587,999 active members*
2,583 visitors online*
Register for free
Login
Results 1 to 16 of 16
  1. #1
    Join Date
    Apr 2012
    Posts
    9

    AC/DC Power Isolation

    First of all, hello, this is my first post here. Sorry it's so long, but it's a pretty specific question that, A: I'm not sure exactly how to ask, and B: haven't been successful in finding a good answer to.

    I have a G0602 lathe that I'm adding a tachometer to. The lathe uses a contactor to de-energize itself when the e-stop is pushed, and a pushbutton to close the contactor and power the lathe up again. A simple 3-position rotary switch controlls FWD OFF REV. The lathe uses a 110VAC, single-phase motor.

    I am adding a PLC controlled, optical-interrupt tachometer with LCD readout that I made. It is powered by 5V DC power supply that runs off of 110VAC. I want the tachometer to be powered on/off with the lathe and not have a separate switch.

    My first attempt was unsuccessful. I wired the DC power supply up to the load side of the contactor. The tach works perfect, except every time I rotate the switch from OFF to FWD, or FWD to OFF, the PLC resets. I'm not sure what's happening, but I belive it has to do with sharing the load side of the contactor with the motor, even though the motor has start and run caps.

    For my second attempt, I found another identical contactor, new for $15. I plan on having them share the same line voltage and wire the e-stop and start buttons in parallel to both contactors. My goal is to isolate the AC motor from my DC power supply.

    My question is, is this the best way to do this, or will I still have the same problems?

    Thanks,
    Daniel

  2. #2
    Join Date
    Dec 2003
    Posts
    24220
    First I assume the term PLC you are using is for a embedded dedicated Micro and not the normal industrial packaged version of a PLC?
    The first thing you may want to do is put a snubber across each contactor coil, if you are using a reverser contactor, this is a Simple RC combo you can either make up yourself or they come cheap from a few suppliers.
    I have had quite a few examples of this prevented by the snubber.
    Another answer may be to reference the 5vdc supply common to the earth ground terminal.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  3. #3
    Join Date
    Dec 2003
    Posts
    24220
    The other thing to ensure is that the +5v supply is De-coupled properly, a 100µf across the supply and a .1µf right at the IC itself.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  4. #4
    Join Date
    Apr 2012
    Posts
    9
    The PLC I'm using is an ATMega328P-PU 28-pin DIP module. The power supply I made has the 100uF and 10uF caps on it. I even went as far as adding a 470uF cap just to see if it would help, and it made no difference.

  5. #5
    Join Date
    Dec 2003
    Posts
    24220
    Here is an example of a snubber to place across the coils.
    CNC deviation error - CNCzone.com-The Largest Machinist Community on the net!
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  6. #6
    Join Date
    Apr 2012
    Posts
    9
    Thanks for all your help. I found this, I think it will work...

    If you don't mind another, probably stupid, question. I read that it needs to be installed across the coil...? I think that means on the line side of the contactor, not the load side...? Correct?

    I am using a 3TB41 Siemens contactor. Here is a post with some pics of the G0602's wiring and the contactor. Contactor pics are about 3/4 way down the page. There's also a schematic.

  7. #7
    Join Date
    Dec 2003
    Posts
    24220
    That appears to be a RC snubber, it is intended to go across the coil terminals, not the power contacts, although there are versions that are intended to be wired across the power conductors but are usually larger capacity.
    Did not see the contactor in the link.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  8. #8
    Join Date
    Feb 2009
    Posts
    6028
    Does your contactor not have aux contacts?

  9. #9
    Join Date
    Apr 2012
    Posts
    9
    Yes, I belive they are labeled A1 and A2. I'm just now beginning to understand this contactor.

  10. #10
    Join Date
    Dec 2003
    Posts
    24220
    A1 and A2 is usually the designation for the coil, which you would connect the snubber across as in post #2.
    I am assuming you have the tach powered all the time but when you go from FWD to REV the contactor reversing action causes the display to reset?
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  11. #11
    Join Date
    Apr 2012
    Posts
    9
    Here are some pics of the contactor. I labeled the wiring diagram with the contactor terminal numbers(in blue). Also, pay no attention to the wires labeled L and N. They seem to be labeled backwards in the manual. I know mine and crevicereamer's lathes, at least, have Black as Load and White as Neutral.





    Here's the wiring diagram...


  12. #12
    Join Date
    Apr 2012
    Posts
    9
    The way I have it now, I have my DC power supply connected to terminals 2 and 6 (with the motor). Circuit 13/14 is normally open (same as 43/44)(21/22 and 31/32 are NC). I think that I can connect a jumper wire from 1 to 13 and connect my power supply to 14(Line, when closed) and 5(Neutal, from wall). Also, I'll place the snubber across A1/A2.

    Or should I connect a jumper from 5 to 13 and connect the power supply to 1(Line, from wall) and 14(Neutral, when closed)?

  13. #13
    Join Date
    Feb 2009
    Posts
    6028
    Why not just switch the dc line through 13 and 14?

  14. #14
    Join Date
    Dec 2003
    Posts
    24220
    If you still have problems, try grounding the 5v common of your DC to the ground terminal in the contactor box.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  15. #15
    Join Date
    Apr 2012
    Posts
    9
    Quote Originally Posted by underthetire View Post
    Why not just switch the dc line through 13 and 14?
    That would work fine except the 5V power supply would have to be on all the time.

  16. #16
    Join Date
    Dec 2003
    Posts
    24220
    You could use it to switch the AC side, also you don't normally have to switch the Neutral, you would only have to switch the hot side.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

Similar Threads

  1. Galvanic Isolation, What does it mean for my power supply
    By gavztheouch in forum CNC Machine Related Electronics
    Replies: 9
    Last Post: 06-22-2009, 07:47 PM
  2. LPT Opto isolation BOB
    By shulio in forum CNC Machine Related Electronics
    Replies: 0
    Last Post: 03-12-2009, 01:01 AM
  3. DO YOU HAVE TO USE AN ISOLATION TRANSFORMER
    By racer652 in forum HURCO
    Replies: 4
    Last Post: 02-29-2008, 05:00 PM
  4. PCB Trace Isolation - Minimum Track and Isolation
    By ahoodlum in forum CNC Machine Related Electronics
    Replies: 11
    Last Post: 09-26-2006, 05:02 AM
  5. Electrical Isolation? (p.sup.)
    By FLUTE HEAD in forum CNC Machine Related Electronics
    Replies: 4
    Last Post: 08-24-2004, 07:55 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •