Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: john larkin Newsgroups: sci.electronics.design Subject: Re: dumping a lot of heat Date: Sun, 08 Dec 2024 07:47:25 -0800 Organization: A noiseless patient Spider Lines: 109 Message-ID: References: <1r434eh.9mfcivcsdztaN%liz@poppyrecords.invalid.invalid> <1r43vdz.1x4sizvmjxptrN%liz@poppyrecords.invalid.invalid> <1r490yz.1xraied16vto76N%liz@poppyrecords.invalid.invalid> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Sun, 08 Dec 2024 16:47:28 +0100 (CET) Injection-Info: dont-email.me; posting-host="72093d06a0b0410a65e8b14975ebf113"; logging-data="4083670"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/CVwIHGcIBqOJ/xjZ4b1eX" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:shy+YQerx7nkG+AANR8Qu6QhWIs= Bytes: 5764 On Sun, 8 Dec 2024 14:15:54 -0000 (UTC), Phil Hobbs wrote: >Liz Tuddenham wrote: >> john larkin wrote: >> >>> On Sat, 07 Dec 2024 11:49:14 -0500, legg wrote: >>> >>>> On Thu, 5 Dec 2024 22:03:41 +0000, liz@poppyrecords.invalid.invalid >>>> (Liz Tuddenham) wrote: >>>> >>>>> legg wrote: >>>>> >>>>>> On Thu, 5 Dec 2024 08:55:32 +0000, liz@poppyrecords.invalid.invalid >>>>>> (Liz Tuddenham) wrote: >>>>>> >>>>>>> john larkin 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. >> >> > > Yup. EF91 is known as "Not the valve that won the war." Pentodes are *noisy*, at least 10 times more than some 3 cent jfet. There are a couple of high-voltage tubes that were, until recently, worth at least considering. But multi-KV fets are more sensible these days. I used to use the 1B3 HV rectifier diode as an amp, with the filament voltage modulating the conductivity. The downsides were the bandwidth and the X-ray hazard. One could make an optocoupled tube half-bridge, or full-bridge, with maybe a 30 KV supply. I guess you'd use batteries for filament power.