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From: Jan Panteltje <alien@comet.invalid>
Newsgroups: sci.electronics.design
Subject: Re: dumping a lot of heat
Date: Sun, 08 Dec 2024 15:28:31 GMT
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On a sunny day (Sun, 8 Dec 2024 13:33:39 +0000) it happened
liz@poppyrecords.invalid.invalid (Liz Tuddenham) wrote in
<1r490yz.1xraied16vto76N%liz@poppyrecords.invalid.invalid>:

>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.

Thermal overloads depend on teh heatsink.


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

Dissipating a lot of heat with the heater without any net output.
So low efficiency by default.


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

 https://www.google.com/search?make+your+own+transistor


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

Depends.
The glass may easily break of the impact shock.


Tubes need high voltages to work usually..
I can run a JFET on 10 mV or so
 https://panteltje.nl/pub/lighting_a_LED_with_a_candle_IMG_3604.GIF
  than transform the voltage up.
Useful when the grid is down in nuculear war

Tubes had strong aging, studio equipment using tubes needed re-calibration all the time.



>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.

Lots can be done and have been done with dual gate MOSFETs

LIfetime of transistors is way better than tubes, especially in power applicatioin like for example TVs
and related equipment.

That said, my 1979 Trio oscilloscope's CRT still works (used it yesterday)
But it is a transistor scope.
Color CRT transistoir based monitor in the attic likely works too, have not tried it in years

Fluorescent toobs work too, have some for above the workbench upstairs

But the LED bulbs are nice, some are better.