584,837 active members*
5,517 visitors online*
Register for free
Login
Results 1 to 11 of 11
  1. #1
    Join Date
    Feb 2005
    Posts
    8

    PSU Voltage seems high.

    Hi
    I've just built up the hobbycnc 3-axis kit. Driver board is working fine off a bench PSU. I've built up the Psu with capacitor and rectifier supplied in kit and a 300VA transformer (30V,10A). Before I connected it up to the driver board I checked the output voltage and i'm getting 47vDC. I've checked it on a scope and i'm getting no ripple. This seems to be a bit high for the driver board. I've checked the input to the recrifier and it 29.7vAC so the transformer's giving the correct voltage out. Has anyone got any ideas why i'm getting such a high output and can the driver board take it or will it fry the chips?.

    Thanks in advance

    Matt

  2. #2
    Join Date
    Jun 2003
    Posts
    1365
    This is because it rectifies the peak AC voltage which normal AC outlets peak ac voltage is around 170v. After it is rectified and filtered, typical output is 1.414x the transofmer output voltage.

    Jon

  3. #3
    Join Date
    May 2003
    Posts
    550
    As Jon said, (apart from UK voltage being 220 ) rectified AC to DC increases the voltage by around 1.4. If you're after 30vdc you need a transformer output of ~21vac before rectifier. Don't connect your driver to 47vdc because it will blow it.

    Andrew

  4. #4
    Join Date
    May 2004
    Posts
    198
    Quote Originally Posted by mnewsholme
    Hi
    I've just built up the hobbycnc 3-axis kit. Driver board is working fine off a bench PSU. I've built up the Psu with capacitor and rectifier supplied in kit and a 300VA transformer (30V,10A). Before I connected it up to the driver board I checked the output voltage and i'm getting 47vDC. I've checked it on a scope and i'm getting no ripple. This seems to be a bit high for the driver board. I've checked the input to the recrifier and it 29.7vAC so the transformer's giving the correct voltage out. Has anyone got any ideas why i'm getting such a high output and can the driver board take it or will it fry the chips?.

    Thanks in advance

    Matt
    30VAC(notDC) rectified makes @ 1.41 times DC volts. 30 times 1.42 equals 42.6VDC NO load. It will drop a volt or two under load. Your line voltage is over the input voltage. DO not use above 46VDC.

    Dave Rigotti

  5. #5
    Join Date
    Feb 2005
    Posts
    8
    Ok I guess I'll swap out the 30v,10a transformer for a 24v,12a one. That should drop voltage down to a safe level but still have plenty of current in reserve.
    cheers :cheers:
    Matt

  6. #6
    Join Date
    Dec 2003
    Posts
    24216
    You could always take some turns off, lowering the voltage will up the Current capacity, providing the wire size is OK, but 2amps extra coverage should be possible.
    Rough guide for a full Wave Bridge.
    VAC = 0.8 x (VDC + 2)
    IAC = 1.8 x IDC
    Al
    CNC, Mechatronics Integration and Custom Machine Design

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

  7. #7
    Join Date
    Feb 2005
    Posts
    8
    ok changed the transformer for 25vAC, 12A . getting 38.5vDC now.
    absolutely no ripple
    Matt

  8. #8
    Join Date
    Dec 2003
    Posts
    24216
    Quote Originally Posted by mnewsholme
    ok changed the transformer for 25vAC, 12A . getting 38.5vDC now.
    absolutely no ripple
    Matt
    Remember the ripple has to be measured at the maximum load current to obtain the degree of ripple.
    Al
    CNC, Mechatronics Integration and Custom Machine Design

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

  9. #9
    Join Date
    Apr 2003
    Posts
    19
    Quote Originally Posted by mnewsholme
    ok changed the transformer for 25vAC, 12A . getting 38.5vDC now.
    absolutely no ripple
    Matt
    Matt, I have just finished my 3 axis board which works great off a bench psu (great job Dave :cheers: ), where did you get your transformer from?

    Steve

  10. #10
    Join Date
    Feb 2005
    Posts
    8
    got it from rswww.co.uk. bit expensive but it was exactly the right rating i wanted. Part number is 223-3207. 25v, 6a toroidal. I think it was about 20 quid but then another tenner delivery :frown:

  11. #11
    Join Date
    Apr 2003
    Posts
    19
    Thanks Matt, I'll get one on order

    Steve

Similar Threads

  1. High Voltage wire
    By cncadmin in forum Laser Engraving / Cutting Machine General Topics
    Replies: 9
    Last Post: 02-27-2012, 03:19 PM
  2. Power supply voltage too high
    By spoiledbrat in forum CNC Machine Related Electronics
    Replies: 8
    Last Post: 06-14-2005, 03:39 AM
  3. Transformer voltage seems too high for Gecko
    By bgolash in forum CNC Machine Related Electronics
    Replies: 14
    Last Post: 12-25-2004, 05:35 PM
  4. 5vdc from paralell port, not enough???
    By thuffner3 in forum CNC Machine Related Electronics
    Replies: 8
    Last Post: 11-23-2004, 01:07 PM
  5. Active High/Active Low
    By Sanghera in forum CNC Machine Related Electronics
    Replies: 21
    Last Post: 11-07-2004, 03:47 AM

Posting Permissions

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