How would you wire the external power in a modern full glass cockpit airplane?
What wiring is ‘safe’ for the battery & avionics?
The idea is to use the External Power to:
-start the engine in case the onboard batt is depleted
-power avionics and el devices on ground for maintenance & annuals
-charge the onboard batt if needed
Thanks!
Supik
Is this for adding an external power connector for a certified aircraft, or a homebuilt one?
There are various approaches to it. In most recent (say 1970s onwards) designs, when you plug in the external power, a relay opens which disconnects the battery. This prevents the battery being charged from the external power source.
supik wrote:
How would you wire the external power in a modern full glass cockpit airplane?You can get some inspiration from this schematic of (part of) the G1000 Cessna 172S electrical system. It uses external power for all the purposes you mention.
What wiring is ‘safe’ for the battery & avionics?The idea is to use the External Power to:
-start the engine in case the onboard batt is depleted
-power avionics and el devices on ground for maintenance & annuals
-charge the onboard batt if needed
In most cases connecting a live ground power plug will disconnect the battery ( whatever the master switch position ) and connect the ground power to the battery bus.
(Stand by for examples of aircraft that don’t have this function ).
A_and_C wrote:
Stand by for examples of aircraft that don’t have this functionThe C172S as evidenced by the schematic I posted above. :-)
It’s a homebuilt. I am asking exactly because some of the well established TC aircraft go one path and the others go different..
Is this assumption correct?
Option 1: battery stays connected:
Protects avionics to some degree as the batt serves as a ‘capacitor’, but on the other hand can damage the battery if it was depleted.
Option 2: disconnects the battery:
No protection to avionics -but safe approach for the battery.
Battery cannot be charged.
Risk of ‘welded’ EXT Power relay
Looking at the Cirrus SR22 wiring diagram, the ground pwr relay does not disconnect the battery either:
The TB20 system is described here and the weakness (which Socata failed to address) is that the starter motor current flows through the normally closed contact of the ground power relay, and normally the N/C contact is not able to carry much current because it is held closed by a spring, not by electromagnetism which the N/O contact is closed with.
As the article shows, this aspect is hard to solve because a relay big enough to carry say 200A with its N.C contact is a big relay…
The other important feature is a relay which disconnects the avionics when the starter motor is energised. This is to prevent avionics damage due to the large spikes produced by starter motor inductance. There have been numerous cases of somebody blowing their avionics when they didn’t have this feature, and didn’t follow the checklist for starting. The more recent TB aircraft have this. Some avionics are claimed to be ok to be connected to the bus regardless but I would not trust it. One recent case I saw was the control unit for the TKS system which absolutely is not capable of such, having two 78xx voltage regulators connected directly to the power input and these absolutely will blow up at around 50V. So an installation which is compliant with the STC in this area is dumb. A better way is to wire it to an avionics bus (which is protected from starter motor spikes) and perhaps have a switch which can bypass the avionics relay(s) to feed power to the TKS control unit directly (because TKS failure in icing conditions is very serious).
Peter, my homebuilt is a Vans RV10, no TKS. The avionics is controlled by two 20amp switches respectively. The avionics relay disconnect during engine start is a nice and safe feature! Maybe I’ll implement it.
I am concerned about the battery -how much harm can be caused to the battery if the ground pwr relay DOESN’T disconnect the batt?
Not much… An external power pack (the red boxes in common use) is just a lead acid battery or few, so it outputs just 12/24V, not 14/28V which is what a proper ground power unit would output and what is needed to charge a battery. The only way 12V will damage a 12V battery is if the latter battery is shagged (say down to 2V) in which case it will draw a heavy current which may damage it. But if your batter is down to say 10V then 12V will not damage it. You will need to have a procedure whereby if the voltage is really low, you somehow charge it with a limited current (say 2A) until it is up to say 10V.
My TKS example was only to show that there is stuff out there which is “ok” to connect directly to the unswitched bus but actually isn’t, and if it doesn’t get blown up it is due to luck. I have designed electronics to withstand the spikes (there is an ISO spec for automotive electronics) and it isn’t trivial if the equipment draws a bit of power.
Also see this.
