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i need help with my monostable circuit!!

Discussion in 'General Electronics Chat' started by toh liu wang, May 3, 2017.

  1. GromTag

    GromTag Active Member

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    that attempt when the IC worked for you, then no longer did not, the Transistor may have been taken out by the relay that very same time possibly because of the flyback diode not being able to properly send the inductive spike to the supply rail.
     
  2. GromTag

    GromTag Active Member

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    For post #80, It would be best to use an Transistor for this. It's just that with the 555Ic connected in place of the NPN Transistors Collector, the IC was sinking the relay when off, I will provide a chart that shows this.
     
  3. GromTag

    GromTag Active Member

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    When the IC trigger gets falling edge operation, The Trigger is sunk via the switch connected to Common GND, then the output pin 3 goes High supplying power out from the IC. then the resistor capacitor time elapses at the bottom of the chart, the output pin 3 goes low again, Sinking.
     

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  4. dave

    Dave New Member

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  5. toh liu wang

    toh liu wang Member

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    Where exactly is the relay positive point? Is it the one right above the coils?
     
  6. GromTag

    GromTag Active Member

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    The relay coil, either can be the positive, then the other point the common, the coil is bidirectional. you can choose which one you want to be the positive.
     
  7. GromTag

    GromTag Active Member

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    these are the pins circled in red.
     

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  8. GromTag

    GromTag Active Member

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    It's just the contacts that can be an issue if the relays type is not directly known. Normally open, the relay is off until powered on. Normally closed, the relay is allowing operation when off, then when coil powered the relay is off as the contacts have opened.

    Then the double throw types as well, they can be either as well, tho in either power state those can be configured for either N/O or N/C operations based on powering state.
     
  9. toh liu wang

    toh liu wang Member

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  10. toh liu wang

    toh liu wang Member

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    You mean like this?
     
  11. GromTag

    GromTag Active Member

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    The diode anode would be placed at the relays coil connection, cathode to the rails positive supply.

    And the Violet wire from the relay coil on the right side needs to be paced to the Common GND rail.
     

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    Last edited: May 6, 2017
  12. GromTag

    GromTag Active Member

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    lengths of wire and circuit traces can act as small inductors themselves, as current flows along the wire it can be pressured to an end connection point, the Diode placement puts its anode right at the coils common.
    ......
    The Anode needs to be located at the relays left side at the Brown wire point, apologies. I will remake the image correct, please disregard image post # 90!
     
  13. GromTag

    GromTag Active Member

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    Correct Diode placement.png This has the Diode correctly placed for the relays fly back, giving direct line travel to the power supply!
     
  14. GromTag

    GromTag Active Member

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    Rechecked,

    once the relay flyback diode is connected as in post #92, then the other connections show to check out.

    Motor flyback diode is good, the ic is grounded, powered,
    stabilized with the capacitor, the timing bank with pins 6 threshold and pin 7 discharge tied together.

    Resistor R1 from power supply positive to pins 7, 6, then to capacitor.

    pin 5 control cap, right.

    trigger button, right.

    pull up resistor for trigger pin, right.

    checks out.

    At the most the inductive issues would be the most to be cautious of, in the most event when the IC is directly driving the relay.
     
  15. GromTag

    GromTag Active Member

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    Oh and also, in the event the circuit works, and the relay and motor starts up after the button is pressed yet does not seem to want to shut down (if), then the resistor for pin 7 threshold would need the 51k resistor back in place, as the 100k would run for quite some time before shut off.
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The 51k resistor and 470uF capacitor would cause the 555 to timeout in about 26 seconds. Did you measure the battery voltage while the 555 is timing and the motor is running? How low did the voltage drop?
     
  17. GromTag

    GromTag Active Member

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    Long winded questions and thoughts removed for shorter reading.

    What would be the likely current handle for the discharge pin for an (and I would thing this can vary) 555 IC discharge pin to internal driver>

    My shortened thoughts are, what of the internal 555's depicted transistor NPN from the "block diagram" being direct connected to the capacitor as the schematics per example mostly only showing small time values used thus an small cap value would not produce high risk current for the internal NPN's collector, say 10n or 100n if pulled at cap charge would allow some impressive current flow directly into the sinking method even with the resistor being quite large in impedance, unless their is an current limit within the IC itself and not displayed in the "block diagram" then I'm onto nothing then. Tho both the Fairchild and TI do not show any limiting factors per again the block diagram.

    An larger microfarad, 10uF - 470uF, at charge then adequate (quick) discharge would lead to aggressive current draw and would damage the 555 discharge component potentially causing the device to state on at all times via the output.

    In testing the Fairchild LM555, per 100K and 100uF cap for random time, each shut down of the LM555 pulsed my TPS2049 power switch OC indicator implying a short had minutely occurred at the LED's switch off driven by an 2N7000 FET with the 555 chip pin 3 at the FET Gates helm for direct gate control sinking the 3mm LED with 1k resistor for the LED itself, to comprehend the chips push pull ideal in my testing.

    The current draw exceeded 150mA quickly before the TPS2049 switched to constant current mode and flagged the OC indicator as an pulse.

    :using 5 V only here for testing:
     
  18. GromTag

    GromTag Active Member

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    Found an topic back to 2011 about the discharge pin limit... 200mA stated as assumed max current.
     
  19. GromTag

    GromTag Active Member

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    My concern is, what if the OP chip pin 7 has been damaged via over current draw via the 470uF cap.

    Post #13, thanks alec_t , this topic seems familiar to you does it not?

    I will not mention what day it was:)
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The datasheet for the LM555 has a note that says, "No protection against excessive pin 7 current is necessary providing the package dissipation rating will not be exceeded".
     
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  21. GromTag

    GromTag Active Member

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    Yes, I was going along with the no need of current limiting via the methods of timing use, an current limit would impede the use for many potential configurations for the IC.

    This datasheet for the LM555 I have dates to 2002.... reviewing the data overly.
     

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