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BITS N BYTES

FUSE Protection for Sinking Input and Ouputs

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Looking for suggestions as how best to protect groups of sinking I/O with fuses. With sinking INPUTS the common of the PLC is tied to +24VDC. Will a fuse in this line provide a suitable level of protection for the inputs? With Sinking OUTPUTS the common is tied to 0VDC. Will a fuse in this line provide a suitable level of protection for the outputs? Any ideas welcome. Thnx B & B

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Perhaps but I do not generally bother. There is good protection on the inputs and I normally use the G2RV relays on the outputs and they have flywheel diodes built in. I also use IDEC (IZUMI) indicating lights and they have diodes built in as well. Have never had an output fail and only ever the very odd input from an induced voltage surge or some idiot putting 240VAC on a 24VDC input! Boom! I suspect that all this fusing of inputs and outputs would not work that well as the electronics would probably fail before the fuseeven murmured.

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plc inputs draw few mA when on. using fuse to protect inputs is not practical. using fuse (or circuit breaker) to limit current in case of a fault is another story. if your circuit is powered by large power supply (i have done plenty of jobs with 20A or even 40A power supplies), and you have 800 I/O points. you don't want to spend lifetime to find the problem. and you don't want to use thumb thick wire because you were too cheap to put couple of fuses. i would say this is wrong. inputs common goes to "DC common". outputs common goes to "DC power". always...! in case of sinking I/O, "DC common" is the positive psu terminal, "DC power" is the negative psu terminal. in case of sourcing I/O, "DC common" is the negative psu terminal, "DC power" is the positive psu terminal. any over current protection goes on a "DC power" side (or AC power if that's what you use), not on a common (or neutral). same goes for any switching, all switches are on power side (power switches, circuit protection, sensors, plc outputs...). one side of the load always goes to "common", never to "power" rail. well, "never" is such a harsh word. of course completing circuit without regard to this is possible too and it can work but this is not good, such circuits are not fail safe. non-fail safe circuits are quite common and brake many rules. in many cases that is ok because they are low power etc. however anyone doing industrial automation should be wiser and learn the difference. nobody cares if some kid playing with breadboard and LEDs uses switch on positive or negative side of the battery or if this circuit is grounded. same goes for any low power devices like cellphones, laptops, vending machines, office printers etc. simply potential for damage or injury is non-existent or very remote because energy levels are low. this is why mix of sink and source I/O while using same reference is also ok. so go ahead and Charlieplex LEDs and PWM some small DC motors or desk lamp all you want... it is another story if this is something industrial where energy sources and actuators are considerably larger and you are not the only user. this is where rules come in and designers are expected to be familiar with standards, regulations, fail safe operation, etc. even if signals are low power (sensors, handshaking etc.), the danger is what they can do to influence actuators. this is why one should always consider every aspect of a circuit.

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One has to be a bit careful with 'sinking' and 'sourcing' - different manufacturers describe and use it differently.

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that is absolutely correct. unfortunately industry did not agree on standard. sink/source is not ambiguous when talking about one circuit (at the component level). the problem is that manufacturers have chosen to apply the term relative to internal (IO card) or external (user added) portion of the I/O circuit so one must check connection diagrams in the manual to verify polarity. i realize now that i assumed the "common" mentioned in original post is common on the IO module while it may have been the opposite end (user wiring). sigh, this is why drawings are invented... sorry for any confusion i may have caused

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Different manufacturer, and different people. Thats why I prefer NPN and PNP terms, less confusing.

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agreed...

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consulting-resources:

Please refrain from posting to topics that are more than one year old.

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i always prefer NPN and PNP terms, surveyzop

Edited by eonsur
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