XPost: sci.electronics.components, sci.electronics.design   
   From: Sophi.2@invalid.org   
      
   On 8/7/2015 10:59 AM, Ralph Mowery wrote:   
   > "John S"  wrote in message   
   > news:mq2f9s$9qs$1@dont-email.me...   
   >> On 8/7/2015 12:52 AM, DaveC wrote:   
   >>   
   >> A VFD (Variable Frequency Drive) is usually designed to be an industrial   
   >> controller. It usually has isolated inputs. That is, the inputs for the   
   >> enable function is usually passed through an optocoupler inside the VFD.   
   >> Furthermore, those digital inputs usually accept dry switch contacts.   
   >>   
   >> If the manual recommends otherwise then ignore my response.   
   >   
   > That has been my limiated experance with the VFDs.  Some of them have   
   > adapter boards that use optical isolators.   
   >   
   >   Ran into that problem when the company I worked for was in a big rebuilding   
   > project.  The engineer had just used the solid state output of a PLC to turn   
   > on a VFD.  The VFD wanted a hard contact, so I used an interposing relay as   
   > a quick fix as it was the weekend and needed to get it up and running.   
   >   
   > It is really difficult to say the resistance of a solid state device is so   
   > many ohms.  As pointed out is is the ammount of currrent it can sink and how   
   > low the voltage will go.   
      
      
   You are correct, AFAIK. The important thing is to know the input circuit   
   of the VFD. Without that, you are forced to do tests. Which is what I   
   would do.   
      
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