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0.20 delay in 6v control circuit

Discussion in 'Electronic Basics' started by Bob Fraser, Nov 6, 2004.

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  1. Bob Fraser

    Bob Fraser Guest

    I have need of adding a 0.20 delay into an existing 6 volt control circuit.
    Basically a switch activates 6 volts that energizes a relay, and I need to
    delay that relay by 0.20 seconds. Any ideas?
     
  2. Lots, but they sprawl all over the place given the sparse info here. Is this a
    0.20 second delay?? And assuming this is a mechanical switch right now feeding
    into some circuit, is it a momentary type? In other words, once switched and
    released, does your relay continue being activated? Or does the relay only stay
    activated so long as the switch is held down? If the later, do you also want a
    ..2 (sec?) delay after the release, too? There are other questions, as well.

    It helps a lot if you spend a little time being as detailed as you possibly can.
    It's your question, after all, and the proximity to what you need will depend on
    how well you have expressed yourself.

    Jon
     
  3. The simplest solution would be to use an RC circuit:


    6V
    |
    |
    C|
    C| Relay Coil
    C|
    |
    -------.
    47k | |
    |/ |
    Input -----/\/\/---o----o-----------| -----
    | | |> /^\ diode
    \ | | -----
    100k / | + | |
    \ ### | |
    / --- 22uF | |
    | | cap | |
    '----o-------------o------'
    GND
    created by Andy´s ASCII-Circuit v1.25.250804 www.tech-chat.de

    The RC keeps the voltage down for 200ms.

    This won't cause it to delay when the input goes to 0, because
    the transistor will keep the base voltage at about 0.8V. When you
    drop the input, it'll immediately drop off.

    The 100k resistor is there so the cap drains down to ground when the
    thing is turned off. It takes 3 or 4 seconds to drain all the way down,
    so the startup delay will be affected if its turned off and back on in
    less that that time.

    The diode protects the transistor from getting whacked by a reverse
    transient when you shut off the voltage.

    The accuracy is +-20% or thereabouts, due to the accuracy of the
    capacitor. If you use a 100k trimmer instead of the 47k resistor, you
    can adjust this to compensate if you care.

    If you need an exact delay each time, regardless of cycle time, you can
    use a monostable like a 555 to keep the signal from getting through to
    the relay for a bit. It's also a fairly simple circuit. Let me know if
    you need to see it.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  4. Bob

    Bob Guest

    Thanks Robert.

    I like the idea of being able to vary the time constant with the 100K
    trimmer. Is that an NPN transistor? I will go to radio shack
    tomorrow, pick up the parts and give it a try,
    Thanks
     
  5. Yes, it's an NPN transistor. Any small transistor should work. A 2N2222
    or 2N3904 or 2N4401, for example.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  6. As Jonathan pointed out, your spec is vague. But, assuming the switch
    is a basic SPST type, making permanent contact, then the simplest
    approach is to place a small resistor between switch and relay coil,
    and a large capacitor to ground. Trial and error is probably the
    easiest way to arrive at satisfactory component values. But assuming
    the 6V relay has a coil resistance of say 400 ohms, and pulls in when
    the applied voltage reaches say 5V, then this circuit would do what
    you ask:

    http://www.terrypin.dial.pipex.com/Images/RCdelay.gif
     
  7. Bob

    Bob Guest

    Thanks for all the suggestions.
    Hope this clarifies the picture:
    Control voltage is 6v.
    The switch is basic permanent contact type.
    The relay is a Potter & Brumfield PB336-ND with 4.5 volt pull, 0.3
    volt drop. Relay current is 60mA and coil Power is 360mW.
    The switch normally stays on for 1 second. Then after another second
    we would like the circuit ready to close the switch again.

    Bob
     
  8. Here is a slightly better circuit for you needs:

    6V
    ----------------------------o--o
    | | |
    | - C|
    | o ^ C| Relay Coil
    |=| switch | C|
    | o | |
    | '--o
    | |
    | 10k |/
    o---/\/\/--o-----o-----------|
    | | | |>
    | | | |
    o----|<----| | + |
    | * --- 220uF |
    / --- |
    \ | |
    / 1k | |
    \ | |
    | | |
    | | |
    '----------------o-------------'
    GND

    * Schottky Diode

    created by Andy´s ASCII-Circuit v1.25.250804 www.tech-chat.de

    The base will drop immediately from 850mV to about 200mV when the switch
    is opened, due to the 1k resistor and the schottky diode. When the
    switch is closed again, the base will come up from 200mV to about 850mV
    before the transistor conducts, so

    V(t) = V ( 1 - exp(-t/RC) )

    thus

    t = -ln(1 - .65/6) * 10k * 220u
    = .25s += 0.05s

    Once the cap is charged up, the current will be about 500uA, so the
    current through the transistor can go up to 25mA (assuming a beta of 50).

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  9. Bob

    Bob Guest

    Thanks again Robert,

    Do you have any idea on the specs for the diodes needed. I went to
    pick up the Schottky diode and the one in parallel with the relay but
    was not able to choose from the variety available. Thanks for the
    design.

