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.
Replies
I'm really just wanting to make sure you don't have the +5v middle wire connected on the RSSI cable.
Thank you.
Onur
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!
The recommended diode is a 1n5339
http://ardupilot.org/copter/docs/common-powering-the-pixhawk.html#p...
A sample datasheet for the 1n5339 says that the actual zenner voltage may be from 5.3 V to 5.88 v,
http://www.mouser.com/ds/2/308/1N5333B-D-102336.pdf
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 !
Where in the spec sheet does it say it should not go over 5v? Would explain why mine gets hot, but according to the table, the nominal voltage is 5.6v.
I am fairly sure that the intent is not to exceed 5V input voltage when using the1N5339 Zenner. Anything over that and with measurement errors and Zenner tolerance, you are getting quite close to things getting hot :)
http://www.mouser.com/ds/2/308/1N5333B-D-102336.pdf
For tolerance of the zenner, See the "Electrical Characteristics" section on Page 2
I though the idea of the zenner was to limit voltage to the 5v pixhawk rail by not allowing voltages above 5.7~ volts. A 5v zenner would block valid voltages of 5v to the rail from a 5v bec.
I am confused about the zenner recommended though. It is a zenner in reverse?
I plan on testing all this on the bench before adding to the pixhawk. Have ordered the 1n5339 but have not recieved it yet. The 1n5339 is the recommended diode on the official site and it is 5.7v.
previous comment edited for clarity.
A Zenner diode is intended to be used "upside down" compared to a regular diode.
https://en.wikipedia.org/wiki/Zener_diode
Note: In previous posts in this thread clearly my spelling was wrong.
Should be Zener not Zenner !
Added an issue about it to ardupilot wiki
https://github.com/ArduPilot/ardupilot_wiki/issues/411
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