I moved over to utilizing the PTT instead of the thermostat disks after I'd had a few thermostats fail. But I agree with your thinking.
In my mind the slight inefficiency of running my fans earlier than the heat builds up is not significant to me. I like to think it slows the heat buildup. Once everything reaches a stable temperature and the repeater goes idle, there is no new heat generated and things cool more gradually without the fan blowing. But this is nit-picking. Either way works fine without adding unnecessary circuitry. Chuck WB2EDV ----- Original Message ----- From: "wb6fly" <wb6...@verizon.net> To: <Repeater-Builder@yahoogroups.com> Sent: Sunday, December 21, 2008 4:27 PM Subject: [Repeater-Builder] Re: fan timer circuit >I mostly agree. My thinking is that a fan that runs continuously > wastes power and draws dust and dirt into the equipment. I also > believe that a fan that runs only during PTT is not doing much good > except on very long transmissions- when the heat of the PA has made > it to the fins. It may take several minutes of key-down time before > the fins start getting warm, so running the fan before then is > ineffective. > > My practice is to fix a normally-open thermal switch to one of the > heat-sink fins, using heat-sink compound for good heat transfer. I > have found that a switch that closes at 122 degrees Fahrenheit (50 > degrees Celsius) works best, since it opens about ten degrees lower, > and provides near-ideal hysteresis. The thermal switch directly > controls a small AC or DC fan that blows on the fins. Not only is > this an extremely simple solution, but it ensures that the fan will > run only when needed and for as long as needed. > > I use a Cantherm switch that Digi-Key sells under catalog number 317- > 1094-ND for about $7. > > 73, Eric Lemmon WB6FLY > >
