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MOSFET ratings

Discussion in 'Hobby Electronics' started by Peter Howard, Sep 20, 2005.

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  1. Peter Howard

    Peter Howard Guest

    Making a timer to switch off a soldering iron in a telephone exchange after
    an hour. Irons are 50V Wellers fed from exchange battery. Timer prototype is
    working fine but I'm hoping to get away from use of a relay to switch the
    iron on and off, mainly because inexpensive small relays with contacts rated
    at 50VDC are scarce. Every dollar counts when I want management to blow the
    moths out of the petty cash tin and pay for parts for half a dozen copies
    for half a dozen exchanges when the prototype is finalised.
    I thought a power mosfet was the way to go so I have been experimenting with
    a $2.00 MTP3055 rated at 60V and 12A. The current rating is well within
    requirements but 60V seems to me to be uncomfortably close to the actual 54V
    of the exchange battery. The exchange battery voltage is, at least, well
    regulated and filtered. How robust are mosfets generally? How conservative
    are manufaturers when they quote ratings? Should I go for a more expensive
    100V type like IRF50?
  2. Peter,

    I applaud your enthusiasm and I'm assuming the project has a safety slant to
    it besides saving energy. Seriously though, one would think that a few
    dollars spend on such a worthwhile project would not be beyond the budget of
    the big T? Having said that I'm not surprised you are having trouble
    convincing management to come up with the $$$ to fund a small project.

    Several years ago when I was still with the Big T, I developed (in my own
    time) an inexpensive device to interface with a specific piece of test
    equipment and a laptop PC. The Big T has well over 1000 of these test units
    in use in the field and a lot of the field techs had laptops on which to run
    the software the Big T had purchased from the test equipment manufacturer at
    a huge cost. What they couldn't do was get the laptops to talk to the test
    equipment. I offered them a prototype for nothing but management had closed
    eyes and weren't interested at the time. Later I was approached by someone
    within the Big T to supply a few units for evaluation. Well the rest is
    history because since then I've sold them hundreds of units at a nice tidy
    profit. I still get orders for units from time to time. :) Stupid thing is
    they could have had it for nicks. Their loss = my gain :)

    Nothing wrong with using the 60V MOSFET, but the 100V type just gives you
    that extra margin. Not that a drain-source short would be a big deal. It
    would just leave the soldering iron on - and aren't they already that way?

  3. Phil Allison

    Phil Allison Guest

    "Peter Howard"

    ** The 100 volt rated IRF520 or IRF530 would be good choices - only a
    couple of dollars each from Farnell in One.

    Mosfets are sensitive to voltage spikes created by inductive loads so it is
    wise to include a zener across the drain and source pins. In your case, a
    62 volt one like the BZT03-C62.

    ........... Phil
  4. Peter Howard

    Peter Howard Guest

    I'm amused by the war story, especially when it rings so absolutely true. I
    always think that Big T management to workforce communications are like a
    diode. The threats, exhortations and useless bumf flow in one direction
    only. Try to communicate a good idea to higher authority and you can see the
    manager thinking "Will there be any comeback or will my career be affected
    if I choose to ignore this liney?" I find it better to commit a submission
    to writing so it can be binned straight away. Saves time for everyone.
    My timer idea is for safety and also to save wear and tear on the soldering
    irons. Much as I'd like to see it done on a national basis with a properly
    developed custom PC board, the reality is that it will only happen, if at
    all, in our local patch and I'll be flat out getting funds for the parts,
    let alone work time to assemble them.
    P.A. pointed me in the right direction, to Farnells. I was gratified to see
    that the Farnell website and search facility is lots better than it was and
    I found a suitable 100V mosfet for a mere 37c more than the one I can source
    locally. Just to be on the safe side. Only problem with Farnells for me is
    the $10 overnight satchel delivery so I bought up lots of other bits to make
    it worthwhile.
  5. Good to hear you are making some headway with the idea and can source the
    FETs you are after.
    Lots of luck convincing management though.....

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