Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Fri, 27 Sep 2002 11:02:12 -0600
NNTP-Posting-Date: 27 Sep 2002 17:02:48 GMT
X-Mailer: Mozilla 4.7 [en] (Win98; I)
> >(Mike wrote):
> >The key to eliminating spike-induced damage to avionics and/or the OVP/VR
> >is having said equipment off line at the instant the spikes are generated,
> >and also by installing "catch" diodes across the starter and master solenoids,
> >as well as having a "transorb" (MOV or Zener) across the main bus...
> Peter wrote:
> Is there a US STC for these devices? Or are any "FAA-certified"? I am
> in the UK and cannot do anything although if there is a STC the UK CAA
> respects that (in trivial mods like this).
Cessna realized that they screwed up by producing aircraft from
the 1950s thru the 1980s with no catch diodes across the master,
starter, and external power solenoid, and they issued Service
Bulletins instructing owners to install a Cessna Part
No xxxxx diode(s) across the coils of the solenoid(s).
As I recall, the diodes turned out to be 1N4007...
My mechanic installed the diode per the service bulletin instructions
and maked a log entry "complied with SBxxxx" in the airfame log.
In the US, an AP mechanic can also make a "minor" mod, like adding a
transorb or a catch diode, and just make a log entry to return the
aircraft to service. My mechanic let me install other spike
catching components, and then just took care of the logs.
The spikes I recorded in my 1968 Skylane (14V) during cranking,
(prior to installing a catch diode across the starter solenoid)
were more than 400 V peak to peak, sort of a highly-damped sine
wave, that lasted about 10msec. My Skylane came out of the factory
with a catch diode on the master solenoid, but not on the aux
power solenoid or the starter solenoid.
It also had a factory-installed normally-closed relay which shut
off all of the avionics while the key switch is held in the
START position. However, the avionics relay reclosed before the
Starter Solenoid released as the key switch is returned to the
BOTH position, making it ineffective in protecting the avionics
from the transient. It did "unload" the battery during cranking,