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.



   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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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

We have a steadidrone Vader and it was shipped with a 5v bec powering the pixhawk servo rail. There was no zener diode or capacitor for that matter providing overvoltage spike protection. The official site here is pretty clear saying this MUST be done:


Steadidrone says they have 5 years experience manufacturing drones and have never had a problem and don't recommend "3rd party" hardware. Should we install this overvoltage protection or not?

Yes, install the diode. I have driven for many years without needing my car's airbag, but I still wouldn't buy a car without one.

EDIT: I didn't realize you were talking about a copter rather than a plane. Bottom line, if you have any servos or motors powered by the servo rail, you need the diode. Most copters do not, and therefore do not need the diode.

I have one copter with esc-bec; without iode but powered the pix from power module, It's recommend iode there? 

If you don't have a servo or any type of motor powered via the servo rail, you don't need the diode.

Thank's iskess, 5v wire esc is connected to pixhawk but not powering anything except in case, if the power module fails, power the pix in an emergency case.

Exactly. Give it a try. Unplug the power module and witness the magic!

I dabble in electronics so please don't take the below as gospel, I do believe I am correct in what I've written, why else would I have written it.

I'm more then happy to be corrected if I'm mistaken on anything...

The zener in that diagram is a functioning as an effective voltage regulator for protection.

The resistor in parallel in the diagram is merely there to represent an effective load within the circuit with a limited current drawing capacity. 

The specific property of a Zener is to go into reverse avalanche breakdown at their assigned threshold voltage, keeping the actual voltage in the system at or below this point. For brief transient spikes within the frequency specification of the diode it will accomplish this with ease.

The other desirable element of a zeners functionality is that typically when they do fail, they go open circuit, so you won't end up with a short circuit destroying tracks, wiring and power sources/batteries in the process.

In the event that the zener failed the voltage protection would have been eliminated however the power circuit would not have been interrupted.

We used zener diodes in out planes until we saw how stupendously HOT they get and took them out permanently. Well in excess of 90 degrees C. Is this normal? I don't normally trust any electronics that get that hot..

Also, using a MBR1545CT caused problems because it would continually switch between power supplies since the voltage would drop of the supply that was switched to, because of current draw, bringing it below the other power supply voltage, causing the MBR1545CT to switch back - this happens many times a second and eventually caused a failure on the regulators!

Interesting. Mine never get hot. I install the diode along with a capacitor. I did get one that came with the 3DR Aero and that thing burst into magic smoke one day. I assumed it shorted internally, but maybe it was getting hot and I didn't notice.

Also interesting what happened to your Schottky Diode array. I guess the key is to have one regulator with a significantly higher voltage output so it is always the primary.

The recommended diode is a 1n5339


A sample datasheet for the 1n5339 says that the actual zenner voltage may be from 5.3 V to 5.88 v,


so the wiki should be changed to say that the input voltage should not be greater than 5v .

The zenner diode voltage  in this application must be rated over the highest continuous input voltage. else it will go pop !

Hi Iskess... yea face palm ... I double checked the one I removed and saw that it was reversed installed (positive and negative swapped..) so that's why it got insanely hot.. I'll re-install the others and fix that one.

Regarding the rectifier, fortunately the castle BECs we use can be programmed at a voltage, so I'll put some margin in. However I got such a shock seeing what happened, I can't get myself to try the process again. It's horrifying seeing a 3m 16kg UAV go down in it's maiden !

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