    Bob
     
  10. (fixed top posting below. Please post replys after the main body of
    text. It's just convention here)
    You can use any of 1N4002-1N4007 diodes for both of them. A Schottky
    diode would be nice where the * is (it doesn't have to carry any
    appreciable current, so any of them will do) but isn't necessary. It
    just makes the recovery time a bit better, because it has a lower
    forward voltage drop.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  11. Bob

    Bob Guest

    As you can probably tell, this is the first time I ever posted to a
    news group. Anyway I built the circuit as you designed. What
    actually occurs is the relay is energized for only .20 seconds instead
    of a delay of .20 seconds then energized until the switch opens. (Did
    you expect this? I checked the path of the circuit 3 times)

    Bob
     
  12. If you've wired it up correctly, then it should perform as Bob said.
    (Although it gives a delay of about 0.33 s with that 220uF cap; lower
    it to about 140uF to get the 0.2 s you specified.)

    Have you connected the protective diode across the relay the wrong way
    around? Its cathode (usually the marked end) should be connected to
    6V.
     
  13. Bob

    Bob Guest

    I used silicone diodes in both locations and they are in the correct
    orientation. The one accross the relay has the marked end (cathode)
    on the +6v side. Is there a way I can check to see the Diode is
    marked correctly?
     
  14. John Fields

    John Fields Guest

    ---
    Still vague.

    Do you mean that at the same time the switch turns off you also want
    the relay to turn off, then after the switch stays off for at least
    one second you'd like the relay to come on 0.2s after the switch turns
    on again?


    Like this?:

    |<----1s---->|<----1s---->|
    ____________ _______
    SWITCH ______| |____________ _ _ |

    0.2s --->| |<--- 0.2s --->| |<---
    _________ ____
    RELAY __________| |____________ _ _ ___|


    What if the switch turns on again before it's been off for one second?
     
  15. Bob

    Bob Guest

    Exactly. When the switch turns off I want the relay to turn off, the
    switch will be off for at least one second. Then when the switch is
    turned on I want the relay to be turned on 0.2 seconds later.

    The switch will not be turned on again for at least 1 second.

    Thanks
     
  16. When you close the switch, it should take 0.2 seconds to charge up the
    capacitor enough to turn on the transistor. Thus, assuming the relay is
    open when you start, the following should happen:

    1) relay open, switch open
    2) switch closed
    0.2 seconds
    3) relay closed
    ....
    4) switch open, relay opens immediately.

    It sounds like you have the 10k resistor and 220uF capacitor swapped.
    That would cause it to

    1) relay open, switch open
    2) switch closed, relay immediately on
    0.2 seconds (or less!)
    3) relay open
    ....
    4) switch open, relay still open

    From your reply, I can't quite figure out whether that is what you are
    seeing, though.

    (You probably know this, but closed means passing current, open means
    unable to pass current.)

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  17. Bob

    Bob Guest

    Thanks so much for all the assistance. I am amazed with your patience
    with an electonic baby. It appears the Transistor is the culprit.
    Maybe undersized? I replace a Radio Shack #276-1617 (tiny) with a
    Radio Shack #276-2020 also known as a TIP3055. The ciruit works like
    a charm. Using a digital video camera, (consumer grade (Sony)), I am
    able to count a .22 second delay now compared without this delay
    circuit. Thanks again and my hat comes off for your patience and
    expertise with this circuit design.

    Bob Fraser
     
  18. Ok. I'm glad it works for you. I'm suprised the other transistor didn't
    work, though. It's just a 2N2222 equivalent. Maybe it got toasted at
    some point, or was just bad to start with.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  19. [snip]
    I've mentione this in the past. The 2N3904 is only capable of handling
    100 mA max, and that's on a good day. Because the circuit had a 10k
    resistor to its base, it was a really bad day, there wasn't enough
    current for the transistor (2N3904 or even a 2N4401) to fully saturate,
    and the relay gets current starved also.

    Remember that this is 6V, so it takes twice as much current to pull in a
    given relay compared to 12V, and the relay coil is 1/4 as much DC
    resistance. That puts four times the demand on the transistor compared
    to 12V.

    So use a decent transistor such as the 2N4401, or better yet a 2N2219 or
    2N3053. And feed the base enough current to keep the transistor fully
    saturated. If you can't reduce the base resistor to 1k or so, then use
    a pair of transistors, but if you connect them darlington, the voltage
    drop collector to emitter will be at least 1V, maybe more. You can
    connect the collector of the first transistor to a 1k resistor, and then
    to the +6V, and that will help.
     
  20. The OP mentioned that the relay was closing, then after .2 s, was
    opening again. If the transistor was toasted by excess current, that
    wouldn't happen, and the relay would probably just stay open or closed.
    Also, if there isn't enough current, a relay simply won't close. It
    definitely won't close for a short while, and then to open back up,
    since relays take more current to close initially than to stay closed.

    In addition, the TIP transistor has a much lower beta than the 2N2222 he
    was using, so if anything, the current issue should have gotten somewhat
    worse with the larger transistor.

    Simulating with 6V, a 2N2222 and a 5V relay coil with a small 100 ohm
    resistance, I got max about 30mW in the 2N2222. If the relay was
    moderately big, then the 60mA might not be enough, but if this were
    true, using the TIP wouldn't help.

    One possibility is a misconnected transistor (2N2222s have non-standard,
    CBE pinouts). I haven't analyzed why a reverse connected transistor
    would do this, though. Seems like it would just have a much smaller
    beta, and thus the relay would stay open. The other possibility, that I
    mentioned before, is a reversal of the timing cap/timing resistor. That
    fits his description (the relay energizing initially for a short while,
    then opening until the next cycle.)

    However, without more information, it's a mystery.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
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