|
Hi Bob,
Don't know if you don't ask. A diode is a
device that allows current to flow only in one direction (DC circuit.)
Schottky is a particular diode construction. One desirable feature of a
Schottky is that it has a low forward voltage drop. All diodes have a drop
in voltage (like a small resistor.) In this case, the lower voltage drop
allows the injectors to continue firing slightly longer than other diode types
in the event of an alternator failure and lowering battery voltage.
On the 20B, the injector control lines are wired
together, installing the diodes prevents the current / voltage from back feeding
between the primary and secondary circuits and the possible failure modes I
noted earlier. Application of diode(s) in this manner is typically
referred to as isolation diodes.
Since the EC2 controller internal jumps the 2
controller power feeds, I installed isolation diodes (Schottky) to prevent
similar problems between the 2 supply circuits.
You may also find diodes useful in
making annunciator press to test circuits. If you have 2
devices such as a transponder and a GPS that both need to share altitude
encoder inputs, isolation diodes would be used. Of course with any
avionics installations, follow the manufactures guidelines.
Joe
----- Original Message -----
Sent: Sunday, October 14, 2007 10:16
PM
Subject: [FlyRotary] Re: EC2 20B Injector
Power Circuits
Al
Hate
to continue to display my ignorance but what is a Schottky diodes. When
it comes to electronics I’m as dumb as a stump.
Bob
K
From: Rotary motors in
aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Al
Gietzen
Sent: Sunday, October 14, 2007 5:42 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re: EC2 20B Injector Power
Circuits
Joe;
You
are right. We discovered this crossfeed issue some time back. Is
applies only to the 20B version as the 13B has separate drivers for each set.
Install Schottky diodes in the circuits for each injector. Tracy
is aware of this, and has installed the diodes in his as
well.
Al
-----Original
Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On Behalf Of Joe
Ewen
Sent: Sunday, October 14, 2007 3:20 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] EC2 20B Injector Power
Circuits
I have been testing
my wiring harness from the EC2. I found a behavior that I did not
expect. The manual drawing (EC2 manual) show a single injector supply
that splits at the Injector primary and secondary disable switches. On
my installation, I have 2 separate feeder circuits, supply Primary comes from
battery 1, and supply Secondary comes from battery 2. I did this
for redundancy. If I had a failure such as a short to ground in either
circuit, its related overload should trip, but the other circuit should remain
active.
While testing the
injector circuits, I noticed what I would term an abnormality. With both
injector disable switches in the enable position, I turned on the power for
the primary circuit (I have LED indicators on all circuits that annunciate
power present) and the secondary circuit also showed power present even though
the power switch for the secondary circuit was off. The same is true
when the secondary circuit is engaged and the primary is off.
Initially this led
me to think I had miss wired something in the circuit. After checking
out the wiring, I found no issues. So I took a closer look at the wiring
diagram in the manual. It turns out the power was being back feed
through the injectors to the opposite circuit. On the 20B version of the
EC2, the injector control lines for each rotors injectors are junctioned going
into the EC2. For example the Rotor 1 Primary and Secondary control
lines are junctioned into a single input point at the EC2.
I can see several
situations where this may lead to undesirable operation. For example, if
the Primary circuit had a hard short to ground, it would take out the primary
circuit's overload (CB or Fuse.) Since the primary and secondary
circuits are electrically connected on the control line, the other circuit
would also trip. Another situation that could occur is a medium to high
resistance short. Current would then flow uncontrolled through the
unaffected circuit's injector to the affected circuit's injector then to the
resistance short. This could keep both injectors on 100%. While
this situation is unlikely, it seems
possible.
I believe the
13B version uses separate control lines for each injector, so this in not
likely to be an issue on the 13B version.
It appears that
isolation diodes are needed to prevent these possible problems. Before
making any changes I have to look the circuit over more closely, as well as a
call to Tracy on the next service day. My question to the group,
especially anyone with a 20B, is am I looking at this
wrong?
|