Message text written by "LTS"
>Circuit breakers and fuses behave quite differently. For that reason I
always use fuses on the basis they don't blow without reason so shouldn't
need replacing in flight and when something is wrong they blow far quicker
and near their specified value giving the protection you think you have.
Conversely circuit breakers release very slowly and at much higher values
than might be expected. (Fuses are also light and cheap - can't believe
cars
would have fuses if circuit breakers were better).<
I generally agree with Jerry's comment above. From William's experience
this is perhaps one case where it is better to use a c/b. Should the
crowbar unit operate it will have no problem operating either fuse or c/b
as it intentionally shorts to ground. I am a little concerned why the fuse
blew in the first place. William has checked his crowbar unit and it
doesn't operate until 16.5v is reached. I assume that unless the fuse
failed without the crowbar unit operating there was a transient regulator
problem .
As can be seen from this case, on its own the capacitor may not survive for
very long. If the regulator did malfunction temporarily the high voltages
produced could well have been high enough to cause the capacitor to fail.
If this was the case this might be the first crowbar unit to operate in a
Europa. Fred's comment suggesting a higher rated capacitor is a good one. A
35v rated capacitor may well survive a regulator malfunction. In this case
I don't think this is the issue for two reasons. Firstly the problem has
reoccurred with a new capacitor. Secondly, although these capacitors might
be operating close to their limit there hasn't been reports of others
failing which infers that they are generally up to the task.
If a crowbar unit is not fitted, this type of failure will damage the
battery and could result in total electrical failure. I would suggest this
is particularly relevant to any aircraft using just electrical fuel pumps
(eg Rotax 914 and Subaru powered Europas).
---From William's latest fuse failure I would suspect a regulator fault. With
care by fitting a c/b, inflight (or ground running) analysis can be
achieved by monitoring system voltage closely. Digital voltage indications
will help spot any small trends. However a short duration spike that cannot
be controlled by the capacitor might trip the c/b without a voltage rise
being observed. The other possibility is of course the crowbar unit. This
is less likely also for two reasons. Firstly Bob Nuckolls ( aircraft
electrics expert) who developed this product reports that these units ar
very reliable. I would support this having set one up to operate at 5 times
a second for over an hour during a presentation. Secondly the fact that the
capacitor failed suggests that the regulator output voltage increased to or
above the capacitor's limit (25v if my memory is correct).
Nigel Charles
|