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Linear or Switching BECs

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  • Linear or Switching BECs

    I know nothing about BECs, so was intrigued when Al Nucci asked what type our BEC was.
    Subculture piped up with this response and I thought it was worth sharing.

    A switched mode supply basically works by switching an inductor coil in
    and out very quickly (usually about 400 times a second). They can be
    configured to drop or increase voltage, called buck and boost respectively.

    A magnetic field is induced in the inductor when it is doing this, and
    it is that which makes them electrically 'dirty'.

    Best quality units will have a grounded metal shield around them, steel
    being best, but aluminium works and is lighter. That deals with the
    inductor on the unit. Also a ferrite is usually wound around the output
    lead to nix any nasty frequencies induced into the wiring by the conversion.

    A simple analogy would be to imagine you're watering the lawn with a
    hosepipe. The water pressure is too high, and you can't adjust with the
    tap, so what could you do? Well you could add a bypass, and shunt a bit
    of the water off down the drain, that's your linear BEC. Or you could
    get some one to squeeze the hose on and off with their foot, that's the
    switched mode. The latter mode doesn't waste any water, although energy
    is expended by the person doing the squeezing- hence switched mode
    supplies are never 100% efficient.

    Linear converters don't create any dirty fields, but they generate a lot
    of heat when the voltage differential is high and the current demands
    are great.

    For example. Lets say we have a model submarine operating off 12 volts,
    and we want our usual 5 volt feed to the radio kit. Let's also assume
    the current required is 1A, which is adequate for two or three servos if
    they're not stalling, or thirsty models.

    To see how much power is wasted in heat, we subtract 5 volts from 12
    volts, which is 7 volts, and multiply that with the current, which gives
    us 7 watts of power wasted in heat. That's 140% greater than the power
    being delivered. Can you say inefficient? Plus you'll need a sizeable
    heatsink to get rid of the heat if the regulator is to survive.

    If the model was operating on a lower voltage, say 7.2 volts, then
    things become much more benign. 7.2-5= 2.2, times that by the current
    equals 2.2watts, under a third of the power wasted. Hence why linear
    regulators work best in low voltage boats.

    An equivalent switch mode supply will only use about half a watt.
    Stop messing about - just get a Sub-driver!

  • #2
    A good indication of whether a BEC is switched or not is to look at the voltage range. If it's very wide, and can work at a figure well outside of the regulated voltage, that's a pretty solid indication it's a switched BEC, because linear BEC's cannot supply a lot of current at voltages far above their regulated feed, because of the heat issue.
    DIVE IN! Go on, go on, go on, go on, GO ON! http://www.diveintomodelsubmarines.co.uk

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