----- Original Message -----
From: "Mark Burton" <markb@ordern.com>
Hello Mark,
Thank you very much for going to the trouble of logically going through the
different failure modes with explanations that go a long way to helping
those with only basic electronic knowledge like myself.
In the main I am confident I understand what you have explained but there
are a couple of points where I am not so sure so I would be much obliged if
you could see fit to clarify my queries below please.
You said:-
> Assuming that the switch has operated as it should, the pitch will now be
> fully fine. When the controller next commands the pitch to go coarse, the
> switch is already open so the diode will have to carry the current. That
> current can be large as it's basically the pitch motor stall current
> (initially around 25-30A, falling quickly as the motor spins up). Now the
> diode is only rated for 5A continuous current but it has a good pulse
> capability (check the data sheet) so it probably won't fail (we will talk
> about what happens if the diode fails next) - assuming the diode did not
> fail and the pitch motor is actually moving, the switch will close shortly
> afterwards. Exactly how much current will flow through the switch is
> difficult to predict because we don't know how much the motor has spun up
> so it could be quite a lot - mind you, there's only around 1V across the
> switch (the diode drop) so I should think the switch could hack it OK
> (perhaps with a reduced life).
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?
Secondly . . . .
> Now, let's consider some failure modes:
> 2 - the diode fails short circuit - in this case when the switch opens as
> the prop reaches fully fine, the pitch motor keeps going and will drive
> the pitch all the way to reverse position (and the reverse switch would
> operate). If the controller subsequently tried to coarsen the pitch, it
> would succeed unless the diode decided the current was too much and died
> in which case the pitch would freeze. Now as it has been reported that the
> pitch of the prop was not in the reverse position this probably didn't
> happen. Again, the C/B would not blow.
In this case the diode fails short circuit . . . . . later you say that the
controller would succeed when trying to coarsen the pitch "unless the diode
decided the current was too much and died"
In this case, I would have considered the diode to have already died when it
short circuited . . . . . am I correct in my understanding that what you
are saying is that given enough current, the already shorted diode may then
go open circuit causing the resultant "freeze" ? OR is the reference to
the diode dying (dying open circuit) another diode across the reverse switch
?
Sorry if I am a bit thick Mark but I'd rather not understand at all than
only half understand hence my plea.
Best regards
Kingsley in Oz.
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