IRF7319 Mosfet latching switch

Discussion in 'The Projects Forum' started by dumpystig, Nov 13, 2013.

  1. dumpystig

    Thread Starter New Member

    Apr 10, 2013
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    I have breadboarded the circuit below but can't get it to work as intended. As soon as power is connected the load (an LED just for test purposes) illuminates and stays lit. Operation of the momentary switch doesn't have any affect. I have transposed the Mosfet pin numbers from the datasheet to the diagram in red, but think I may not have done it correctly.
    Can anyone help me out with this?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Remove the capacitor, at switch on it will be discharged and will conduct to put the P-channel mosfet on (pins 3-5), also connect the switch in place of the capacitor.
     
  3. dumpystig

    Thread Starter New Member

    Apr 10, 2013
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    Thanks for the quick reply. Doing as you said doesn't have the desired result - the LED is still illuminated when power is applied, although the switch does turn it off until released and then it's lit again.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The load cannot be a LED in-series with a resistor. The load must be have an effective resistance of 1000 Ohms or less.
     
  5. dumpystig

    Thread Starter New Member

    Apr 10, 2013
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    Is that with reference to the original diagram or to the ammended suggestion by Dodgydave?
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The circuit is sensitive to the turn-on threshold voltages of the FETs.
    In your original circuit try increasing the 100k resistor value (e.g. to ~200k)
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Did you move the switch as i said earlier????
     
  8. dumpystig

    Thread Starter New Member

    Apr 10, 2013
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    Yes, I removed the cap and put the switch in its place.

    Have I transferred the pinouts from the datasheet to the circuit diagram correctly?
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    The original circuit.

    I simulated it, and posted the results of the simulations on ETO under the title "Momentry Power Switch Troubles"
    If loaded with just the LED, the forward drop of the LED screws up the circuit. The impedance of the load must be low enough. The circuit was intended to switch a good fraction of an Amp...
     
    Last edited: Nov 13, 2013
  10. crutschow

    Expert

    Mar 14, 2008
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    The capacitor is needed for the alternate action latching operation. The switch is momentary.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    My simulations agree with MikeML. If you add a parallel resistor across the LED of about 1kΩ or so the circuit should work.
     
  12. dumpystig

    Thread Starter New Member

    Apr 10, 2013
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    Thanks guys, adding a 1K resistor across the load has got the original circuit almost functional; but still when initial power is applied the circuit is on instead of off, although subsequent switch presses work as intended.

    What about changing the cap value? Incidentally, the cap I'm using is a ceramic that I bought specifically for this build and I don't have a stock of any other values - would an electrolytic (polarised) suffice, providing I have the correct value?
     
  13. RamaD

    Active Member

    Dec 4, 2009
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    While switching on, the capacitance across the DS of the P MOSFET, makes the N MOSFET to conduct, turning on the led!
    Maybe, a 10kpF cap on the GS of N MOSFET should limit the gate voltage from turning on, assuming 320pF of Coss.
     
    dumpystig likes this.
  14. crutschow

    Expert

    Mar 14, 2008
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    My simulation shows about 10nF across R1 causes the circuit to start in the OFF state. Much larger or smaller value and the circuit doesn't work properly. You may have to experiment for the optimum value.

    A polarized type will work fine for the bottom capacitor.
     
  15. dumpystig

    Thread Starter New Member

    Apr 10, 2013
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    Which resistor are you referencing as R1?
     
  16. dumpystig

    Thread Starter New Member

    Apr 10, 2013
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    I've sorted the output side of the circuit by using a relay to switch the load. The relay coil is 160R so seems to be of low enough impedance for the circuit to operate almost as intended. But there's still the problem of the circuit being on when power is applied instead of off.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    Sorry I forgot to specify the resistor in my simulation. It's the 10kΩ resistor on the top left of his schematic in the first post.
     
  18. dumpystig

    Thread Starter New Member

    Apr 10, 2013
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    Cheers, that did it. As I only have a few values scavenged from old boards I had to try what I had. I couldn't find a 103 (0.01uF) with long enough leads but it turned out that it works with a 104 (0.1uF).

    So thanks to you guys for helping me out on this, you have provided some very expert knowledge and I'm grateful.

    I've also attached an updated diagram.
     
    Last edited: Nov 16, 2013
  19. igeorge

    Active Member

    Jul 2, 2010
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    i tried the proposed circuit, but when i simulate on proteus, at power up the switch is on
    Any suggestion ?
     
    Last edited: Dec 15, 2013
  20. igeorge

    Active Member

    Jul 2, 2010
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