, allowing the motor to run up to full speed. The system worked flawlessly, up to 8 times a day, for several years.

There are those Edison base ceramic heaters . Maybe you can cobble up something with those.

Cheers

If you are using elements at near red heat, remember you need to keep the radiant heat away from the outer walls of the cabinet. Reflectors just throw the problem elsewhere and eventually will tarnish, the best system is several spaced blackened steel baffle plates with vertical air passages between them (visual black is not always IR black).

If you need a rapidly-controllable load, valves can dissipate energy at a much higher temperature than transistors, so they might be worth considering.

HVAC heater elements are available, premounted in ductwork.

Typical value 10R xKw. Tap, cut or series/parallel to your heart's content.

The components to build similar structures, ceramic insulators, and terminals and coiled resistance wire (of selected tempco) are all available off the shelf. Punch your own metalwork.

RL

Seems to me that black baffles will absorb IR and get hot, so devolve to air-cooled heat sinks.

The solution to the IR problem is to reduce the element temperature so's to shift the dissipation from radiation to air heat transfer. I think the math on that is good.

I don't much mind if a user sees a dull red glow through the slots on the back of a rackmount box. Maybe we could add a cosmetic black baffle, but it can't be allowed to absorb much IR or it becomes the heat sink. Maybe a few percent would be OK.

If I reflect the glow down, only very short people will see it.

Probably not practical. Tubes are big and full of expensive vacuum which doesn't conduct heat well. May as well use some giant tubular incandescent lamp which has the same glass and heater as a big tube. Most are filled with a gas that conducts heat better than vacuum, so would bet more air cooling from the glass.

Tubes radiate heat from the plate, so I'd need to step up my source to high voltage.

Fun to think about.

If I'm going to use a finned circular heat sink, why put a tube inside? Resistors would be cheaper.

But as noted, it would be hard to dump a kilowatt or two into a finned heat sink. Well, unless I let its surface temp approach incandescence.

I'm considering using a class-D amplifier that can sometimes be back-driven as a dummy load, in which case it pumps up its own power supply, normally something like 70 volts maybe. I need to synthesize the equivalent of a two kilowatt, 75 volt zener.

We're just brainstorming a product at this point, toying with goofy ideas. With the right people, that is as Phil says, the most fun you can have standing up.

Yeah, some commercial heating element would be great.

I'd rather not have to terminate the ends of a red-hot nichrome wire, if I can buy that already done. Electric clothes dryer elements look interesting. I'll look into duct heaters too; thanks for that tip.

Yes, all the energy is eventually going to finish up heating the air, the only question is the pathway it takes. One way to avoid that would be to construct a massive infra-red searchlight beaming the energy away from the earth - or a broadcast transmitter beaming upwards.

To dump heat into the air, you either have to have something very conductive with a large surface area or you need another way of spreading the energy across a suface, such as heat radiation. A big sheet of thin, blackened steel plate for heating by radiation is a lot cheaper than a thick die-cast aluminium lump with fins for heating by conduction.

The economics of mechanically-forced air cooling are better than convection unless you are able to use a tall 'chimney', so that the energy of the waste heat is used to generate the draught.

The economics aren't so good.if you have to spread the heat over a large area by conduction alone.

Yes, I did say 'rapidly-controllable'; if you just want a fixed resistor, a valve would be a highly uneconomical way of doing it.

Most of my valve designs err on the safe side. Putting electrolytic capacitors where they can get hot is just plain stupid. I have seen loads of shoddy radio/audio designs where the cathode resistor was strapped along the side of the bypass capacitor or the main smoother was stood up right next to the rectifier or the output valve.

A full sized rack cabinet could be remarkably similar to a chimney if it had baffles in the right places.

John

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You could do what large battery analysis equipment does and feed the energy back to the AC supply. Then you wouldn't have to dissipate the heat within your equipment.

Grid-tied inverters for solar power are relatively low cost (< $1/Watt) and don't use large transformers so it shouldn't be too difficult, although there are probably lots of rules they would need to honour.

kw

Yes, as long as the ceiling of the room isn't lined with polystyrene tiles. It might not be very good for the air conditioning system in the building where this is installed, regardless of which dissipation method is used.

In the days of germainium transistors, one firm used to make cabinets with a 'clerestory' roof, like an upturned tray suppoted on spacing pillars above the ventilation holes in the roof proper. This allowed the heated air to flow out under the lip in case some idiot put the instruction manual on the top of the cabinet.

Another possibility, especially if there aren't going to be many of these on sale and the installation will be done by the firm that makes them, is to make a hole in the wall and stick some stainless-steel boiler flues up the outside of the building. Even better, in an old building. use a redundant fireplace and put the contol box in a decorative housing on the mantlepiece.

We have in mind some commercial rackmount products, 1U to maybe 5U dummy loads that people would buy and bolt into their 19" racks like any other instrument. They would blow hot air out the back, like most other gear.

My users typically have cold forced air enter a rack from below and expect it to exit near the top. I have one customer that controls the air temp to milliKelvins, in what may be the world's biggest clean room.

You are going to need an awful lot of air if the exit temperature isn't to be too high. Ordinary domestic fan heaters seem to be the best starting point and some models would fit comfortably as a side-by-side pair into a 5U 19" rack dissipating about 3kW each. Any more than about

Yeah, small shops often do this kind of thing. Cheap, and works just fine. Probably the original brake resistor had failed.

Joe Gwinn