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1. 08:46 PM - Protective Regulator Wiring for Rotax 912ULS Installation (David Bloomfield)
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| Subject: | Protective Regulator Wiring for Rotax 912ULS Installation |
I'm at the stage of refitting the whole aircraft with iLevel EFIS / Auto
Pilot, GRT Engine Monitoring and a UAvionix AV-30 / TailBeacon
Transponder. My aircraft has a PowerMate regulator (a discontinued
model with no specifications at hand).
There is conflicting advice online about the wiring of non Rotax voltage
regulators for Rotax 912 ULS. I have yet to look into
available alternatives to the PowerMate I currently have. Regardless there
is a concern about suitably protecting these new avionics. I have drafted
a simple mod, as a precautionary facility and catch all solution, and I am
open to informed engineering comment on it.
If the Master Switch is turned OFF while engine is running, it leads
to a *No-Load
*condition for the generator & regulator. Subject to the product make,
type (and whether there is inherent built in protection for such an event),
this could result in high voltage and/or current spikes onto the DC power
bus and its connected avionics. I recall reading that some have acuurately
measured this and found to be true. Furthermore if a Filtering Capacitor
is fitted (for DC smoothing or AC noise suppression for VHF radio), it may
be left in a charged state or could even be overloaded with voltages above
its rating, leading to a potential capacitor fail, possibly with a lot of
smoke.
>From what I read, most say that Regulator Terminals B+, R and C are all
connected to the load side of the master switch (or master relay), so the
battery is totally isolated and cant drain back through either of the
regulator terminals. Other schematics show the *B+* permanently connected
to the battery, and only the *C *& *R terminals *are switched off.
But there are other comments about the high potential for damage if the
Master switch is turned OFF before engine is stopped, due to over-voltage
and large current spikes via the generator and regulator still producing
power.
In the attached schematic, The Master switch (negative switching) drives
a (SPST) relay (firewall side) whereby the Load (12V Bus and Regulator) is
connected to the COMMON terminal, the N.O. terminal to Battery + and the
N.C. terminal to GND via a Resistor (Ceramic 100 Ohm, 10W) .
The theory is that it effectively provides a "Dummy Load" for the
Regulator/Generator, and also a resitive load to discharge the Capacitor,
supressing spikes into the Avionics and Pwr Bus. It's not 100% proof as
there is still a tiny time frame (microseconds) of open-circuit when the
Master relay contacts switch from Battery to Resistor Load. Also shoud
the relay fuse and short the contacts then the worst case is a .8 Amp /10
Watt Resistive load dead across the battery , which will drain it slowly if
left unattended. A warning light can be paralled across the resistor to
indicate this condition.
I have yet to wire up and test the isea so are welcoming commentary.
*David Bloomfield B.E. *
*Sydney, Australia*e: bloomfield.dh@gmail.com
mob: +61 414775225
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