×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Strange problem on VFD board, any ideas?

Strange problem on VFD board, any ideas?

Strange problem on VFD board, any ideas?

(OP)
Hi I am having a strange circuit problem and I'm wondering if anyone has any ideas how to fix it. I am designing an AC variable frequency drive using a PIC. My PIC is optically isolated from the driver section. The board is mounted in a metal chassis. If I connect the earth ground to the chassis ground and touch the pic ground (isolated from earth) at various places on the board the circuit works erratically. Also if I try to probe the circuit with the scope as soon as I connect the scope ground(earth) to the pic ground the circuit stops working correctly so I can't even look to see whats going on with the scope when this happens.   If I remove earth ground from the chassis I can probe with the scope but then the circuit works fine so there's nothing to see.Also if I just attach one lead of my voltmeter to the pic ground the circuit doesn't work the way it should. I think it is an rf problem of some sorts but not sure how to fix it. Anyone have any thoughts on this one??
Jim

RE: Strange problem on VFD board, any ideas?

Suggestion: The added path by touching the PIC ground creates a apath for the common mode currents and voltages.

RE: Strange problem on VFD board, any ideas?

(OP)
Thanks for the suggestion. What is the best way to get rid of this problem. I should be able to connect the PIC ground to earth ground shouldn't I and still have the circuit work? It actually isn't necessary for me to connect my PIC ground to earth ground but devices connecting to my board may have their ground connected to earth. Is an optoisolator the only way?

RE: Strange problem on VFD board, any ideas?

Hi, you should have all your circuits referenced to ground somewhere, then you will be able to measure and find where your problem is.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources