Dual redundant BEC + overvoltage protection (crowbar circuit) to power the flight controller?

or ?

Hi, when i fly multirotors one of my main worries is to avoid it from crashing. Mechanically I've practically solved it because I usually fly an octocopter (motor, prop and ESC redundancy). My main concern right now is how to power the flight controller safely.

Initially I've connect all my 8 linear becs (in parallel) to my flight controller because I thought that if one fails, the rest would supply the required power. Later on I found out that a linear regulator may fail and may supply voltages higher than 5v and that would burn the flight controller.

A fuse is the easiest approach to solve this but I've read that sometimes it doesn't work well (fast enough). So i found the Crowbar Circuit.

So, i would use 2 linear BECs for redundancy and the crowbar circuit after each of the becs so in case it fails it will not burn the flight controller:

Another option:

I had the idea to use a switching BEC but i don't know if it's the best alternative. I've read that you cannot put 2 switching BECs in parallel because the voltage is generated in waves that may interference each other. So to solve this, one switching bec would be in use and if it fails, the backup switching BEC would take place.

The diagram for this is shown below:

If the SBEC number 1 fails, the Schottky diode would alternate to the SBEC number 2. I think that using 2 diodes would be better for the redundancy (i don't know if it's ok to do that). Could I use a relay instead of this diode?

What do you guys think? Will it work? Any idea is very welcome! ;D

Does anybody have the part list to do a crowbar circuit for the APM? I think it should supply 5v and 500mA, is that right?

Best regards

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Comments

Bernardo M. A. October 28, 2013 at 7:09pm

Thanks for the great explanation Thomas and Phil ;)

eduardo, thanks for the tip. I´ll use this SBEC. May be I´ll use together with a schottky diode to have some redundancy, i still don´t know.
eduardo October 27, 2013 at 8:22pm

Bernardo, this SBEC is verygood.

I am using some of this parts in my lanes.

Sometime i modify some resistors to change the output voltage.

Take care about the ESC BEC´s.

If you connect all ESC BEC in parallel without equalization network you will have problem.

One bec will work very hot and it can cook itself.

When this occur the input voltage will pass direct to output and across the double-diode burning you AP.

Some time a good designed AP is much better than some redundancy.
Admin

Thomas J Coyle III October 27, 2013 at 7:16pm

@Bernardo,

The transistor(s) inside the 7805 are called "pass" transistors and are used to regulate the current from the 7805 input to its output which in turn maintains a constant 7805 output voltage of +5vdc.

It is a linear regulator because it dissipates the voltage difference from its input to its +5vdc output as heat. Think of it as a variable resistor that instantly changes resistance to maintain a +5vdc output no matter what the input voltage is or current that is required by the circuit is driving.

Regards,

TCIII ArduRover2 Developer
Phil P October 27, 2013 at 7:12pm

Bernardo, the transistor is internal to the 7805, but its still there!
Bernardo M. A. October 27, 2013 at 7:08pm

Phil P, thanks for the explanation ;) But isn´t the 7805 a linear regulator (no transistor)?

eduardo, that´s nice to know that =)

Could you recommend a good regulator?

I bought the following SBEC, do you think that´s a good option? There are lots of goods comments

http://www.hobbyking.com/hobbyking/store/__10313__Turnigy_5A_8_40v_...

What about using this SBEC for the main powersupply and the BECs from the ESCs as the auxiliary powersupply? Schottky diode would alternate between them.
eduardo October 27, 2013 at 1:08pm

Bernardo, i realy dont know way you are worried to used this kind of redundancy in rotor crafts.

i fly rotorcraft for years (more than 5 year) i never had problem with controller power supply.

Its more easy that you get a problem in speed controller / motor / propeller than with stabilization power supply.

I use this kind of redundancy only in my big planes.

BTW, i have 3 2.6m, 8kg flywing and never had problem with regulator.

PS: use only good regulators.
eduardo October 27, 2013 at 1:03pm

The problem is not if the output transistor cook itself but way it cook.

Never user 7805 or any other regulator like this in poarallel.

To use in parallel you need to put some resistor to equalize the output.

My english is very poor to explain way the output transistor cook when more than one 7805 is used in parallel.

I not remember where i view this info (7805 or lm317 detasheet) but it exist.

This kind of thing is from old school electronics.
Andrew Chapman October 27, 2013 at 12:01pm

Thanks Phil - the person I'm hoping to convince has even less electronics knowledge than me though, so I was hoping for the "Idiot's guide as to why you don't connect multiple BECs in parallel", a web page, or definitive post explaining it ;)
Phil P October 26, 2013 at 7:46pm

A 78L05 data sheet will give you the answer. It will show an "equivalent circuit" diagram of what goes on inside the regulator. A linear regulator can be seen as a transistor that is in between fully on and fully off, with feedback circuitry that senses and regulates this transistor. The output of the transistor is your 5v output. So if the transistor cooks itself, it can short the input and output leads, causing the full supply voltage to be on the output. Also if the feedback circuitry fails it can drive the transistor fully on, causing the same result.
Andrew Chapman October 26, 2013 at 6:15pm

"Initially I've connect all my 8 linear becs (in parallel) to my flight controller because I thought that if one fails, the rest would supply the required power. Later on I found out that a linear regulator may fail and may supply voltages higher than 5v and that would burn the flight controller."

Can someone point me at some definitive info on why this is a bad idea? I agree, and know the theory, but there's someone I need to convince so really want something I could point them to and be done with the argument ;)

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