jpx(at)qenesis.com
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 Posted: Thu Mar 11, 2010 1:00 pm Post subject: AeroElectric List: Low Voltage Module |
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Joe,
Wow - sharp eyes. I have modified the Power diagram to move the wire
from the Essential Bus Relay to directly feed the Essential Bus. No
use having independent paths that are not independent ! Thanks Joe !
Yes, I did design the Low Voltage Module myself. I decided there
wasn't room on my panel for two ammeters that I was rarely going to
look at anyway and that a low voltage light would tell me (and my
wife, who is also a pilot) everything we need to know in flight. I
expect a I can connect a shunt to the Dynon and see what the load
current is if need be.
I wanted something that monitored both charging systems.
I wanted something that the indicator lights blinked, making them
easier to notice.
I graduated many years ago as an Electrical Technologist, although I
have spent most of my career in IT.
There are a lot of fancy new customizable chips that could probably do
this entire circuit, but I don't know anything about them, so I used
old components I am familiar with.
Did you really want an explanation of how the circuit works ? If not,
skip to the next message in this forum.
Keep in mind as you read this, that I have drawn the schematic, but
not breadboarded this circuit, so it may require changes to function
as intended.
Diodes D1-D4 bring the battery voltages into the module, while keeping
them isolated. Any one of D5 to D8 provide ground to the module to
turn it on. This means I will be reminded to turn off any switch that
would allow a battery to discharge if I accidentally leave it on.
D2 and D3 take power from whichever battery has the highest voltage to
power the module. I chose to run the electronics at 5 volts, so the
module will complain about low voltage at battery voltages far below
what a functioning battery will produce. U1 is a transistor-sized
voltage regulator needing no external components to set its voltage.
R1 and R2 form a voltage divider to create a 2.0 volt reference, which
is used by all the comparators.
D4 feeds the voltage divider for the primary battery/alternator.
R3 and R4 could have been a single resistor, but I split the value.
This makes it easier to make a small change to tweak the exact voltage
for the comparator. I think fixed resistors are more reliable than
adjustable ones. Also, by putting the C4 between R3 and R4, and
voltage variations that still occur will be divided by two before
being seen by the comparator.
R6 provides positive feedback for hysteresis. So the comparator
functions like this. Assume all is well with the alternator, so the
battery bus is say 14 volts. The voltage at the +ve comparator input
will be above the reference of 2.0V. The open collector transistor
output of the comparator is off. So R7 pulls the output up toward 5V.
It won't get there, since a voltage divider is created with R15,
which is turning on Q1, which holds Q2 off and there is no current
through the indicator LED.
Now an alternator failure. As the voltage on the battery bus
decreases to 13.0V, the +ve comparator input decreases and as soon as
it decreases barely below the 2.0V reference, the comparator flips and
turns on its output transistor, taking its output to ground. This
changes the direction of current through the feedback resistor R6,
changing the voltage divider, which now presents 1.85 volts to the +ve
input, keeping the comparator from flipping back. The bus voltage
must rise to 13.8 volts before the +ve input reaches 2.0V.
Q1 is now off, which means Q2 is driven by U3, which is a timer chip.
R8, R9 and C6 form a voltage divider. When the power is first applied
to the circuit, there is no voltage across C6, so the Trigger input is
low, producing a high Output. C6 charges, until the Threshold voltage
is reached, approximately 2/3 of 5V. The Output changes to low and
internally, an open collector transistor pulls the Discharge pin to
ground. C6 discharges through R9 into the Discharge pin. When the
voltage at the Trigger pin reaches approximately 1/3 of 5V, the output
goes high, Discharge open circuits and the charging cycle repeats.
C6 charges through R8 and R9, but discharges through only R9, so
charging takes longer. This means the LED will be blinking, but will
be on about 80% of the time. I figured the blinking would attract
attention, but having it mostly on means that a quick glance is most
likely to find the LED lit. Decreasing R8 and R9 will increase the
blinking frequency. Changing the ratio of R8 to R9 will alter the
duty cycle.
D1 feeds a voltage divider for an identical comparator circuit for the
Aux battery/alternator. Since the blinking circuit is common to both,
if both indicators are on, they will blink in unison.
The third comparator U2-C and the second voltage regulator U4 are to
control the brightness of the indicators. I find indicators bright
enough to be seen daytime to be very annoying at night. Manual
control to dim them could mean that I might not see a dimmed indicator
in sunlight. So the solution is to automatically dim at night.
R10, R11 and R12 form a voltage divider, which is compared to the same
2.0V reference. R11 is a resistor that changes it's resistance in the
presence of light. The one I have has not been manufactured for a
very long time. I will need to find an alternative part and tweak the
voltage divider accordingly. The current values are chosen so that
during daytime the voltage at the +ve comparator input causes the
output to be high.
U4 is an adjustable voltage regulator the size of a transistor. It
has no ground reference, but adjusts its output voltage so that the
voltage across R17 is always 1.2V. So during the daytime, the voltage
divider of R17 and R18 causes the regulator output to be 7.7V,
producing 23mA through the LED, making it quite bright. At night, the
output of the comparator is low, including R16 in parallel with R18,
changing the voltage divider so that the regulator output is 5.3V,
producing 12mA through the LED, so it glows dimly. Depending on the
LED, R18 and R16 can be changed to set the desired brightness.
Since a 14 pin dip includes four comparators, I had one left over. I
thought perhaps the day/night auto dimming might be useful for other
instrument lights. So U2-D just duplicates U2-C. To guarantee
isolation between the Primary and Aux busses, I used an opto-isolator
U6 to provide an open collector output that can be connected to
another circuit someday.
The capacitors and voltage regulators should make this circuit
relatively immune to voltage variations and noise on the busses.
The circuit as shown may be damaged by a high voltage spike. A change
I made since posting the original diagram is to add 20V zener diodes
in parallel with C4 and C9, which will protect the comparator inputs.
However, the maximum ratings for the regulators is 30 to 40 V. So I
need something to protect them. Does anyone have any recommendations ?
Jeff Page
Dream Aircraft Tundra #10
| Quote: | Jeff,
If wire 2223 or its connections fail, power will be lost to the
essential bus.
Wire 2222 should be connected directly to the E-Bus.
Now I see what those 4 wires are for that connect between the low voltage
module and grounded switches. Any one of the switches can enable the low
voltage module. The diodes isolate the 4 switches from each other. Did you
design the low voltage warning module yourself? Are you an electrical
engineer? Let us know how it works.
Joe
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