Power Resistor Causing Relay To Freak Out!

Discussion in 'The Projects Forum' started by jwilli, Aug 5, 2009.

  1. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    I have a standard "Bosch" type SPDT relay that will activate and send 12v of power to a power resistor (to automatically drain a battery). All I'm trying to do is to send power through the resistor and also turn on a LED to let you know that it is receiving power.

    Let me apologize for the bad drawing, but this is what I have:

    [​IMG]

    It works fine if I connect it directly to a 12v source, but when connected through the relay, the relay will spasm and open and close non-stop very, very quickly.

    The circuit leading to the relay works fine as I can activate the relay and turn on a simple light bulb- the problem is something to do with this little resistor setup.

    Any idea why this is happening?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    Do you have the complete schematic (including the relays) ?
    The part you gave does not tell us how the relays is connected.

    Greetings,
    Bertus
     
  3. jwilli

    Thread Starter Active Member

    Apr 21, 2009
    42
    0
    I don't have the entire thing drawn up but hopefully this will help you:

    [​IMG]
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    How is the relays coil powered?
    Is the 12 Volts stable when the relays is "flapping"?

    Greetings,
    Bertus
     
  5. jwilli

    Thread Starter Active Member

    Apr 21, 2009
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    0
    Yes, the coil voltage is stable.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Are you powering the relay coil from the battery that you are discharging?

    Consider using (an) incandescent bulb(s) instead of a power resistor. When an incandescent bulb's filament is cold, it has much lower resistance than it does when it's hot; it's resistance is non-linear. This is just the kind of response you want from a load when discharging a battery.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    The first thing that comes to mind is that when the contacts are closed the load is so severe that it pulls the coil voltage down below Holding Voltage. Then the contact open and the voltage rises until the voltage reaches contact Pull In. The end result is a buzzer!

    (1) Are we to assume that the 12V that powers the relay coil is the same 12V battery that you're attempting to discharge, or is this circuit being powered by a wall wort?
    (2) Are you building a Battery Cycler, ie: a circuit that charges a battery and then discharges it repeatedly until it's lost its memory?

    The pictorial you posted leaves many unanswered questions, like what is the value of the power resistor? Why do you have a Schottky diode directly across the battery when the contacts are closed?
     
    Last edited: Aug 6, 2009
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