So after much reading and research into the whole "backup" BEC saga. I think I've found a solution!
My first idea was just to run a 5.5v backup lipo in parallel with the 5v UBEC output.
This idea has been vitoed by most. Too many unknowns apparently.
The other was to parallel all the BECs up, but this would cause harmonics due to all the square wave interaction.
So I figured we cant mix power supplies. Other than using a microproccessor controlled mosfet to switch
power supplies when and if the bec dies.
We use a relay :) Bear with me.
The main power supply powers the relay. With the relay closed (NO) power is routed via the bec to the apm.
If the bec fails, the power to the relay fails and is switched to the auxillary power supply (battery or another bec)
This completely seperates the two power sources.
Will the relay be able to switch fast enough to the auxillary power source without the APM losing power?
If not, im sure a power capacitor will keep the APM alive long enough for the bi-pass surgery to take effect?
Above is a recording of the ciruit just in case my explaining skills are dodgy :)
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Comments
HobbyKing has this. It's a nice start, but 1) 5.9v, not 5; 2) you have to push buttons to set it; 3) shipping weight is 49g, not sure how much the gizmo weighs in at, but probably more than we want to add to small quads..
I'm guessing the APM relay will not be fast enough without a sizable capacitor, so the solid state IC route would be better. Just my $0.02 worth. :-)
Dean, I didn't even know something like that existed, cool! The 4358 looks like it isn't the right voltage to work for us. The LTC4415 however seems to be exactly what we need. And it's dual channel. One problem is it's incredibly tiny. Too small to DIY, I think it's surface mount.
I think something like this with twin linear voltage regulators would be perfect.
Gareth, I can't imagine you'll find a relay that will switch on before 5V, and off at 4.8V. It's just way too tight.
I wanted to try and keep it as simple as possible :)
@Robert: I would put a buzzer on the auxillary line, so if the bec failed a buzzer would go off.
Also, just get a relay that wont function below 4.8v.
Iv thought about using a mosfet. But you dont get double pole mosfets (do you?)
This was just an "analog" idea. Doing it digital would probably work better but would be a shlep...
Using diodes will lower the BEC output voltage by the diode drop voltage (generally 0.4v or more for a low drop diode).
Running BECs in parallel (with BECs that support paralleling) can provide backup power if you have a separate battery on each BEC, but will probably not help if one of the BECs fails.
If you want to implement a very high reliability system, take a look at the Linear LTC4358 or another ideal diode controller (http://cds.linear.com/docs/Datasheet/4358fa.pdf). They switch a mosfet to act like a diode without the forward voltage drop associated with an actual diode. Other ideal diode controllers are available that use an external MOSFET, but the 4358 is probably a better match for BEC or'ing.
http://www.dimensionengineering.com/ParallelRegulators.htm
I'd rather use a semiconductor to do the switching than a relay, but I don't think a FET or transistor will work.
Another thing to think about, how you are going to protect for situations where the primary supply is failing, providing less than 5V, but still providing enough power to hold that relay closed. That would take more circuitry. Again, it might be easier to control this with the APM logic.
there are relays that can switch very fast.
the same type like in UPS devices.
why not just connecting a battery to the BC output with a diode ?
that way the battery charges constantly and if the BEC fails, the battery keeps on feeding the APM
the problem is to find a suitable battery that can be fed 5v non-stop...
maybe 5xAA batteries in series ?
it's impossible with lipo
(5v @ 2 cells = 2.75 per cell)
I think that's a great idea. I've thought about something like that myself. I may just use your idea. Can I suggest adding a green LED to the primary supply, and a red LED to the backup? Then you can see when it has changed over so you know to land.
I wonder if this could even be implemented using the APM relay that is built in. Use the ADC inputs (battery voltage) to know when the primary has failed and changee over to the secondary power supply. Then it can also signal RTL or something like that.
Yes, a large capacitor bank can definitely keep the APM alive during the changeover. I have a capacitor bank from Hobby King (they call it "voltage protector") which can keep the APM alive for about 2 seconds.
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