Switching Current

Discussion in 'General Electronics Chat' started by scorca, Jun 16, 2009.

  1. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    Hi all,

    I have a double pole switch with max. switching current at 100mA. One pole is connected to a 24 VDC on positive side and a relay on the other side. Relay consumes 2 W, so the current through the switch is 83.3 mA. The other pole is connected to Digital input of an IC. This input delivers 24VDC, 350 mA max. My question is: Will my switch survive?

    Take a look of the scheme.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I question the digital IC input pulling 350 ma. Most digital IC's need well under 24 volts - like a maximum for COMS at 18 volts - and less than a milliamp. Perhaps you could say what that IC is.
     
  3. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    Maybe I shouldn't say it an IC because it actually is a part of a motor controller. I don't know what is inside the controller and got that value from datasheet.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If the motor controller's output happened to be a ULN2003/ULN2803 Darlington array, that would agree completely with said IC's datasheets.

    Operating a relay at 83% of its rating is OK. Just keep in mind, the closer you get to its maximum rating, the shorter its lifespan will be.

    If you were operating at 50% of rating instead, the contacts would last about 3x longer.

    However, you really aren't stating what the load between the relay and the motor controller is?

    Or is the load the coil of the relay?

    [eta]
    OK, this schematic really doesn't make much sense:
    [​IMG]

    What exactly are you trying to do?
     
    Last edited: Jun 16, 2009
  5. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    Yes, the load of the relay is the coil.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    OK, it still doesn't make much sense.
    Are the contacts shown actuated by the relay coil shown?
    If so, when the relays' coil starts energizing, the left (N.C.) contact opens, causing the current path to the relays' coil to be broken, causing the left contact to close. It'll be a buzzer.

    The controllers' "digital input" through the N.O. contacts doesn't make sense, either.
     
  7. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    I redrew the schematic. It is actually a safety relay.

    What's wrong with N.O. contacts at digital input?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    A safety relay? What safety function does it perform?

    This:
    [​IMG]

    ...still doesn't make sense, I'm afraid.

    I really don't know what you're trying to accomplish.

    Why don't you explain what you wish to do?

    Do you have datasheets for your motor controller and your relay? Or at least the part numbers?
     
  9. scorca

    Thread Starter Active Member

    Oct 31, 2008
    36
    0
    The motor is fed by 24VDC source via the safety relay (through built-in switch that is controlled by coil of the relay). The button is an emergency switch. Thus, if you push the button, coil of the relay will be de-energitized and the built in switch will be open. The motor will be no longer fed.

    The relay is PNOZ 2.1 from Pilz and for the motor controller, I guess it is a custommade motor controller.
     
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