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Inexplicable voltage drop across high-current relay/mosfet controller — Parallax Forums

Inexplicable voltage drop across high-current relay/mosfet controller

FlyingFishFingerFlyingFishFinger Posts: 461
edited 2006-05-18 02:53 in General Discussion
In the process of testing my controller (for which I'd liked to have used the DOS editor, metioned in the other thread, now I have to shuttle my board from the garage to my room every time I need to modify something smile.gif ) I found that in one direction the controller works fine, getting me pretty much the input voltage back. But when I change the polarity with the relay, I consistently see a 0.3V drop on the whole system. I have no load on the thing except the voltmeter, so that should not be the problem. The resistances on all the wires seem to be fine, so I can't see a problem there either. On the other hand, I dont entirely trust my meter, so that may be the problem. That aside, can anyone see other problems?
Thanks
Rafael
Sorry to bug ppl alot...this is my first project as mentioned before somewhere (although it has been dragging on for over a year, due to timelessness and partial Stamp-lessness, the current controller is not the only thing on the board)

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-05-16 14:36
    From your message I don't have any idea how to help you because you haven't provided any details about the connections or parts you're using.· A schematic would help.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-05-16 21:57
    Aahhh...I'm sorry
    Will try to post one later today
    Rafael
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-05-17 13:50
    An inexplicable voltage drop is usually a short circuit somewhere.

    Changing polarity on the relay?
    Do you have a protection diode that you are forgetting about?

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

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  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-05-17 14:24
    Kramer has a point...You don't normally change the polarity of a relay...It's either on or off...Is there a reason for doing this on your relay?· As he points out, if the relay has a protection diode in it, then reversing polarity would cause there to be an inherent short across the relay terminals.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-05-17 22:50
    Change the polarity of the relay...possible miscommunication on my part probably. I'm using 2 DPDT realys wired up to polarity change, in one Motor output direction they're off, to reverse the motor direction they turn on. I'm working on a schematic right now (I'm using PowerPoint, so it might take a while). I'll try getting it up as quick as possible.
    Thanks
    Rafael
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-05-17 23:20
    Here's the schematic. I apologize for the bad part placement.
    The basic design comes from Robot DNA "Building Robot Drivetrains"
    I've had pains with the diodes, getting different information from all sides. The ones shown were recommended by PJ Allen (as far as placement goes).
    Thanks for help. As I have mentioned before, this is my first motor controller, so please bear with me.
    Rafael
    Circuit.jpg
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  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-17 23:36
    Is this how ulcers begin?
    The input to the FET (Gate) will be the DIR control.· Let's go this route --
    attachment.php?attachmentid=41704

    Post Edited (PJ Allen) : 5/17/2006 11:42:59 PM GMT
    relays_polswitch.JPG
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  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-05-17 23:44
    Wait - I have the FET controlling speed and on/off...the relays do direction. So why would I have the FET drive the direction switch...the other transistor's doing that...?
    Sorry[noparse]:)[/noparse]
    Rafael

    Post Edited (flyingfishfinger) : 5/17/2006 11:47:29 PM GMT
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-17 23:46
    The FET should be sinking the Relay coil/s.· The relay contacts (switches) action should be switching a voltage on their own, but not wired to the FET circuit otherwise.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-05-18 00:07
    Here's a possible routing scheme for direction changing through the FET which you are using to vary speed with (PWMing.)

    Post Edited (PJ Allen) : 5/18/2006 12:24:30 AM GMT
    changeit.jpg
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    raf.JPG
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  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-05-18 02:53
    Yeah...that's about what I had. I kinda found out what the problem was: Very idiotic. I'm using an unregulated power supply, which outputs 17.6V under no load at the supposed 12V setting. Of course, as soon as I turn on the relays, which constitute a load, the voltage begins to drop. Basically problem solved, I think.
    Thanks PJ for your insightful insights.
    Rafael
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