lml@lancaironline.net Mailing List Archive

Content-Disposition: inline X-Mailing-List: lancair.list@olsusa.com <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> Welded contacts and coil duty cycle are to some extent separate issues. You want coils that can take some abuse, even if its just during the building and testing process (cycling equipment x times in a row starts looking like continuos duty). You want contacts that won't weld shut under any circumstances. You can find continuous rated relays with poor contacts as easily as intermittent duty relays with great contacts. Buy duty cycle relative to the task to be performed. Buy quality contacts (brand, reputation, etc) at all times. The contacts have no clue what caused them to come together. The force that closes and the springs that open the contacts do have an impact on contact welding however. Also the distance the contacts travel away from each other in the open state is important. The greater the distance the less arcing potential exists, and consequently the lower the chance of welding shut. Welding starts at a minor imperfection that usually results from arcing. You can reduce arcing with diodes, MOV's, capacitors, and small circuits designed for the purpose. The more forceful (read faster) you can make and break the contacts the better. Power almost always equates to size, so if you have two relays that appear equal to the task via their specs, knowing nothing more than that one weighs more than the other one, take the heavy one. Solid state relays are available that eliminate relay contact welding, but they are expensive and don't offer the range of load carrying capacity and contact options the mechanical units do. They are electrically relatively noiseless. Mechanical relay coils are about as dirty electrically as you can get, so solid state relays eliminate much of the snap crackle and pop we hear when we cause relays to function. To a large extent, with relays you get what you pay for. Bill Gradwohl IV-P Builder LML homepage: http://www.olsusa.com/Users/Mkaye/maillist.html