Sink Wiring Questions

S

Thread Starter

spring_daffodil

Hello,

I was working on a plant over an year ago and I noticed some control wiring that struck me as odd.

The plant had an MCC (motor control center) where they were using MS 116 motor starters in the electrical panels to start DOL (direct on line) motors. Apart from a contactor they had mounted a few relays, memory fails me, and I do not remember what kind of relays they were. However the function of these relays was to pass on a signal regarding status (motor run, circuit break etc) of the contactor, back to the PLC via a DI Module (Digital Input). The Digital Input module they used was an STBDDI3725 Advantys digital input from Schneider Electric.

For the wiring part what they did was: From the control panel they took the red lead (SMPS- +24VDC) and ran it to the motor panel and connected the lead in one terminal of the relay and then took another lead screwed in to the other terminal of the relay and ran it back to the DI module and plugged it in to the Negative terminal of a channel (Channel # 1, -ve Terminal). The SMPS Negative terminal was connected to the electronic ground in the panel.

At first sight I did not think it would work, but it was working. They told me that it was a 'sink' connection, but I was not entirely convinced as if it was a true sink then I think the +24VDC ought to have been connected to the + terminal of the DI module as well to complete the circuit.

I apologise for the fact that perhaps my post is probably not very clear but I'd appreciate it, if someone could shed some light on this one and that perhaps will clear up something I have been trying to understand fully.

One more thing is that the DI Module (STBDDI3725) was mounted on a plane that can give it access to a power source if needed.

Regards,
JM
 
If its a sink connection all the Di +ve terminal will be shorted to common, which in turn is shorted to the +24 supply for your controller.

Only the -ve terminal of Di needs to be connected to the relay to accept controls/signals. The other lead of the relay needs to be connected to the SMPS -ve terminal (As -24V is the desired input signal).

The basic confusion that arises from the question is if the terminal was a DI (Digital input) or a DO (Digital Output).

As the only DI are used for feedback purpose and DO are used for actuating purpose.
 
The STB DDI 3725 module is looking for a (+) input from sensors (field relay contact in your case) as an active signal vs. no signal for inactive (no current). This means it "sinks" the source +ve to the -ve for active signal determination. Typically the +ve is obtained from the STB DDI 3725 module connector, but it will work if the v source is paralleled and -v is common.

http://www.schneider-electric.cn/do...4C21852577F80063CB74/$File/31002952.03-EN.pdf
Pages 142-3.
 
Top