Power supply fusing can be tricky. It has been my practice to use ptc devices embedded into a transformer primary as the "fuse," they're sometimes called everlasting fuses. They detect temperature in their immediate environment (eg transformer winding) and due to self-heating. They trip at a particular temperature and suddenly go high resistance, reducing current flow to very low (safe) level. They can only be reset by removing the power completely for a short time, if the environment has cooled down enough. They enable very stringent Eu safety regs to met with relative ease. Mass produced transformer bobbins are often made with a little pocket in the moulding to hold the device.
Ordinary thermal fuses are often mis-used. Some common fuses are rated to blow in 30s with 2.1 times their current rating. Many circuits cannot generate sufficient current so the fuse will never blow except in times of a dead short. In a supply primary, thermal lag fuses are fitted to prevent them blowing at start-up which exacerbates the problem. Answer: use a ptc device, it will protect that expensive transformer from cooking itself to death. David G3UNA > Just a comment and a question. > I take care of our club's repeaters. One of the power supplies failed this > weekend. I watched as smoke came out of it. It is 20+ years old. Here is > the comment; make sure the fuse on the primary is properly matched, even > if > you choose to go lighter. (It's not good to work the PS to the max > anyway.) > It may not be possible to fuse a power supply to protect everything in it, > but at least you can protect the transformer. (I realize this thread is > talking small for the KX3, but the subject is power supplies.) > > Question; Elecraft radios are protected against low voltage, but what > about > high voltage? I'm wondering if a FUSED Crowbar circuit would be practical. > The fuse (if any) on the PS 12V leads is behind the crowbar. It seems to > me that a versatle fused crowbar circuit would be good. > Has anybody been there and done that? Maybe someone sells one. I'm past > the > age when I quit keeping up on components and their specs, so I appreciate > comments aye or nay. > > Rich, n0ce > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
