alternating polarity output with delay

Discussion in 'The Projects Forum' started by kleinesbear, Jul 15, 2009.

  1. kleinesbear

    Thread Starter New Member

    Jul 15, 2009
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    I am having an issue with an alternating polarity output. I have a 12V supply and when triggered i am supplying 5V to a solenoid then after 4 seconds supplying the opposite polarity to the same solenoid. I can successfully get the voltage and polarities correct with the attached diagram but the problem i am running into is that the solenoid does not have time to discharge the coil when switching polarity via a DPDT relay and does not change state.
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
    782
    Kleinesbear,

    You haven't used any protective (relay coil back emf) diodes on the LM555 output (pin 3) terminals. It might be good practice to include them, as I don't think there is an internal diode in the chip itself.
     
  3. kleinesbear

    Thread Starter New Member

    Jul 15, 2009
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    t_n_k

    You are correct about the diode. I have not added any circuitry protection at this time. This is a prototype (proof of concept stage 2) project. Thank you for the reminder.

    Alberto

    My only issue with closing and re-opening K2 is that I am having a brain freeze on how to re-trigger the lower 555 on K1/upper 555 state change. I did replace R2 with a 10K as C2 is a 100uF and manually triggered the 555 and it worked. But now I have no idea how to trigger it once at the beginning then automatically at the end of the upper 555 and still return the trigger voltage to high at then end of the sequence. I am also limited on space/components. The only other thought I had was to introduce a third 555 as a Schmidt trigger or a circuit control timer controlling the supply to the lower 555 and changing the lower 555 to an astable. The problem with that is my application may trigger for longer than the monostable duration and I still need the second state of polarity no matter how long the trigger duration is.

    I was wondering if there was something I could put on the output with a diode structure that would interrupt the flow for a period of time on K1 normal state.
     
  4. kleinesbear

    Thread Starter New Member

    Jul 15, 2009
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    are you saying to change K2 to a DPDT and discharge C2 with a resistor to ground through the second pole on activation? I am not sure how that would re-trigger the activation once K1 is normal again. or am i misunderstanding your suggestion?
     
  5. kleinesbear

    Thread Starter New Member

    Jul 15, 2009
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    I have attached my thoughts on a delay switch on polarity change. any thoughts? or am I heading down the wrong path?
     
  6. kleinesbear

    Thread Starter New Member

    Jul 15, 2009
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    I have put more thought into the timer for the polarity change. I haven't run the math to figure out exact capacitance or resistance, but I have what seems to me a way to make it work. I am awaiting some transistors on order to find out if it actually works. If anyone can provide insight I would appreciate it. I have attached the revised delay timer for the polarity switch.

    Again, I am not sure at all if this will work.. I am certain I have gone beyond my initial knowledge and am in a completely learning mode at this point.
     
  7. kleinesbear

    Thread Starter New Member

    Jul 15, 2009
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    I put the lead wrong... Q1 is supposed to supply to the coil of the relay... not the switch.. regardless.. it won't work without removing the charging circuit from the picture when supplying voltage to the load coil. the load coil has such a low resistance that the charging circuit turns into inline resistance and reduces the supply to low to activate the load coil. I am just at a complete loss on how to solve my problem in this manner. I am going to look at a 3 timer sequence.
     
  8. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    I'm not following this...

    If it is a conventional solenoid, changing polarity will have no effect and it will stay energised. A delay in the changeover will allow it to release, but it will operate again when the current flow restarts.


    For polarity to be relevent, it would need to be a permanent magnet assisted type (most likely bistable).

    These do not need a 'no current' delay to operate, just the correct current and the appropriate polarity will do it.

    One other (rarer) possibility is a Remanence solenoid.
    These are engaged by magnetising the core with a 'high' current and released by demagnetising the core with a low current in the opposite direction (or low current AC).

    If it's one of these, you need a diode in series with the solenoid so full current passes in one polarity, then a resistor across the diode to set the lower release current (25% ??) when switched to the opposite polarity.

    Again, no delays are needed.
     
    Last edited: Jul 24, 2009
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