Installing a Zener Diode and Capacitor for HKPilot32 (PixHawk)

Installing a Zener Diode and Capacitor to help bullet-proof your Pixhawk is easy. Both components may just help save your plane or multicopter someday.

The Pixhawk design allows for the servo power rail to be used as a secondary or backup power supply for the AP.  However, the design has very low tolerances such that if power is to come from servo rail (via the ESC or an independent 5V UBEC – the latter is recommended) the servos must not pull more than 5.7v.  Note that most micro servos operate in the 4-6v range (with digital servos going > 10v) and when the voltage pull goes beyond 5.7v, the Pixhawk will reboot, resulting in disaster!

To mitigate voltage spikes beyond 5.7v on this servo rail, the recommended mod is to add a Zener diode.  When a zener diode is  installed in reverse (ie the anode coupled to the ground and the cathode to the current) it acts as some kind of breaker,  activating and capping the voltage spikes once its rated voltage is reached.. this means if the correct zener diode is attached in reverse polarity in the servo rail, it will prevent over voltage and therefore not result in the AP resetting midflight. Installing a capacitor in parallel* to the zener is also recommended as it helps smooth the voltage “ripples” further.

To make a Zener Diode and Capacitor for Pixhawk or HKPilot you need the following:

1)  Two (2) servo leads
2)  One 1N5339 Zener Diode
3)  One 6.3v 220uF capacitor

Zener Diode and Capacitor
Capacitor (L) and Zener Diode (R)

1)  For the Zener – make sure that the polarity is reversed, ie the side with the silver band must be soldered to the current (ie middle) servo lead and the other to the ground.
2)  For the Capacitor – install as normal.. ie the positive to the current servo lead and the negative to the ground lead.

Here’s how it looks like installed:

Capacitor and Zener Diode Completed
Capacitor and Zener Diode Completed

and installed in the HKPilot32:

Zender Diode and Capacitor installed in HKPilot32
Zender Diode and Capacitor installed in HKPilot32

* Coming from an non-electronics background, understanding what parallel connections should be, is confusing.  Thanks to “turdsurfer” over in diydrones.com for pointing out what this meant! =)

Updated Version (2014-11-21): 

It turns out my original intent of just combining both into a single component is possible (http://ardupilot.com/forum/viewtopic.php?f=21&t=6463&start=20), here’s the Diode and Capacitor before being soldered together:

zener and capacitor v2
zener and capacitor v2

and the finished component:

zener and capacitor v2 complete
zener and capacitor v2 complete

The higher capacitor 1000uf 25v, used for filtering,  means less ripples and noise.  It’s ok to place a higher voltage cap in parallel for filtering (ref1, ref2).

4 thoughts on “Installing a Zener Diode and Capacitor for HKPilot32 (PixHawk)”

  1. Just wanted to say thanks for the great post!

    I just did this on my pixhawk. Wasn’t really having any issues but noticed my “servovoltage” logging (its under POWR in the logs, as well as in the list in Mission Planner when doing real time logging) was quite bouncy. It would at times swing by as much as 200mv in real flights, and 100mv on the bench.

    I installed the zenner + cap, and the swings at least on the bench now are about 10-20mv at most.

    Thanks!

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