Buffer packs, BEC and schottky diodes

[rorywquin]

Calling all electronic gurus / fundi...........................

The Schulze web site recommends use of Schottky diodes when using a buffer battery pack with a BEC.

They recommend any of these diodes SB340, MBR0340, SB350, MBR0350..

ie 3 amp 40 volt or 3 amp 50 volt with voltage drop of .5V .......

I was unable to obtain the exact recommended diodes but got 3amp 40 volt doides with a voltage drop of .525 instead of .5 volts.

Being an electronics idiot - I assume the the 25 thou difference in voltage drop is not going to make any diffference - am I correct! Is there any other part of a diode specification that may cause me problems?

Oh yes....the bec has a diode in the + line with the ring toward the receiver and the back up battery has the same - ie diode in the + line with the ring (cathode??) toward the receiver!

I cannot read German very well but assume the diodes are to prevent the BEC charging the backup and so that the BEC battery does not drain the backup if you run the BEC too low.

Seems this set up will help eliminate perceived glitches caused by voltage collapse when the BEC is under strain.

This setup will go into my new Simprop Excel Competition 2 (4 servos / AXI motor and speed controller on 12 cells) and I want to get it right.

Thanks

[Mr.RC-CAM]

Oh yes....the bec has a diode in the + line with the ring toward the receiver and the back up battery has the same - ie diode in the + line with the ring (cathode??) toward the receiver!Those are steering diodes. The highest voltage source is the one that will supply current to the Rx and servos. The one that droops will be in full isolation. The two batteries will not "see" each other.

The diode's forward voltage (Vf) drop will cause issues if your Rx cannot tolerate the reduced voltage. For example, some PCM Rx's will failsafe under 4.2V. So, be sure to test under worse case conditions (low battery, full mechanical loads on servos).

RC-CAM

[rorywquin]

RC-Cam

Thanks for the input - I am using a Schulze receiver which, operates on a nominal voltahe of 3.8V. As the BEC will supply 5v minus the .525v drop - seems I will still be well in on that side. On the buffer battery side, 4.8V minus .525v still leaves me at 4.275v so all seems OK.

What you say helps me make sense of what I could glean from the German language article I read. Seems the receiver will receive power from the BEC (being the higher nett voltage @4.475V) unless it drops below 4.275 when the buffer pack will supply the power.

[Bill Glover]

I'm a big fan of the UBEC now, so simple and efficient ... gives a solid 5V output no matter what the load.

[rorywquin]

Bill - who imports them or, did you import directly ?

Cheers

Rory

[Bill Glover]

Gordon Tarling was advertising them before he 'shut down', but I never actually managed to get one from him ... bought online from E Cubed in the end:

http://www.azarr.com/

They are available from various other places in the US, including Hobby Lobby and Koolflight (who make them):

http://www.koolflightsystems.com/ultimatebec.htm

[rorywquin]

Cheers Bill