Lord Riverside wrote: » Rule no. 1 in this world. Don't trust anyone, never mind rely on them.
Keplar240B wrote: » Your application determines what you want to use. This means you need an understanding of the design and characteristics of different relay types, and your loads. First, I will describe a brutal application where I applied a P&B KUP 11D15 24 VDC coil relay. This application has been in a substantial number of machines in automotive production over the last 40 years. The relays need to be replaced periodically. A good time frame is 3 months, but some have worked for a year. The cycle rate on some may be 3 to 4 times per 24 seconds, or about 6 to 8 per minute. This relay has contacts rated at 10 A AC and uses silver-cadmium contacts. These contacts are very poor at low voltages, but at 120 V provide better durability than plain silver. In my application I paralleled the DPDT contacts, and the contacts were used as a SPST relay. This relay switches a highly inductive load fed from DC at 110 V. The steady state current is slightly over 1 A. There is an RC snubber across the load. Our goal was the shortest drop out time of the electromagnetic clutch. Thus, a shunt diode across the load was not feasible. The failure mode is unidirectional transfer of contact material from one contact to the other. One advantage of not doing the following to lengthen the life was that this gave the plant electricians something to do. To lengthen the life a second relay could have been added that during the off state of this switching relay the second relay would reverse the direction of current that would flow thru the switching relay. What happens when switching DC is that on average (meaning always) some contact material transfers from contact A to contact B. This produces a conical cavity in contact A and a conical mound on contact B. Change to an AC current thru the contact and on average for a randomly operated relay or switch there will be metal flow in both directions. In the old days when breaker points were switching the DC to an ignition coil there was this one way transfer of contact material. Filed May 17, 1943 is patent http://www.freepatentsonline.com/2402543.pdf by Henry Ford and Emil Zoerlein for a distributor design that automatically reversed current direction each cycle. I do not believe it was ever put into production. In the early 50s Ford Motor developed a breaker point set with a hole in the center of the contact that grew the mound. This somewhat increased breaker life. An electromechanical relay that switches moderate voltage AC at a relatively low current will probably fail of mechanical fatigue. At high currents it will probably be contact failure. Contact size and cycle life will determine the relay sized. Lots of AB machine tool relays, back in the days of relay logic, probably had cycle life in the 10,000,000 range. One cycle every 12 seconds is 2,628,000 cycles per year for 24 hours per day. I suspect a lot of machine control panels had some relays that were never changed (repaired or replaced) in 10 years, but not all panels ran 24 hours 365 days per year. However, some relays would operate more that once per 12 second machine cycle. Now to SSRs. For your type of application almost all will have optical or other high resistance isolation between the control signal and the output switch. The output switch is a transistor or equivalent device for DC circuits. Any appropriate solid state device can be the output power switch for AC loads. The amount of leakage current thru this output switch will vary, but is many times significant, and results from the switch device itself, or shunt circuitry around the switch. Major failures of solid state switches will result from excessive current, or overvoltage. If operated within reasonable limits the cycle life can be very great compared to an electromechanical relay. Also SSRs may be operated rapidly. Some normally do not turn off when commanded, but wait until the next current zero crossing of the load current. You need to learn more about the various devices and evaluate the requirements of the load.
lila1 wrote: » I could not believe what I was hearing but she seemed sure of what she was saying.
Fuzzy Clam wrote: » Whew! All that and you didnt read the question. Not what type of relay........ Always read the question.
lila1 wrote: » I have been through the mill for a number of years healthwise and ...
DEFTLEFTHAND wrote: » You find out who your real friends are in such situations.
lazybones32 wrote: » It sounds cold - and it is - but some people cannot cope with it.
endacl wrote: » Your world sounds crap.
Priscilla Beautiful Weight wrote: » What a sad view of life and humanity.
Lord Riverside wrote: » Actually once you learn that, life becomes very happy
Priscilla Beautiful Weight wrote: » But it's a very false perception. I'm over 70 and one thing I have learned in life is that there are people who most definitely are trustworthy and reliable. My life is full and happy because family and friends all help support and assist each other. It's called love.
Lord Riverside wrote: » Firstly I wouldn’t trust anything you've just claimed. Secondly, very few get to live such a charmed life. That's the random lotto of life. I've learned more experiencing both sides of life. I help and support people, but I don't trust or rely on anyone. “Love all, trust a few, do wrong to none.” - William Shakespere.
Priscilla Beautiful Weight wrote: » As long as there are the few in your life that's all that matters. As for not trusting anything I've said; you have no right to not trust me. You have no authority to base that on. I do not lie. I do not claim anything but the truth. The question was asked and I answered it openly and honestly. You don't know me, or those I know can be relied on, so please don't try to paint my world with your imagined view of it. I know it for what it is. You are only making unfounded suppositions. At that, I out.
Lord Riverside wrote: » I'll decide who I trust. Trust is earned, not demanded.
