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Choosing best mosfet for low voltage application

Discussion in 'Electronic Basics' started by smpowell, Dec 31, 2004.

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  1. smpowell

    smpowell Guest

    I'm building a circuit that sends power to a photographic flash
    unit at certain times. Basically the output of a 7555 timer is
    connected to the gate of a mosfet, which acts like a switch to
    send 6 volts to the flash unit.



    I'm not familiar with using mosfets; how does one pick the
    best one(s)? Something cheap, easy to find, and a low
    voltage drop in this type of circuit.

    I'm thinking of a logic level mosfet like a IRL 2702-ND
    (Digi-Key page 641, $0.70 US).

    The schematic and description are at:
    http://octopus.freeyellow.com/slaveflash.html


    Stephen Powell
     
  2. Stephen Powell (smpowell) wrote...
    I see nobody has answered, so I'll chime in. Ding!

    Did you mean the IRL2703, which is a 30V 60-milliohm (with 4.5V
    gate drive) TO-220 part? This is pretty respectable for 56 cents,
    qty 10. Your 7555 circuit is OK, I suppose, but I suggest you add
    a say 100-ohm gate resistor to slow the FET's rise/fall time a bit.

    Normally we'd ask you to characterize the load, but with a nice
    60-milliohm switch, I'm not sure if that's necessary! But I am
    curious about your 6V power source. What's that, a 9V battery?
    Four AA cells? Usually it's important to analyze your circuit's
    performance when the battery is dead or nearly dead. Your 7555
    and logic-level FET should work down to say 4 volts, although
    the Ron rises to 80 milliohms or so...

    Hmm, "Trail cameras are cameras placed out in the woods to take
    pictures of wild life, tripped with an IR sensor." Tell us more.
     
  3. Archilochus

    Archilochus Guest


    Hi Stephen,
    Have you checked out any of the forums that discuss trail cams?
    Lots of knowledgeable people over at http://www.hagshouse.com

    I'm wondering what will happen with the circuit in this situation...
    Animal walks by, a picture gets taken (with flash), and the "delay"
    between pictures is shorter than the 555's 'flash recharge' timer.
    Seems like the next picture would then have no flash, since the 555
    didn't allow another recharge cycle.

    Maybe you could incorporate some sort of reset logic to force the flash
    to recharge immediately after the flash is discharged?

    On the MOSFET - couldn't find that part # listed, but look for a one
    that can handle the Maximum current the flash will require (maybe 2
    Amps or so?).

    Arch
     
  4. smpowell

    smpowell Guest

    Have you checked out any of the forums that discuss trail cams?
    Lots of knowledgeable people over at http://www.hagshouse.com

    Yes.
    between pictures is shorter than the 555's 'flash recharge' timer.
    Seems like the next picture would then have no flash,

    The camera controller controls the minimum time between pictures.
    flash
    to recharge immediately after the flash is discharged?

    While there is a small possibility that a second photo might only get
    the flash that is built into the camera, unless there is a very
    simple way to force the recharge, I prefer the K.I.S.S.
    principle.

    Stephen Powell
    Electronic Hobby Information
    http://octopus.freeyellow.com/
     
  5. smpowell

    smpowell Guest

    I see nobody has answered, so I'll chime in. Ding!

    Thanks!
    gate drive) TO-220 part?

    Yes.
    a say 100-ohm gate resistor to slow the FET's rise/fall time a bit.

    Is there a reason for wanting to slow the FET's rise/fall time?
    It's a common Vivitar 2000 Flash. I measured a load of
    80 ma after it was charged. I'm guessing 2-3 amps
    from a cold start up. The flash uses 4 AA's; we might
    use an external pack of 4 D cells instead.

    I expect the circuit to reduce power consumption by a
    factor of 10.

    The IRL2703 was my best guess after learning what I could
    about these parts and looking through the Digi-Key catalog.
    Mostly I figured something cheap with a very low voltage
    loss when the flash recycles.

    I'm hoping that some experts will second guess me, in case
    it turns out that I really don't understand as much as I think
    I do :cool:.
    For example, is there any reason to add an external component(s)
    to protect the MOSFET, or perhaps there is another part that would
    be more tolerant of any transients that might occur.
    pictures of wild life, tripped with an IR sensor." Tell us more.

