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Question for the EE's among us.. — Parallax Forums

Question for the EE's among us..

Oldbitcollector (Jeff)Oldbitcollector (Jeff) Posts: 8,091
edited 2008-06-02 17:27 in General Discussion
This is a Propeller/General Electronics question..

I'm playing with three servos.. Why does the following happen?

If I set the input voltage to 7.5 DC (walwart) I seem to over draw too much current and reboot the Prop.
If I set the input voltage to 9v DC (walwart) I seem to over draw much faster and reboot the Prop.
If I set the input voltage to 6v DC (walwart) everything works with power seemingly to spare.

This seems backwards to what I would have expected.. What's actually happening here?

Thanks
OBC

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New to the Propeller?

Getting started with the Protoboard? - Propeller Cookbook 1.4
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Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-06-02 00:58
    Servos can draw a lot of current (an Ampere each maybe). If you're driving them with 7.5V or 9V, you cause the 5V regulator to overheat and it shuts down taking the Prop with it. At 6V, the regulator doesn't overheat. Note that 3A x (6-5V) = 3W, 3A x (7.5-5V) = 7.5W, 3A x (9-5V) = 12W.
  • Oldbitcollector (Jeff)Oldbitcollector (Jeff) Posts: 8,091
    edited 2008-06-02 01:32
    And yet it works perfectly when I connect a 9v battery.
    (Less current draw?)

    OBC

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    New to the Propeller?

    Getting started with the Protoboard? - Propeller Cookbook 1.4
    Updates to the Cookbook are now posted to: Propeller.warrantyvoid.us
    Got an SD card? - PropDOS
    Need a part? Got spare electronics? - The Electronics Exchange
  • pwillardpwillard Posts: 321
    edited 2008-06-02 02:39
    this would lead me to believe that your walwarts are junk and do not deliver what is written or printed on them.

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    There's nothing like a new idea and a warm soldering iron.
  • RobotWorkshopRobotWorkshop Posts: 2,307
    edited 2008-06-02 02:49
    Oldbitcollector said...
    This is a Propeller/General Electronics question..

    I'm playing with three servos.. Why does the following happen?

    If I set the input voltage to 7.5 DC (walwart) I seem to over draw too much current and reboot the Prop.
    If I set the input voltage to 9v DC (walwart) I seem to over draw much faster and reboot the Prop.
    If I set the input voltage to 6v DC (walwart) everything works with power seemingly to spare.

    This seems backwards to what I would have expected.. What's actually happening here?

    Thanks
    OBC

    You need to specify what the current output for each of them as well at least what they say it should be able to provide) It should be in a ma rating. The ones you tried with a higher voltage may have a much smaller current rating. Under a load the voltage may drop a lot more than you would expect.

    Do you have an extra cap placed near the connectors for the servos?

    Robert
  • FranklinFranklin Posts: 4,747
    edited 2008-06-02 16:54
    Stick your meter on the input and see just what you are really getting. Servos work best if they have their own source of power.

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    - Stephen
  • Oldbitcollector (Jeff)Oldbitcollector (Jeff) Posts: 8,091
    edited 2008-06-02 17:17
    All wallwarts are junk by design... This one is a Radio Shack multi-voltage special.
    So I'm guessing it qualifies, despite my paying too much for it. [noparse]:)[/noparse]

    It's my contention that we should standardize the DC voltages used by electronics
    and wire new homes with dual wiring, one 110AC and another 6v-9v DC.
    Stupid walwarts just suck juice, create heat, (that I have to pay AC to deal with)
    and waste energy. (Sorry about the rant.. )

    OBC

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    New to the Propeller?

    Getting started with the Protoboard? - Propeller Cookbook 1.4
    Updates to the Cookbook are now posted to: Propeller.warrantyvoid.us
    Got an SD card? - PropDOS
    Need a part? Got spare electronics? - The Electronics Exchange
  • uxoriousuxorious Posts: 126
    edited 2008-06-02 17:20
    Sounds like your walwarts are the "cheapie" kind, and only work properly at the specified current/load. I just ran through a situation where a 12 volt, 200 mA walwart tested at 20.5 volts with a meter, when unloaded. However, when connected to the PCB that draws a constant 180mA, the power was measured at a rock solid 12.08 volts.

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    ~~ dRu ~~
  • Mike GreenMike Green Posts: 23,101
    edited 2008-06-02 17:27
    dRudRudRu,
    The Wall Warts may be cheap, but this behavior is normal for unregulated power supplies. Unloaded, they will have voltages 50% or more above the fully loaded voltage. This voltage drops rapidly with increasing load to near the fully loaded voltage, then stays near there to full load, then drops again with overload.
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