Discussions

Zener diode in PIXHawk

Posted by Ravi Gupta on August 6, 2014 at 1:20pm

On the power connection page, it is suggested that we use zener diode for protection when using back up power supply. The connection suggested is to connect zener across the +5v and GND of the servo rails directly.

http://plane.ardupilot.com/wiki/common-pixhawk-wiring-and-quick-start/

However when I read about zener diode, then the circuit to control voltage is suggested as follows.

Notice the resistor in series with the zener. I understand that this resistor will consume the voltage which is excess to the required voltage. However in the connection suggested for PXHawk, there is no resistor. In this case how is it able to regulate the voltage? My knowledge of electronics is limited, so I would appreciate an explanation of this.

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Replies

Michael Scott July 26, 2015 at 9:13pm

Hello, I'm installing a zener diode with a capacitor in parallel for a Pixhawk application with a 5V 5A UBEC. Would appreciate some advice on capacitor size. I have the option of using a 220 microfarad, a 330 microfarad and a 100 microfarad capacitor (all rated at 10V or greater). Will the 100 microfarad capacitor by suitable or should it be larger capacity? The 100 is really convenient because of its small size - about the size of the zener.

Advice would be appreciated. Thank you
Sandy Sound May 12, 2015 at 7:05am

Hi guys,

Is this a suitable capacitor to use in parallel with the zener?

https://www.elfaelektronikk.no/elfa3~no_no/elfa/init.do?init=2&...

Cheers,

Sandy
json November 17, 2014 at 8:56pm

In relation to this..

http://plane.ardupilot.com/wiki/common-pixhawk-overview/#Powering
"To complement the diode, it is also useful to add a capacitorin parallel to the diode. The capacitor will smooth out eventual voltage ripples. As advised for the diode, the capacitor should be connected with as short wires as possible. Do not oversize the capacitor."

1) what is the recommended capacitory? Will a 25v 1000uf work or is that 'oversized'?

2) How do we parallel it.. should it be done this way?

--------(+)-----/[capacitor]----/[zener diode with rev polarity]
--------(-)------/------------------/

or this way:

--------(+)-----/[zener diode with rev polarity]----/[capacitor]
--------(-)------/-------------------------------------------/

thanks for the help!
- turdsurfer > json November 18, 2014 at 3:56am
  
  I use a 6.3V 200uF capacitor.
  
  Actually, in my case when using a redundant backup 5V supply to power the ESC output rail, my PixHawk will only boot with the capacitor added.
  
  If I don't use a capacitor, the Pixhawk just makes cricket-like sounds with crazy flashing of the LEDs instead of booting. See it happen after 40secs here:
  
  https://www.youtube.com/watch?v=jODj9UzkG1Q
  
  I suspect that it has something to do with the sequence in which voltage rises at the power module input and the ESC connector during power up, and the power selection mechanism in the Pixhawk.
  
  The capacitor in my case, serves to slow the down the initial voltage rise at the ESC input.
  
  I've reported it a month ago to 3DR support, but haven't heard anything of it since.
  - Chris > turdsurfer February 15, 2015 at 1:13am
    
    Hi,
    
    I have the same issue on my Tricopter, I've had to power the servo rail via an external BEC. Cant explain why the BEC from the ESC is not liked yet the external one is all good.
    
    I've tried a 200uf capacitor but it didnt work. I tried them 2 in parallel (for 400uf) and it didn't work, did you get a resolution in the end?
    
    Chris
  - json > turdsurfer November 18, 2014 at 6:18am
    
    thanks.. may I know how you soldered it up? is it this way:
    
    A) capacitor then zener in reverse
    
    --------(+)-----/[capacitor]----/[zener diode with rev polarity]
    
    --------(-)------/------------------/
    
    B) zener in reverse then capacitor
    
    --------(+)-----/[zener diode with rev polarity]----/[capacitor]
    --------(-)------/-------------------------------------------/
    
    thanks in advance!
    - turdsurfer > json November 18, 2014 at 6:54am
      
      If I'm interpreting your ASCII art correctly, then B makes no sense, because either your circuit is open, or you want to use the capacitor in series.
      
      The capacitor must be connected in parallel, so it doesn't even matter in which ESC/servo plug you stick it. You can make one plug for the zener diode as shown in the documentation, and another one for the capacitor.
      
      The only reason I'm even using a capacitor is simply to make it boot. Otherwise I wouldn't even bother, because I don't have noisy equipment connected to my Pixhawk. My ESCs are opto isolated too, so it's as clean as can be.
      - json > turdsurfer November 20, 2014 at 3:36pm
        
        Actually, A or B will work:
        
        http://ardupilot.com/forum/viewtopic.php?f=21&t=6463&start=20
      - json > turdsurfer November 18, 2014 at 7:34am
        
        *slap on forehead* =) thanks for taking time to clarify that! I've been searching exactly what it means by 'parallel' and I just never thought of doing it in a separate servo lead!
turdsurfer October 18, 2014 at 4:32pm

I just want to point out a mistake in the photo of the servo connector: it shows that the diode is connected between positive and signal, instead of ground.

You can probably flip a JR connector around in the pixhawk, so for safety sake, I'd recommend using a Futaba connector and the diode in the correct pins.