Page 8
PPS Installation and Operating Manual
Rev. C.2 (January 19, 2018)
3.6 Switch Nomenclature
The chart below shows the most common switch types. A parenthesis
() around a switch position indicates it is a momentary, spring-loaded
position.
Switch Type
Designation
Symbol
Mechanism
Single Pole,
Single Throw
SPST
OFF-ON
OFF-(ON)
Single Pole,
Double Throw
SPDT
ON-NONE-ON
ON-OFF-ON
(ON)-OFF-(ON)
ON-OFF-(ON)
Double Pole,
Single Throw
DPST
OFF-ON
OFF-(ON)
Double Pole,
Double Throw
DPDT
ON-NONE-ON
ON-OFF-ON
(ON)-OFF-(ON)
ON-OFF-(ON)
PPS Control input pins
3.7 Current Sensing (Shunt)
When planning your aircraft electrical wiring you must consider whether
to wire an ammeter (usually a shunt or hall effect sensor provided with
the engine monitor) on the wire connecting the alternator(s) to the
main bus. The ammeters indicate the amount of current the alternator is
providing.
A shunt is not required to tell if the alternator is working. It is very easy
to tell if the alternator is working correctly by simply looking at voltage.
If you see 14 (or so) volts with the engine running then it is working. If
you see 12 (or so) volts it is not working or not turned on or the devices
are drawing more current than the alternator can provide (note, engine
must be running). If you set your low voltage alarm on the EFIS at 13
volts, then you will get a low voltage alarm if the alternator fails.
The PPS provides the alternator current (shunt) output, which shows
the amount of current the alternator is providing to power the devices
and charge the battery. The VP-X total current reading shows the total
amount of current the devices attached to the VP-X are using. The delta
between the two is the battery charging current, which goes to zero after
re-charging any loss from starting the engine or charging a run-down
battery.
If the battery charging current is important to you, the PPS also provides
a battery current (shunt) output as well. If not, then simplify your wiring
and don’t utilize this PPS output. Your call. And of course each builder’s
needs are different so there is no absolutely right answer.
The EFIS displays a VP-X page which shows individual device current
as well as total current through the VP-X. The EFIS also has an ‘Amps’
gauge that is used to show the readings from the shunt. In some cases the
EFIS ‘Amps’ gauge can be used to display total system current from the
VP-X. Please check with your EFIS manufacturer for details.
If you don’t use the PPS current outputs then the shunt wires on the
engine monitor/ EFIS are not used.
