What Do I Need and What Is It Called?

Discussion in 'The Projects Forum' started by Nirrad, Oct 24, 2009.

  1. Nirrad

    Thread Starter New Member

    Oct 24, 2009
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    Hello everyone!

    I have been searching all over, and decided I truly need some assistance. I have a feeling I'm not wording it correctly, or searching the correct terms.

    I am looking for a device that is like a relay, but changes output polarity every time power is applied.

    I am trying to add an actuator to my truck tailgate. And would like to use the truck release feature on the car alarm. The problem is that it only sends 12v (+pos) and I also need (-neg) to move the actuator the other direction.

    What relay do I need in order to get the output polarity to change with every press of the button?

    Please point me in the right direction; I don't really want to use a SPDT Center-Off switch!

    Thank you!
     
  2. Mike2545

    Active Member

    Mar 26, 2009
    116
    3
    For automatic operation you will need limit switches to tell the linear actuator to stop at the end of travel. A relay would work for changing polarity to the unit.
     
  3. Nirrad

    Thread Starter New Member

    Oct 24, 2009
    3
    0
    Hi Mike,

    I think I understand what you mean about the limit switches. The alarm unit only sends a short burst to the actuator which moves it very slightly. So would a limit switch still be needed?

    The thing that I would like automatic is the switching of the output from the relay, from - to + with each burst.

    Could you explain the relay circuit as to how it takes that 12v + source and as it passes through the relay it changes the output to neg and back to + then back to neg each time?
     
  4. Mike2545

    Active Member

    Mar 26, 2009
    116
    3
    You may need a friend that is familiar with your alarm/linear actuator that is fluent in electronics to design a circuit for you.



    If you come up with a design, post is here or ask questions about what you already have but as far as a free design service...I cant help you there.
     
    Last edited: Oct 24, 2009
  5. Nirrad

    Thread Starter New Member

    Oct 24, 2009
    3
    0
    Thanks for the input Mike. But the reason I'm here asking is because I thought this is a place full of people fluent in electronics.

    I'm not looking for a free design service or any service. I'm in need of knowledge, and I don't think I'm searching/asking the right questions.

    I have a design, but I'm just wondering if there is such a device (or circuit incorporating what I believe uses a relay) to reverse the phase, like a SPDT Center-Off switch. I move the switch up = positive flow I move the switch down = negative flow The switch is in the center = no flow

    But I'm wondering if there is something that can do this automatically, change the output depending on the current state?

    That's all I'm asking. Thanks.
     
  6. Mike2545

    Active Member

    Mar 26, 2009
    116
    3
    I think a circuit with a DPDT relay (Double pull, double throw) would work in your case but you would have to incorporate the limit switches.

    Do you have pictures/model numbers/schematics to share?
     
  7. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    You might also be able to use a flip-flop for this application.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Try this:

    [​IMG]

    Pressing S1 energizes relay 1, causing the motor to rotate in the forward direction.

    Pressing S2 energizes relay 2, causing the motor to rotate in reverse.
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    I've taken a little "poetic license" with SgtWookie's circuit. It provides latching of a given direction so a pulse would start it. His limit switches would stop and reset the circuit but by using a self resetting circuit breaker of the proper rating, the system would not require the extra wiring and physical positioning of the switches. I think some automotive power window systems uses something similar.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, what you've done is cause either the pushbutton switches and/or the relays to get fried instantly when either pushbutton is pressed. The relay coils would never get energized, because there is now a dead short across them.
    [eta]
    That problem can be corrected with the addition of a couple of diodes, like this:

    [​IMG]
     
    Last edited: Oct 24, 2009
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You would need a second set of contacts in parallel with the switch to perform latching.

    Otherwise, you have performed the basic "aw $hit" that sgt wookie described.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    I fixed the problem with a couple of diodes. ;) Much cheaper than a 2nd set of contacts.
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    I sure missed that one! Thanks for catching it. It seemed so logical on first look. What are your thoughts, Sgt, on using the self resetting circuit breaker to reset rather than the llimits?
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Trust me, I've done worse. :rolleyes: :D

    Yeah, I know. If only there wasn't that dead short problem...

    The circuit DOES need at least a fuse. A self-resetting circuit breaker may be preferable, but they are expensive, and if it's a "real" failure, it could lead to a dead battery in a very short period of time.

    However, using a self-resetting circuit breaker to control the limits is IMHO a very bad idea. :p The system will be mechanically stressed, as the motor will be subjected to operation at stall loading periodically, the circuit breaker will have to make/break that stall current load (which will arc it's contacts), and the battery will rapidly become discharged due to the periodic high load currents that it would be subjected to.

    The limit switches breaking the ground path for the relay coil is a significantly "cleaner" design. Once the coil's ground path is broken, the SPDT contacts toggle and create a short to ground across the motor, which acts as a brake for brushed DC motors. This brings the motor to a more rapid stop than if the motor were allowed to slow by itself, and compared to slamming into a physical stop, it's very easy on the motor.
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Sgt,

    Do you think there is enough time for the latching diode to conduct after the push-button switch is released?

    For some reason, and it could be because simulators are faster, but I'm not getting the latch to work. I can, however, connect the anode to the 12V source to force the connection.

    I can produce a video if you wish to see ...
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Joe,
    The simulator I'm using has analog and digital modes.
    In analog mode, the pushbutton switches don't work.
    Diodes can't be simulated in the digital mode. :rolleyes: So, I don't have a good way to simulate the circuit. However, if the relay coil draws less than 1A, it should work just fine.

    Real-world relays may take 20mS to 100mS for the contacts to toggle states, and then the contacts may "bounce" for a period of time afterwards. However, after the bouncing stops, the current flowing through the closed contacts and the diode should be sufficient to keep the relay contacts pulled in.
     
  17. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Sgt,

    I know they bounce a little. I also know the second set of contacts make a positive connection ... likely because while they are in parallel with the switch, the contact resistance is such that the current is split between the switch contacts and the relay contacts.. With the diode in parallel, the switch handles alot more current than the diode.

    Tina does analog simulations as well as digital.

    I guess I'll set this a similiar condition on the breadboard to check the diode bypass.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    You've lost me with the "second set of contacts" statement. I only have SPDT relays and SPST switches in my schematic. I guess you're using DPDT relays?
    As long as the pushbutton switch is closed, the diode will be carrying virtually zero current. However, if the pushbutton has been held long enough for the coil to energize and the NO relay contacts to close, the diode will start to conduct within nanoseconds of the pushbutton switch contacts opening.

    I've have Tina 7 TI installed for a couple of years, but haven't used it much. It seemed a bit awkward to get the hang of.

    I suppose I should check to see if there's a newer version available.
     
  19. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    In TINA I can add as many sets of contacts as needed, by telling which relay controls them.

    Here's the schematic of the one that actually simulates.

    Tina 7 is the latest version.

    When I test drove circuitmaker student version, the learning curve was impeded by my knowledge of electronics workbench and TINA simulators. I would expect the same impediment going the other direction.

    I agree with you concerning the diode conducting upon release of the switch. I do have a five volt relay I will be checking later to see the times.
     
  20. BillB3857

    Senior Member

    Feb 28, 2009
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    Are you sure you have the diodes installed in the proper direction?
     
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