Can we use this circuit to prevent going flyback to supply line?

Discussion in 'General Electronics Chat' started by booboo, Aug 23, 2016.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
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    You know we use such a circuit to drive a relay:

    [​IMG]

    But can we use such a thing?:

    [​IMG]
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    No. The top diode would make the lower diode useless in killing back-emf spikes.
     
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  3. booboo

    Thread Starter Member

    Apr 25, 2015
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    Thanks but the spikes goes to the supply rail when we turned off the solenoid and it would disturb some-things.
     
  4. DGElder

    Member

    Apr 3, 2016
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    Don't know your switching speeds but a schotky diode may help if your diode isn't fast enough. And or consider an RC or diode RC snubber network.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Don't know why you say that Alec. :confused:
    The top diode will not affect the operation of the diode across the relay coil, whose purpose is to conduct the inductive coil current when the transistor turns off until the inductive energy is dissipated.

    But the transient from the relay coil inductance when the relay is turned off is slightly negative at that point and small (one diode drop), so I don't see how that could cause a significant problem in your system. A decoupling capacitor from the power to ground at the relay connection point should take care of that.

    How long is the wire between the relay coil and the transistor collector?

    What value of spike are you measuring on the rail, or is this a theoretical concern?
     
    Last edited: Aug 23, 2016
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  6. crutschow

    Expert

    Mar 14, 2008
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    The turn-on time of a normal silicon junction diode is very fast, which is what is needed for this application
    Their turn-off can exhibit a long reverse-recovery time, which a Schottky doesn't have, but that's not an important characteristic for a diode use for transient protection across an inductance.
     
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  7. booboo

    Thread Starter Member

    Apr 25, 2015
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    About a week ago I heard from a guy who said when he turned on the relays in this circuit, it causes some problems on LM35:

    [​IMG]

    http://sj.uploads.im/UJ6YB.jpg

    I was curios to know what happened to the LM35 in this circuit?
     
  8. DGElder

    Member

    Apr 3, 2016
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    I wasn't thinking, its not like you are going to be switching relays in nanoseconds.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Of course not.
    But the diode has to start conducting as fast as the transistor is turning off to minimize any voltage spike from that turn off.
    In other words as the transistor is stopping its carrying of the coil current, the diode has to take over carrying that current until the inductive energy has dissipated.
     
  10. nigelwright7557

    Senior Member

    May 10, 2008
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    I always put 100uf across the power supply close to the relay to absorb spikes.
     
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