Re: dumping a lot of heat

john larkin <JL@gct.com> wrote:

On Sat, 07 Dec 2024 11:49:14 -0500, legg <legg@nospam.magma.ca> wrote:
 

On Thu, 5 Dec 2024 22:03:41 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
>

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

>
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.

>
By standard 'safe' design, tubes have a pretty limited life.
>
By 1956, the heater was no longer the weakest element in
the life equation for these parts - glass electrolysis was.
>
Electrolytic caps and their use has always been an issue.
Cuffing the tubes not only enforces distance to other
components, but reduces radiant effects in the viscinity.
>
Win, win.
>
RL

 
Tubes were awful. Still are.

The techniques for designing with them are quite different from
transistors and ICs, you have to think a different way; they aren't just
poor transistors, they have a different lifestyle altogether.  They also
have some advantages over semiconductors:

1)  Withstanding short term overloads without damage.

2)  Dissipating energy in a smaller space at higher temperatures.

3)  Easier to make with few exotic materials or processes.

4)  EMP-proof and radiation-proof in the event of nuclear war.

Those properties aren't needed most of the time but when they are,
valves are a lot easier to design with than transistors if you are
conversant and comfortable with the technology.  There are still very
few single transistors that can out-perform a humble EF91 from the
1940s.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk