Thanks for all the advice and the remedial physics course - it's been a while!!
I take on board your comments concerning the relay being a potential single point of failure. However, as I said, I have an AUTO/OFF/ON switch where I can isolate or by-pass the relay so there are 2 ways to connect the standby power to the ESS BUSS. Furthermore, I have a switch breaker between the Avionics and ESS BUSS. Together with the avionics master and the battery master, I can isolate any buss in the event of a dead short. I accept the comments concerning the reliability of relays and maybe I have just tried to be a little too "elegant" with the design, but I have tried the system out and it certainly works as advertised. I am down-route at the moment but will post a diagram when I get home which I hope will show the system better and how it deals with various failure modes.
To the physics! Roee's diagram helped a lot - I was getting there slowly and had already realised it needed to be a parallel circuit. The need for the diode had escaped me. I believe that all that is needed now is a resistor on the positive side of the capacitor circuit to limit the charging current. BUT, won't that drop the voltage to the transponder when main power is removed? So the next question is - would the resistor have the desired effect if placed on the earth side of the capacitor?
CB
|
Diode
|
| ------ GTX327 ------ Earth
|
Capacitor
|
Resistor
|
Earth
Somehow, I don't think that works
I have solved (I think) the maths for the size of the capacitor - amazing what you can do at 3 in the morning when you are jet-lagged and 6000 miles from home!
Max draw of GTX327 is 22W for 20ms = 22x0.02 = 0.44J (say 0.5)
For a capacitor, W=1/2 C V(sq)
So 0.5=0.5xCx14(sq) so C=1/200 or 5mF
However, as the capacitor discharges, the voltage drops. Assuming we wish to maintain at least 10V or about 14x0.7 then the capacitor must be not more than half discharged (0.7(sq)). So I need a 10mF capacitor.
Which is all pretty academic as I am rapidly coming to the conclusion that it will be much simpler just to take the few seconds to switch the transponder back on! Interesting discussion, though
The only final thought I have before heading for breakfast is why does the GNS430 cope with the power transient and the GTX327 doesn't? I would have thought that any certified avionics would have dealt with a short interrupt.