Everything thing your friend says makes sense, except when you design
important aircraft systems, you provide redundancy or fail-safe,
unless the probability of failure is extremely remote. True, the
ultimate fail-safe for trim systems is the flyability in full
out-of-trim, but accidents are usually event chains, "no one of
which...."
This argues against pulse width modulation, which takes a dozen or two
components to achieve (any one of which...), or a carbon pot running
close to its puny 1/4-watt power rating. In either case, you do want
a bypass switch for fail-safe, but that means another panel switch, to
remember what it does in the event of a problem in flight.
The best approach I believe is a 3-terminal series regulator IC
(LM7808T, for 8 volts, or other 78XXT to one's preference for trim
response). It's probability of failure in a low-current,
intermittent-duty application should be very remote. Some may desire
a panel-adjustable response setup (then use an LM317T IC, with the
adjustment pot doing milliamp duty), but from a pilot's viewpoint, I'd
argue that it's best to have trim respond predictably, so that in a
situation requiring attention elsewhere, trim adjustments can be done
without thinking about, or adjusting, the response setting.
Regards,
Fred F., A063
Miles McCallum wrote:
> ....
> I asked a friend about this (electronics engineer) and his suggestion
> (if you are paranoid) is something along the lines of a "pulse width
> modulator" - infinitely variable, and doesn't drop the voltage (so full
> power regardless of speed) - does it by turning the motor off a greater
> or lesser degree per unit of time.
>
> he also sez:
> looks like they are just reducing the voltage in the old Joe Lucas way by
> putting big power Zeners across the driver supply. Not sure if the drive
> circuit would like to see the effective low resistance short that a Zener
> presents when the supply voltage exceeds the Zener voltage. It may be safer
> and better to use a potentiometer or a power transistor to do this.
> Not sure why your man dislikes carbon-track pots - these are the norm these
> days except for power devices.
>
> So, as I read it, he reckons there is nothing wrong with simply
> dropping the voltage with an appropriate resistor or running it through
> a pot... with the trim system switched off/slow/full(bypass)
>
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