I have a engineering question. When dealing with singling equipment for relays, timers, and contactors when should filtering be considered? If there is electronics involved,definitely. When multiple relay coils firing on and off, when does it start to be praticial to start adding filters?
IBM used tons of relays back in the olden days and we never thought of filtering. Even the original TTL and DTL logic using discrete transistors on cards was pretty noise tolerant. It was when we got to miniaturized electronics on chips with fairly low currents and low voltages that the problem arose.
Thanx guys. Good info. That Is what I kind of figured. PLC's definently have their places. Now with them being more readily available, they starting to really get my interest. What I do, though the simplier the end product is, the better the long term of the systems reliability and maitenance is. Even with all the added work up front. "Remote" Alaska is a bit behind the times and will be for a long time just because of logisitics. More input is always appreciated
Various relay and industrial controls manufactures offer a wide assortment of noise suppression components. For DC coils, a 1N4003 diode across the coil will suppress most inductive kicks. For AC coils, a snubber made up of a resistor and capacitor will provide a shunt path to limit the inductive kick.
Seperation and proper shielding is the key to reduce cross talk issues. For sensors with high impedance inputs, twisted shielded pairs are usually the answer. Twisted pairs ensures any noise picked up by the wiring is present equally on both wires. Since the input is typically looking at the difference between the two wires, the same noise on both wires is ignored.
Shielding is a bit trickier. If you are shielding against low frequency noise (DC up to 50-100 KHz) connecting one end of the shield to frame or earth will usually do it. Typically the location where most of the cables come together is where you ground.
If you are shielding against Radio Frequency interference, ground often and everywhere you penetrate a shielded bulkhead. Don't mix sensitive signals with unshielded signal conductors when possible. The unshielded conductors can radiate the noise back into the sensitive signals.
If you are dealing with high current wiring (100's Amps) being parallel with the sensitive signal wiring, magnetic shielding may be of concern. Running the sensitive wiring in steel conduit is usually the cure.
If all the controls are low impedance items like relays and indicator lamps, electrical noise will probably not be an issue. Susceptable signals vould be things like millivolt signals for temperature, position, or the millivolt voltages of current shunts. Shielded cables and separate conduit runs would take care of 99% of those issues.
If all the sensitive instrumentation is mounted in a grounded conductive enclosure, I would not expect for there to be any issues.
#199435 - 02/25/1110:40 PMRe: Filtering signaling and control circuits power
Back in the olden days we did, occasionally have to deal with inductive kick on a relay that cycled 30 or 40 times a second but the diode was a stack of metal disks the size of a quarter, coated in selenium. It really became important when we picked relays with transistors but by then you could get a silicon rectifier (IBM P/N 2311232) We never heard what the "N" number was but they kept getting tougher and it was hard to blow one up. I still have some.
The other one that was tough was a L/L coil on a 3p wye. The problem is most solid state switches want to disconnect on the zero crossing. L/L on a wye is never zero so they always open under a load. We blew up a lot of stuff with that problem. In one case where a fat solenoid was blowing the RC net we ended up with a wire wound and a motor start capacitor. In another it was a little shaded pole motor (36 per machine) cycling a few times a minute that would blow a 2a SSR. After we tried a bunch of RC tricks, they just sent us boxes of 10a SSRs.
On a 3p wye, I don't really think there will be a time when current is zero. I seem to remember verifying this with a current probe on a scope but I could be wrong. It has me wanting to drag a scope down to my wife's place for a look. I probably have one of those motors in my junk box and they have 3p wye.