LED Flasher with Mosfet issues

Discussion in 'The Projects Forum' started by kilgorq, Feb 24, 2011.

  1. kilgorq

    Thread Starter New Member

    Feb 24, 2011
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    I am building a 9 LED flasher circuit with a 555 and 4017 and the first line in the chain output for the 4017 does not light up. If I lift the output from pin 3 of the 4017 to the MOSFET then the output from pin 2 does not work. When I say not working I mean it works but the LED is very dim. The remaining LED light up at full brightness.

    I tried jumping between pin 3 and 7 and the led will light up full brightness when the sequence gets to pin 7 but it will light up dim for pin 3. I have replaced the 4017 chip and it still acts the same way.
     
    Last edited: Feb 24, 2011
  2. Wendy

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    Mar 24, 2008
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    Nice circuit. You don't need R12 though R20, the 4017 will drive the MOSFETs just fine, you might say they were designed for each other.

    The main problem is the LEDs are backwards. Cathode (the bar) needs to face the negative direction.

    Ever read LEDs, 555s, Flashers, and Light Chasers?

    Chapter 11 covers a circuit similar to yours, but I think you use of MOSFETs is better.
     
  3. kilgorq

    Thread Starter New Member

    Feb 24, 2011
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    Without R12 though R20 the Mosfets stick on. They take several minutes to shut off. The LED Backwards is an error in my schematic. The LED's are in the circuit correctly. I Uploaded a corrected schematic.
     
    Last edited: Feb 24, 2011
  4. Wendy

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    They shouldn't stick on, they aren't designed that way. More likely you have the leads mixed up on the transistors. Gate high (+12V), transistor on, Gate Low (0V), transistor off.

    The 4017 is a CMOS type logic. It goes true rail to rail, +12V on/ 0V off. It uses cousins to MOSFETs you are driving internally. The transistor switching time is instantaneous. The resistors are going nothing for you, except pointing out you have a wiring error if the switching on the transistors is slow. MOSFETs also have tremendous input impedance, a little capacitance goes a long way.
     
  5. kilgorq

    Thread Starter New Member

    Feb 24, 2011
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    What is the difference between a Mosfet and a Hexfet?

    I double checked the IRF3415 and it is a Hexfet. I also double checked my pin out and it is correct. I used these because I had several lying around.
     
  6. Wendy

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    None, hexfet is a brand name and a construction style. But I am positive about what I speak.

    BRB, looking up your number...
     
  7. kilgorq

    Thread Starter New Member

    Feb 24, 2011
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    I have very little experience with Mosfets.

    This is how it is hooked up.

    The Gate is the output of the 4017.
    The Drain is out to the LED's
    The Source is hooked to ground.

    Pin out of the IRF3415 is G-D-S Tab-D
     
  8. Wendy

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  9. Wendy

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    Center pin to the LEDs (which are backwards).
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    I don't see any capacitors across the 555 Vcc/GND or across the 4017 Vdd/GND terminals. You should have one 0.1uF ceramic or metal poly film cap in each place.

    If the MOSFETs aren't going off without the resistors, you might have a poor connection from Vss to GND on the 4017.

    Are you certain that your MOSFET source connections are good?

    What MOSFETs are you using? [eta] OK, IRF3415; that's a TO-220 package.
    If they are in a TO-220 package and you're trying to breadboard your circuit, you probably overstressed the sockets in the breadboard, and your connections will be intermittent after such treatment.
    2N7000 N-ch MOSFETs work really well for breadboarding low-power circuits, as they come in a TO-92 package. They're limited to around 200mA sink current. IRLD024's are N-ch logic level power MOSFETs that come in a 4-pin DIP package, and can sink up to ~2.4A. The DIP package makes them great for breadboarding.
     
    Last edited: Feb 24, 2011
  11. kilgorq

    Thread Starter New Member

    Feb 24, 2011
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    The internal long side of the LED is hooked to 12V the Short to the center pin on the Mosfet.
     
  12. kilgorq

    Thread Starter New Member

    Feb 24, 2011
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    I am building on a perf-board solder in type.

    They are 220 type.

    I will try adding the caps and see if it helps.

    As far as I know The caps are optional in most cases. They are usually added if you are far away from your power supply. They are used as noise filters.
     
  13. Wendy

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    All else fails, take a picture and post it. I'm still thinking wiring error.
     
  14. SgtWookie

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    They are not optional; they are required.

    Please see this "sticky" thread: Decoupling or Bypass Capacitors, Why?
     
  15. kilgorq

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    Feb 24, 2011
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    The caps fixed it... Thank you very much
     
  16. CDRIVE

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    A DMM and an inexpensive Logic Probe would make trouble shooting much easier. IMO, perfboard construction shouldn't be done until satisfactory breadboarding has been done by other means, like a Protoboard. Prior to their existence, tack soldering and jumper leads were the order of the day. Protoboards are now indispensable.
     
  17. CDRIVE

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  18. kilgorq

    Thread Starter New Member

    Feb 24, 2011
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    I agree with you on this. I don't have a Breadboard and this a project that when done will be the last of this type for quite some time. I am building sequential tail lights for my 69 Mustang then going back to the rest of the project. I have dozens of perfboards at my disposal. Also this issue was not showing up on my DMM. The 12V to the Mosfet circuit was present but it was not turning on. It probably would have shown up on a scope. I am guessing it was a noisy 12v.

    This is one of those do I really need to buy a ProtoBoard when I will not use it again for a very long time. If I do get into another project like this I will more than likely buy one. Probably when I get around to the XYZ CNC Table I want to build.
     
  19. SgtWookie

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    Please don't try to use that circuit for exterior automotive lighting, particularly not tail lights.

    It isn't rated for automotive transients, which can hit 60v. You will create a safety hazard for yourself and other drivers.
     
  20. kilgorq

    Thread Starter New Member

    Feb 24, 2011
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    This is only the flasher portion. It will have a regulator and a over voltage clamping circuit. The system I plan on using has a zenor that will clamp to ground and blow a fuse if there is an issue that reaches over 75% of the maximum rating of the regulator. I am using a 317 for the regulator. Also the control module will be mounted in a dry area of the car. The turn signal will be on a separate circuit from the brake lights and tail lights. Each circuit will have its own set of LED's.
     
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