On 06/29/2011 12:51 PM, Kingsley Hurst wrote:
> As I understand it, the 1V across the switch would only be present while
> the switch is still open and on the closing of the switch, is it correct
> to say that ALL of the current (whatever that might be) would then flow
> through the switch (assuming closed switch has minute or zero
> resistance) notwithstanding the diode is still forward biased?
That is also my thought.
But let me explain a bit about switches and AC and DC currents.
It is often said that for DC applications the switches need to have a
higher current rating than for AC applications.
The reason is this: If we look in slow motion to the process of opening
a switch, a small arc will form just at the moment of opening. With AC,
the arc extinguishes automatically when the current goes through the
null when alternating. With DC, the arc will continue to exist until
either the contacts are spaced out far enough, or when the source of the
DC is taken away.
While the contacts are closed, or during closing, the current rating is
the same for AC and DC. It is the opening of the contacts which is the
limiting factor for DC applications.
So, in the scenario Mark is describing, excessive current over the
offical DC rating of the switch has no consequences.
Furthermore, in the application of Woodcomp propellers with either
feathering or resversing options, it isn't actually DC which is fed to
the controller. The controller uses PWM (Pulse Width Modulation) to
control the prop, and it is the ratio between the pulses and pauses that
determines how fast the pitch changes. This pulsing DC behaves, as far
as the micro-switch is concerned, exactly as AC, because the DC is
interrupted periodically and any arc when opening will extinguish in a
few milli-seconds.
So, in this application with PWM you can use the micro-switches to their
max AC-current rating. The lower max DC-rating doesn't apply here.
And finally, I like to restate that the controller and prop form
together a RPM-controlled closed loop, and the limiter micro-switches
will never open as the controller will cut the power to the prop when
the target RPM has been established. Only in faulty setups the
micro-switches will open. Such a fault could be a setup error, like a
too powerfull engine which overrevs even with the prop fully coarse, and
the controller will subsequently try to move the pitch coarser than the
design limit of the prop. Another failure mode would be a faulty
controller, i.e. a controller that keeps feeding power to the prop after
the target RPM has been reached. A faulty micro-switch will consequently
only reveal itself when something else goes wrong in addition.
> In this case, I would have considered the diode to have already died
> when it short circuited . . . .
There are multiple ways of dying... I'm pretty sure that Mark meant that
after short circuiting the diode might blow like a fuse and go open circuit.
Frans
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