Power Interrupt

[paul330]

I have a ESS BUSS on my electrical system with a second feed directly from the battery. If there is power on the Avionics Buss, a relay feeds power from there. If not, it flips and takes power from the direct feed. All works as advertised.

However, the relay switching time is specced at 20ms. The GNS430 copes with this. The Dynon Skyview didn't but now does with the back-up battery connected. The transponder (GTX328) trips off. It only takes a couple of seconds to reset it but with other things potentially going on, I'd rather it wasn't necessary.

Can I prevent this by using some sort of capacitor or something?

[airguy]

Yes, but caution is advised.

You're trying to cover that 20ms gap with a capacitor to smooth the transition, and it will certainly do that just fine with no heartburn. And when the engine is shut down and the master turned off, that capacitor will slowly leak down to zero volts and again creates no heartburn.

Now the next day you jump into the airplane and hit the master - and the battery tries to immediately handle that large load on the capacitor by feeding it some 100-150 amps for 20-30ms to charge it - and in the process it will either trip a breaker or weld the contacts on your master switch - there's your heartburn.

What you would need to do is determine the total current draw that the capacitor would need to deliver during the 20ms window, and then determine the lowest acceptable voltage to be used by the most finicky piece of equipment during that 20ms window, and choose a resistor of the correct size (both wattage and ohms) to place in series with the capacitor to limit the current by producing a voltage drop across it according to Ohms Law. That will ensure that the capacitor charges and discharges at no more than a certain maximum amp rate, and then you will need to put a breaker in your panel for about 150% of that rate, and pass the current loop on the capacitor through that breaker.

[PaigeHoffart]

If I understand you correctly, you have a relay with coil and NO contact connected to the avionics buss. The NC contact is connected to a battery feed, and the common terminal is connected to the essential buss.

If you put a capacitor on the essential buss, it will be charged by the avionics buss through the relay. When you remove power from the avionics buss, the capacitor will discharge through the relay and avionics buss, to the rest of the electrical system until the relay hits its dropout voltage (which is probably a lot lower than any of your avionics can tolerate), then the essential buss will switch to the battery feed.

Replacing the relay with a pair of schottky rectifiers will give you the automatic switching you are looking for, but you will sacrifice about a half a volt.

My .02,
Paige

[cfiidon]

Personally, I'd accept it like it is before I'd add more complexity to the system.

A transponder is one of the things I would consider shutting down if I was only on battery power, depending on the situation.

Even in IMC it's not that big of a deal. Transponders fail all the time.

There's my two pennies.

Don

[rocketbob]

Correct me if I'm wrong but the relay sounds like it could be a single point of failure.

[paul330]

Originally Posted by rocketbob

Correct me if I'm wrong but the relay sounds like it could be a single point of failure.

..... except that the system is wired via an AUTO/OFF/ON switch whereby I can manually switch power to the standby route in the event of the relay failing in some intermediate position Aeroelectric Connection circuits involved using a diode to prevent back flow down the standby section of the circuit. I just had a philisophical objection to having 2 sources permanently connected to the ESS BUSS.

I have a standby 20A alternator which can power the ESS BUSS indefinitely so the power drain of the transponder is not an issue. I was just trying to find a way to smooth out the voltage drop so as to keep it on line for 20ms. I seemed to remember from my school physics that a small capacitor might do that. The idea would be to run it in series from the transponder CB on the ESS BUSS to the transponder only. Everything else copes.

<Edited out series diagram - a bit more research makes me think it needs to be in parallel - Wikipedia is a wonderful thing!!>

If it comes under the "too hard" category then I shall live with having to reset the GTX327........

[roee]

Paul,

Time to review capacitors again in your physics book

If you connect a capacitor in series with your transponder circuit as your diagram shows, your transponder will not work, ever. A capacitor acts as an open circuit to DC. Your transponder will not receive power even when the ESS BUSS is powered.

I think what you meant is to connect a capacitor in parallel with the transponder circuit, i.e. positive terminal on the power side and negative terminal to ground. In this configuration, it could supply power to sustain the transponder through the transition. However,

1) A "small" capacitor may or may not be sufficient. The transponder is a substantial load. You'll have to do the math to figure out the minimum capacitance needed to sustain it through the 20 ms dropout.

2) If it ends up being a sizable capacitor, you may have to contend with inrush current issues, as Greg alluded to.

3) Without further provisions, the capacitor will not only feed the transponder, but all other loads on your ESS BUSS as well. Current will travel just as easily "backwards" through the transponder's CB and back down the other CB's to their respective loads. More loads sucking down that capacitor will mean you'll need an even bigger capacitor to bridge the 20 ms dropout.

If you want to have the capacitor only sustain the transponder (issue #3), then you would need to also add a diode to prevent current from flowing from the cap back to the ESS BUSS. Something like this:

ESS BUSS ----- CB ------|>|---------------------
                       diode      |            |
                                __|__+         |+
                            cap _____       GTX327
                                  |  -         |-
                                  |            |
GND---------------------------------------------

Note that the circuit above only addresses issue #3. Issues #1 and #2 still have to be addressed.

On a "philosophical" note , diodes, and solid state devices in general, are highly reliable. Switches and relays, and electromechanical devices in general, are notoriously unreliable. If I were you, I'd reconsider the architecture and eliminate that relay. FWIW.

-Roee

[penguin]

Glider pilots use a 22000 microfarad capacitor to hold up the avionics (typically a logger and nav system) while switching batteries. This is about 1" dia x 4" long.

However, Paul, are you sure you have the system wired in the best way? The whole point in Bob's architectures is to eliminate single points of failure.

Pete

[fehdxl]

I'm no EE, but when I googled schottky rectifiers, this is what came up:

"Schottky rectifiers...carries advantages such as very low forward voltage drop and switching speeds that approach zero time...ideal for output stages of switching power supplies."

Sure sounds like something that would be good to transfer power between a main bus and a standby bus.

Interested to learn what you decide to do.

-Jim

[paul330]

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.