Dealing with Current and Relays

I've got a question for you guys about dealing with current requirements when using relays such as air solenoid valves, reed switches, or even relay outputs on PLC's. Obviously, minimizing the current that has to pass through the contacts of any mechanical relay switch will prolong the life of these contacts. Take a simpe case of using a magnetic reed switch to turn on an input to a plc. Normally, manufacturers will say use a RC network with the switch to control surge. My question is why don't folks use a semiconductor such as Jfets, mosfets or darlington arrays to guard against arcing on contacts. Its been a long time since I was in school and devising these kinds of buffers. I can say that it is extremely rare that I see any thing like this on industrial machinery. My current dilema is I would really like to go to a small hall effect chip instead of a reed switch to turn on an input. The problem is the chip I'd like to use requires keeping the current under 40mA. What would be a good way to buffer this signal to the plc input? Thanks.

Unless you have a specific reason not to, I would use a solid state relay. Companies like Crouzet or Carlo Gavazzi or SSAC have a whole range of SSRs that would most likely work for you.

Keith

The current draw for a PLC's DC input circuit is generally less than 10 mA. You're more likely to have problems because your Hall effect chip's off-state leakage current is high enough so that you get false input signals to the PLC. Usually this shows up as an input that, once it's been turned on, won't turn off.

As to the question of why not use darlingtons or MOSFETS as surge suppressors, RC networks and MOVs are in general use for AC circuits and flyback diodes for DC circuits. My guess is that they cost less than the alternatives you're proposing, especially when you're switching higher current levels.