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Discussion in 'General Electronics Discussion' started by jcajca, Jun 3, 2016.

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  1. jcajca

    jcajca

    2
    0
    Jun 2, 2016
    Hi

    I wanted a circuit with a timer controlling an output preamp relay. A LED should blinking meanwhile and stay ON when the relay
    get closed.
    I designed this with RL3 reseting U4. Q41 also starts RL2, the output relay.
    It works but after a while, D7 restart blinking. Maybe there is not enough current for 2 relays.
    How can I fix this ?
    Mainly, is there a smarter way to achieve the goal ?
     

    Attached Files:

  2. Sadlercomfort

    Sadlercomfort Ash

    424
    55
    Feb 9, 2013
    It might be best to have two transistors for each relay, the base of each transistors connected together and each relay with its own diode across the coil. The relays will still switch at the same time.
     
  3. Miguel Lopez

    Miguel Lopez

    252
    63
    Jan 25, 2012
    I don't think that the 1,7Mohm resistor in the base of Q41 would be enough to saturate it, in order for the relay to work. I think that transistor is working on its limit of saturation.

    You're using an NE555. Couldn't you use another 555 timer in monostable configuration to provide such delay?
     
  4. Sadlercomfort

    Sadlercomfort Ash

    424
    55
    Feb 9, 2013
    The HFE gain for the MPSA13 transistor is very high (5000 min)

    As an example:
    A 12V relay with a coil 960Ω need around 2.5μA going through the base this transistor at 5000 HFE. Therefore R61 would need to be around 1.6MΩ for a single relay of this type.

    So this seems okay.. but not for two relays. What relays are you using?
     
    Last edited: Jun 3, 2016
  5. Miguel Lopez

    Miguel Lopez

    252
    63
    Jan 25, 2012
    In my modest opinion, 2,5μA could be induced by noise y the transistor could get cut, if the noise induced in the oposite direction, so the relay could get "crazy".
    My two cents.
     
  6. Miguel Lopez

    Miguel Lopez

    252
    63
    Jan 25, 2012
    In my modest opinion, 2,5μA could be induced by noise y the transistor could get cut, if the noise induced in the oposite direction, so the relay could get "crazy".
    My two cents.
     
  7. jcajca

    jcajca

    2
    0
    Jun 2, 2016
    OK sadlercomfort.
    Redo the math for I use 2 x 12V-90Ω relays.
    I already thought about one NPN to command one relay. By the way it's not "two transistors for each relay"
    but one.
    I just hoped that there is a more clever way to ground the "reset pin" a the end of countdown.
     
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