Re: [CAD_CAM_EDM_DRO] Digest Number 3599

>Is there a limit to the voltage that I apply to a PLC input. I wish
>to
>detect when the main spindle of my machine is on and need to know if
>it is ok to feed one of the input phases to the motor to the PLC
>input directly .

You can get discrete PLC inputs in 5 VDC, 24 VDC, 5 - 60 VDC, 115 VAC, 240 VAC, and probabally more, in addition to analog inputs in either voltage or current (or RTD, thermocouple, etc.)

The voltage of the motor phase could be accomodated, but this is not what you want to do. The inrush current and electrical noise would not present clean input sensing, IMHO

If you are using a magnetic starter, use a set of auxillary contacts, or put a current sensor on one of the motor leads. Little current sensing ring transducers can be bought for $15 or 20 bucks. You could also power a relay from the motor lead, and use its output for the PLC input drive. A lot of PLC's have a 24 VDC power source you can use for this, or use the control power if you want 120 VAC for your particular input.

Ron








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