Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Delay Circuit with Logical-Quality Output?

  1. Jan 7, 2012 #1
    Delay Circuit with "Logical-Quality" Output?


    First of all, hello everyone (my first post here)!

    I need to make a delay circuit for my car, but I need the transition time of the output voltage to be quick, because I will use the voltage to activate a power MOSFET transitor, which will in turn activate light ballasts, whose maximum current draw is at startup. I need a ~10 seconds delay, so using a simple RC circuit won't do it (because dV/dt would drop with time). I doubt that using a current barrier will do it either, because dV/dt in this case is approximately constant.

    So what I need is a delay circuit whose output voltage stays very low for almost the whole delay time, then goes up quickly to activate the power MOSFET transistor. Is that something possible with an analog circuit? Otherwise, what kind of programmable chip could I use? In fact, I need a couple of delay circuits (with different time constants), I will need to activate 3 of these power MOSFET transistors and I will also have about 1/2 dozen inputs to my circuit (from different car signals). I wanted to go initially with an analog circuit, but space constraints and flexibility now make me lean towards a programmable digital circuit...

  2. jcsd
  3. Jan 7, 2012 #2
    Re: Delay Circuit with "Logical-Quality" Output?

    Is the time delay to be triggered by a positive going voltage or a negative going voltage?
    If the time delay circuit turns on the mosfets, what turns them off?
  4. Jan 7, 2012 #3
    Re: Delay Circuit with "Logical-Quality" Output?


    I would like everything to be triggered by a positive signal.
    I would like that the same delay module turns off the MOSFET (as soon as the delay module input switches off...)
  5. Jan 8, 2012 #4
    Re: Delay Circuit with "Logical-Quality" Output?

    See attachment for a starter circuit. Adjust R1 and C1 to get the desired delay. Once the input pulse goes high, the output should stay low for the duration of the time delay and then go high and stay high until the input pulse goes low.

    Attached Files:

    Last edited: Jan 8, 2012
  6. Jan 8, 2012 #5
    Re: Delay Circuit with "Logical-Quality" Output?

    Thanks. Could you please identify the source, the drain and the gate on your two MOSFETs? I am not sure to understand the circuit. Also would you mind generating an image or a PDF, just in case my OpenOffice is screwing up your diagram...

  7. Jan 9, 2012 #6
    Re: Delay Circuit with "Logical-Quality" Output?

    Attached is the PDF. Mosfets can be NMOS or PMOS and can be configured as common source, common gate or common drain. They can also be enhanced or depletion types. I cannot guess which type of mosfet you intend to use nor the configuration.

    I asked you how you wanted the delay circuit to work and showed you a circuit that fulfills your specifications. It is assumed if you are specifying a circuit, you already know how it will be connected.

    Attached Files:

    Last edited: Jan 9, 2012
  8. Jan 9, 2012 #7
    Re: Delay Circuit with "Logical-Quality" Output?

    Hi skeptic2,

    I think you forgot to attach the PDF. What I want for my circuit is:

    -Input ground -> Output Ground
    -Input +12V -> Output Ground for ~10 seconds, then output goes to +5V as quickly as possible and stays there as long as input stays active.

    What I did not understand with your circuit notably is that you use the gate for the output
  9. Jan 9, 2012 #8
    Re: Delay Circuit with "Logical-Quality" Output?

    The PDF has been attached.
    I don't understand your terminology. How is input ground different from output ground?
    What is Input +12V. Where is it measured? Do you want a short between Input +12V and output ground for 10 seconds? Where is +5V measured? Are the +12V and +5V measured at the same point? What happens to the +5V when the circuit is not active?
  10. Jan 9, 2012 #9
    Re: Delay Circuit with "Logical-Quality" Output?

    The input ground is the same as the output ground

    The input is +12 to +15 V when the headlight switch on the car is closed. I certainly don't want a short to ground for 10 seconds, as it will blow a fuse on the car. Ideally I would want zero current draw during that 10 seconds. The +5V output will be the input voltage for the power N-channel MOSFET used as a relay. I want the output to go to ground during the 10 seconds delay and also whenever the input is ground.

  11. Jan 9, 2012 #10

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

  12. Jan 9, 2012 #11
    Re: Delay Circuit with "Logical-Quality" Output?

    Does this help?

    Attached Files:

  13. Jan 9, 2012 #12
    Re: Delay Circuit with "Logical-Quality" Output?

    A pair of 555 make a pretty delay thingie too.
  14. Jan 9, 2012 #13
    Re: Delay Circuit with "Logical-Quality" Output?

    Hi skeptic2,

    Could you explain a bit in words how the circuit works? I really don't get what you do with the bottom MOSFET, since you use its gate for the output, although it is the gate voltage that controls the current on the left side.

  15. Jan 10, 2012 #14
    Re: Delay Circuit with "Logical-Quality" Output?

    Perhaps you could explain a little of what you do understand about the circuit. Do you know how an AND gate works and can you explain what both and gates are doing? If not, can you identify the parts you don't understand?

    With NMOS the drain current is controled by the gate voltage. The higher the gate voltage, up to a limit, the more drain current. If the drain current is high enough that it drops nearly 12 volts across the drain resistor, what will be the voltage at the drain? What will be the voltage at the output of the 7805 regulator?

    If the gate voltage is zero, how much drain current will there be. The exact answer can be found in the mosfet's datasheet but it should be close to zero. If there is approximately zero drain current, what will the drain voltage be? What will be the voltage out of the 7805 regulator?

    The value and wattage of the drain resistor is important. If they are too low, you can destroy either or both of the resistor and mosfet. If the value is too high, your circuit will not get enough current to operate. How would you calculate the value of the drain resistor?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook