Re: dumping a lot of heat

On Thu, 05 Dec 2024 08:01:01 -0500, legg <legg@nospam.magma.ca> wrote:

On Thu, 5 Dec 2024 08:55:32 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
>

john larkin <jl@glen--canyon.com> wrote:
>

I'm thinking about building a biggish rackmount dummy load box. It
would simulate series resistance and inductance. Part of the problem
is that it will need to dump a lot of heat.
 
We are using copper CPU coolers on PC boards, which are great up to a
couple of hundred watts, but I'd like to do a kilowatt or two.
 
https://highlandtechnology.com/Product/P945
 
It would take a heap of expensive extruded heat sinks and fans to get
rid of a kilowatt. At 1 K/W, a pretty good heat sink, that's 1000 degC
temp rise.
 
A small hair dryer can dump a kilowatt. So some sort of red-hot
nichrome coils and a vicious fan might work.
 
I'd prefer to not use water.
 
I wonder if there is some sort of runs-red-hot power resistor.

>
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.

>
https://ve3ute.ca/2000a.html
>
RL

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.