    My brother wants to build a couple of these things. Apparently there
    is a following for these devices. A couple of web sites on these:

    http://www.jesseshunting.com/forums/index.php?s=cc79c1a0b6a5350b8082cfc5dd025be6&showforum=50

    http://www.hagshouse.com


    Stephen Powell
    Electronic Hobby Information
    http://octopus.freeyellow.com/
     
  6. Archilochus

    Archilochus Guest

    Here's an idea that should cover both the "K.I.S.S. principle" and the
    issue of the missed flashes...
    Use a micropower dual comparator in an 8 pin package (one with a
    built-in reference).
    One comparator goes to the CDS cell, the other to a pair of high value
    resistors forming a voltage divider that monitors the voltage on the
    flash capacitor. The 2 comparator outputs are connected in such a way
    that the CDS comparators output holds the gate of the MOSFET low in
    daytime, at night the flash monitors output drives the MOSFET On or Off
    as needed to keep the flash topped up (use some positive feedback for
    hysteresis).

    Cost and complexity should be about the same as the 555 based circuit -
    and you'll probably see even higher power savings than the timed
    recharge scheme.

    Now it's off to the new year's festivities!

    Arch
     
  7. smpowell wrote...
    Yes, reduces damaging V = L di/dt inductive spikes, reduces the
    dV/dt in the switched circuit, reduces RFI. Recommended.
    Nah, I think you're set to go. The MOSFET is avalanche rated
    in case of a short spike. Go for it (adding in the relevant
    timing refinements suggested by others).
     
  8. smpowell

    smpowell Guest

    From STTOS:

    "&*&% Jim, I'm a doctor, not an engineer!"
    "&*&* guys, I'm a metallurgist, not a EE!"
    Hmmm, I could use a part #, preferably a common part that's available
    as a DIP and likely to be around for decades to come.
    Any tricks to getting this to work with the -300VDC on the cap?

    Stephen Powell
    Electronic Hobby Information
    http://octopus.freeyellow.com/
     
  9. Archilochus

    Archilochus Guest

    From Stephen Powell:
    There are hundreds that would suit the application - I like the
    cheapies from Mouser (about 50 - 70 cents) - http://www.mouser.com
    # 511-TS3V393IN for example - an inexpensive dual in a DIP package with
    a 'standard' pinout - not the finest comparator by a long shot - but
    suitable for the job.
    As far as availability goes - that would be up to the manufacturer -
    but if you use a 'standard' pinout, you should be OK for quite a while.
    I'd be more worried about the 555 supply drying up, the way things are
    heading with micro-controllers and all.
    That's what the high value voltage divider is for - scales down the
    300+ volts to a manageable level, with only minimal current bled off
    the cap.

    Arch
     
  10. Archilochus wrote...
    The TS3V393 was a nice comparator, operating on 20uA or less, down
    to 2.7V. Some parts may be in the distributor pipeline, but sadly
    it was discontinued by ST last year. ST's TS393 and TI's TLC393 are
    similar parts in steady production; either would be a better choice.
     
  11. smpowell

    smpowell Guest

    Use a micropower dual comparator in an 8 pin package (one with a
    parts

    I'm not following the part about "one with a built-in reference".
    Didn't see
    anything about this on the datasheets for those parts. I assume that
    this
    would eliminate the need for an external voltage reference.

    Is there a reason for not using a TLC3702 with a push-pull output to
    eliminate
    the two pullup resistors needed with the other parts?

    I've put a second schematic using a comparator at:
    http://octopus.freeyellow.com/slaveflash.html
    based on this thread.

    I've included a LM4040 2.5V voltage referance in the circuit, this
    choice
    was pretty much guesswork. Is there another choice with less power
    draw?
    By trick, I was thinking of avoiding putting minus 300 volts on the
    comparator when the batteries are removed. I've included a
    Schottky diode on the new drawing that would probably do the job.

    Stephen Powell
    Electronic Hobby Information
    http://octopus.freeyellow.com/
     
  12. Archilochus

    Archilochus Guest

    Ohhh - I dropped that and went with a more general purpose comparator
    when you mentioned wanting a part that would be around for a long time
    ('course the example I picked is already obsolete!)
    Many comparators have a built-in reference hooked up internally to an
    input on one or both comparators. Maxim makes some really fine parts -
    but I hear that availability can be a real problem with them (not to
    mention all those tiny surface mount packages Maxim is so fond of).
    I'll poke around in some data sheets and see if I can find something
    that looks good.
    Not that I can think of - didn't realize the '393 was not a push/pull
    type.
    Most of the stuff I toss together is just hobby level. I usually just
    use a pair of 1% resistors as a divider for the reference (on circuits
    with a voltage regulator).
    That's a great point! Glad you mentioned it, as I'm working on building
    a similar rig for my own cameras.

    Arch
     
  13. smpowell

    smpowell Guest

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