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Help a beginner with a relay problem

Discussion in 'General Electronics Discussion' started by doobiest, Nov 8, 2010.

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  1. doobiest

    doobiest

    4
    0
    Nov 8, 2010
    Hi, I'm hoping this will be a simple one. I don't know enough to figure it out.

    I'll try to explain it simply put like this..

    I have a line with 12v 200ma going down it. There are two things that can occur on this line. Either constant, for a user defined amount of time, or a pulse for no more than a second.

    This line is driving a relay when active.

    I have no ability to control the pulses, but I'm trying to think of a way to get the relay to ignore them. Basically I want a delay before the relay trips.

    The best way I could describe it is, what can I do to make it so the relay needs more than a second of constant power before it will trip. Also I'll add that the relay still has to disengage like it currently does when power is removed.

    I don't have the know how or the steady hand to make a decent circuit. If there's something quick and dirty I could do, I'd prefer that.

    I don't know much about capacitors but I think once they're fully charged the stop outputting til its power source is removed, then it discharges. Is that true? If it doesnt work like that then I'm imaging I could do something like:

    12+ --> resistor -- > capacitor --> relay

    I'm not sure which values I'd use though.

    Any help is appreciated. Thanks.
     
  2. KMoffett

    KMoffett

    719
    73
    Jan 21, 2009
    Can you change the relay, or insert another relay to control the original? What you need is a delay-on-make relay.
    http://www.artisancontrols.com/pdf/2410sa.pdf
    How much voltage/current does the original relay switch?

    Ken
     
  3. doobiest

    doobiest

    4
    0
    Nov 8, 2010
    Thanks for the reply. Yup that looks more like what I want. I wonder if my local store would have one or if I'd have to get it online. Do you happen to know a good place to order one?

    Let me give you some specifics to my setup. I was trying to be as vague as possible to the application, but still give good details on the actual problem. I find forum threads can spin off topic pretty quick.

    Setup:

    -I have a reverse camera that I just installed into my car.
    -I have a 7 inch lilliput 629 lcd in the dash
    -The reverse camera is powered, always on when car has ignition
    -The lcd screen will automatically switch to the proper video input, when I put my car into reverse.

    The way this works is simple. My car's automatic, when I shift into reverse, a switch triggers a relay(original to the car), which then lights up a power line to my reverse lights in my taillights.(the white lights that help you see when backing up at night).

    There's a wire on the lcd screen that is meant to be tapped into this reverse power line. When it gets a signal it switches to the proper video input. When it loses the signal it returns to it's previous input.

    If you imagine what a gear shifter for an automatic car looks like, the order is P, R, D, etc.

    My problem is cause when shifting quickly from P to D, crossing R for a brief moment. It causes the screen to flicker, and worse than that causes my touchscreen to calibrate wrong as the behavior is like re-plugging a usb cable in a split second, it messes up my computer.

    So by what you're saying, with a delayed on relay. I think what I would want to do is this.

    Shifter(in R) --> Trips existing relay so original functionality remains
    AND
    Shifter(in R) --> Split to second ondelay relay, power the lcd's sensor wire after 1 second.

    Is there any considerations regarding the type of power these relays can handle? Once again I'm pretty new but given its a car battery I think it's 12-18V and as many amps as you feel like pulling, for the most part.
     
  4. KMoffett

    KMoffett

    719
    73
    Jan 21, 2009
    Last edited: Nov 8, 2010
  5. doobiest

    doobiest

    4
    0
    Nov 8, 2010
    wow I have no idea how to work with 555 timers. Its intimidating. Maybe I'll pick one up and see how far I get though. Thanks for the links. The cheapest I could find locally was $60 so this is certainly cheaper.
     
  6. doobiest

    doobiest

    4
    0
    Nov 8, 2010
    To add to this, it takes 200ma for the reverse line to trigger, this is what I'm trying to delay. So is there anything quick and dirty that could accomplish this?

    Is there something I could do so that it would slowly build up to a 200ma draw? I don't care if theres some current in the first second, as it wont be enough to trigger the line, I simply just need some way for it to take a moment to build up to that, or maybe it's not simple at all.
     
  7. KMoffett

    KMoffett

    719
    73
    Jan 21, 2009
    I don't have an automotive relay, or a circuit simulator to try this. Place a 33Ω resistor in series with the wire from the reverse switch to the relay. Place a 2200uF/25VDC capacitor across the relay coil. (+) on the capacitor goes to the resistor side. This will allow a maximum of only 2/3's of the battery voltage across relay coil, but the pull-in rating is usually ~7.5V. The larger the capacitor the longer the delay. It will be pretty short anyway.

    Ken
     
  8. KMoffett

    KMoffett

    719
    73
    Jan 21, 2009
    I tried it with a 27Ω series resistor, a 50Ω resistor (to simulate the relay), and a 3300UF capacitor. It took ~ 0.15 seconds to reach 8V across the "relay". Don't know if that would be long enough.

    Ken
     
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