Comment by Dean Franks on January 10, 2012 at 3:25pm

Switching regulators used to be much more noisy, but as switching speeds have gone up, the noise have dropped for a given inductor and output capacitor size.  Also, now that switching speeds are high enough the output cap can be a low esr ceramic which further reduces noise.

If there are no other loads on the battery the 7805 might produce less noise, but I doubt it.  Load transients will dominate the noise and the AVR can generate substantial transients without proper filtering.

Using two OKI-78SRs would protect against a shorted cell.  One battery is connected to each OKI with the outputs tied together.  A short or 0V on the input of the OKI-78SR will just cause it to shut down and it will not load down the other regulator in this state.

Switching regulators almost always fail open, and sometimes fail with the output shorted to ground (highly unlikely, but possible in a standard buck configuration).  Because the regulators are under-voltage protected on the input and over-current, short circuit and over-temp protected the chances of any kind of failure is very close to zero.

We operate tri-rotors with a custom APM on a single OKI-78SR on a separate battery that runs everything except the ESC/lift motors and have never had an incident.  Early in the game we used BECs on the ESCs and had several failures (Turnigy).

These tri-rotors are lifting 2kg sensor packages (with several thousand dollars each) so the cost of redundant power is not an issue, it is just not a probable failure mode when using the OKI-78SR or the older Murata 78SR-5/2-C regulators.

We also run a separate battery for everything except the ESC/lift motors for safety.  Everything can be powered up and verified before a pull pin is removed enabling the ESC power (from a safe distance) (there is also an electronic lockout for the motors controlled by the APM/radio link).

Developer Comment by R_Lefebvre on January 10, 2012 at 5:10pm

Yes, I'm also a fan of seperate lift, and radio system batteries.

So with your dual OKI-78SR setup, do you have automatic switchover?

I can see how the switching noise has been improved over time but it will take a while for the reality of that to percolate down to the users.  Pretty common in the hobby industry.

Comment by Andrew Dunlop on January 10, 2012 at 5:47pm

For significant load currents such as would be encountered on a servo bus, Schottky diodes are not a panacea.  Even the very best Schottky will drop about 0.4V at 5A.  Better than silicon, but still far from ideal.

Ideal diode controllers - that include a diode ORing function - have been available for a while now and represent a vastly superior solution.  An N-channel FET is used instead of the diode, and depending on the FET you should be able to keep the voltage drop well under 100mV.

Comment by Gustav Kuhn on January 10, 2012 at 11:28pm

Define perfectly smooth:-)

It should be no more than a few 10ths of a mV, if that is what you mean.

If you google the design of a typical switching circuit, you will notice that it boils down to a capacitor that is constantly being pulsed up to the required value.

A good design will have ripple of no more that a few mV.

All the square wavey stuff happens earlier in the circuit, hence the RF noise that is inherent to the design.

Comment by Gustav Kuhn on January 10, 2012 at 11:42pm

I see replied rather belatedly :-(

However,

Alrighty then how about a 5.5v NIMH battery with a diode to stop the bec from charging it?

Why do you NOT want the BEC/UBEC to "charge" the battery?

At 5V, 5.5V typical output range, the current flowing into the battery is negligible, if of course it is fully charged before you switch on

Effectively I run my BEC/UBECS through a diode, straight to the battery.

That way, the battery can supply the current spikes demanded by servos, without any drop over a diode.

And in those periods that there is light current draw, the BEC/UBEC can top up the battery.

I'm no engineer, but I've been flying my planes and heli's like this for a few years now.....

Comment by Gareth Rens on January 11, 2012 at 12:32am

Thanks Gustav, its difficultto get a difinitive answer around here ;)

So should i put a 4 or 5 cell NIMH battery in parallel with my bec? And where do i put the diode?

Thanks :)

Comment by Gustav Kuhn on January 11, 2012 at 12:54am

Hi Gareth,

I have 4 cell nicad, or nimh in my different aircraft.

(Older tail rotor gyro's are low voltage only)

And 1A continuous, 8A peak Schottky diodes in the BEC lines.

Can't remember the number, but they look like a 1N4007.

I tease the cable coming from the BEC apart, cut the red wire, solder in the diode with cathode(stripey end) pointing to the plug, and then heat shrink the lot for neatness.

Works for me.

Comment by Gareth Rens on January 11, 2012 at 1:15am

So you put the diode in the BEC + line not the Battery + line?

Comment by Gustav Kuhn on January 11, 2012 at 1:21am

Yes, in the BEC + line.

Comment by Gareth Rens on January 11, 2012 at 1:26am

AWESOME! Thank you Gustav for the real world advice :)

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