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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: dumping a lot of heat
Date: Sun, 8 Dec 2024 13:33:39 +0000
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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