Melsec NPN I/O 1

[MikeKnight]

Hi,

one of our japanese customers plans to replace our standard "Siemens" PLC control by his own PLC make "Melsec" (no idea what type).

Now he requests to have 2-wire NPN type wiring (instead of our standard 3-wire PNP design). Is that normal (or possibly required) for this type of PLC? It will mean a re-design of the field wiring and equipment.

Can anyone point me to a manual or similar explaining that type of connection? What does it mean for
- 24V DC proximity switches
- 24V DC outputs (lamps, solenoids)
- analog 4-20 mA Inputs (is that applicable, anyway?)

Kind regards in advance

Michael Marx
Electrical and Control Engineer
VOITH Paper - Euskirchen, Germany

 

[Skogsgurra]

Dear Michael,

The Mitsubishi PLC family is often referred to as the Melsec PLC. It is widely used in general automation and has a broad support in most countries. Since I know VOITH as a German-speaking organization I give you this link for starters

http://www.mitsubishi-automation.de/loesung/art9.html

and this for the manufacturer's site

http://www.mitsubishielectric.com.sg/factory/faxpcmela.asp

The NPN is not considered the best I/O nowadays and in machine-tools it is not allowed, according to standards. But if the customer wants it and pays for it - he probably gets it. You should perhaps ask him if he is aware of the "obsoleteness" of the NPN I/O. He may even thank you and agree to use your standard.

 

[fangas]

Hi Mike,

Melsec is a Mitsubishi PLC. Pretty common in Japan.

In your normal 3-wire PNP set up, there is a + dc line, a - DC line and a signal line to the PLC inputs. In the PNP configuation, the prox switches the + line onto the Signal line. Your PLC would have your -V power supply line connected to the Input Commons. If you consider the PLC Inputs as the load, your curcuit would look something like this:

+VDC-------Switch(Prox)-------Load(PLC)------0VDC(Common)

In a 3-Wire NPN configuration, the prox switches the - line to the PLC. Like this

0VDC-------Switch(Prox)-------Load(PLC)------+VDC(Common)

A 3-wire PNP switch uses a PNP type transistor that will conduct + to - only. In the 3-wire NPN a NPN transistor is used that only conducts - to +. The leftover line is used only to power the switch.

The 2-wire switch doesn't care. It will conduct in either direction. It does have a higher leakage current however, this is due to it's deriving power from the the same line that it is switching. That is to say, in the 3-wire switch, current required to power the switch flows from the -(source) through the switch to +(source) to complete the curcuit. The transistor that is used to develope the signal is off and current flowing through it to the load is extremely small (micro Amps typically). While current to power the 2-wire switch flows into the switch out of the switch and into the load (your PLC)and back to the source to complete the curcuit. This current used to power the switch is Leakage.

I prefer the 2-wire switch myself. It simplifies the curcuit by removing a wire. There are some things to watch out for when you change over to a NPN to PNP configuration. These include selecting an Input Module that is designed for that type input. Every manufacturee seems to use different nomeclature to denote this. I just look at what they list as a common to the input module. If the Common is shown as Neg then this Module wants to see a PNP type switch, if Pos, a NPN type. It's best to review the manufacture's lit.

Another thing to be aware of is the "leakage" current of the 2-wire switch you'll be using. Your switch specs will list a "max leakage current" and your Input Mod will list a "min on current". The "max leakage" must be less then the "min on" or else the Module will interprete the laekage as a valid "ON" signal and never be in an "OFF" state.

The same holds pretty much true for your outputs. If your customer wants the entire system to be of the NPN type, then you will be using the Pos as your common line to all your loads and switching the Neg line to them. You will most likely have to change your drawings to reflect this.

Your Analog signals (4-20mA) should not be effected by this change.

I hope that helps

Ed