Replies to This Discussion

Permalink Reply by Glenn M on July 3, 2012 at 4:39pm

I just came to post the same thing... Just assembled my board and there are no instructions about where to put these two components.

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Permalink Reply by yovio on July 4, 2012 at 4:17am

Please share here if you got the answer from Tech support team.

 

Thanks

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Permalink Reply by Vernon Barry on July 4, 2012 at 5:27am

Guys, the zener diodes are not normally used. This sounds related to the thread were people were blowing up the boards using BECs (AKA voltage regulators) from the batteries that supplied higher than exactly 5 volts. The instructions specifically say to not apply more than 5 volts. Likely what you have is a fuse and a zener diode. The idea is that the zener is a shunt regulator when you place it in the circuit reverse biased. Is shunts or shorts the voltage and limits is to 5 volts (until the zener burns out). The idea is to put the fuse in series and the diode in parallel across your input voltage. http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm In other words, they don't get soldered to the board, you make your own external circuit.

The original thread where the topic of zeners came up. http://diydrones.com/forum/topics/problem-with-apm2-powering-up-my-...

 

My advice is to just use a regular 5 volt BEC and be fine. If the fuse fails in flight, then you crash for sure, probably ruining the boards anyway in the crash. While it's a novel idea at safety protection, I think alone your confusion should be telling you to just follow the normal directions EXACTLY, use a 5 volt regulator and just don't ever try to load power across the APM (power the inputs and then attach servos to the outputs that are not powered). You best best is to follow the directions and always power the board from the outputs.

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Permalink Reply by Steven G on August 11, 2012 at 2:15pm

Just for clarity, because of voltage drops, the voltage on the output rail should be slightly higher than 5 V as noted in the Wiki, "Therefore, power requirements are as follows: 5.0VDC +/- 0.5V supplied into the PWM input connector, jumper JP-1 removed. 5.37VDC +/- 0.5v supplied into the PWM output connector, jumper JP-1 in place

Warning: Do not exceed 6.0V DC of power supply input voltage or you will damage your board."

That said, not many BEC's are adjustable giving you 5 or 6V. However, some of the Castle products are adjustable via software.

Best,

Steven

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Permalink Reply by Vernon Barry on July 4, 2012 at 12:28pm

Again, here is the direct info from the 3DR team and the "why"behind the etra parts. They are a fuse and a zener diode.

Basically the link I have in my first response explaines the external circuit you must build where the resistor is replaced by the fuse to protect the entire circuit.

Reply by Craig Elder on June 11, 2012 at 2:53pm

Hi Adrian

I have been investigating the power supply to the APM2 for the last couple of weeks and I appreciate you taking the time to look into this as well.

What you have actually identified is an error on the schematic rather than the PCB.

Diode D1 is listed as a RB751S40T1-D on the schematic, but it is actually a MBR0520LT1G.

The RB751S40T1-D has the same footprint as the MBR0520LT1G, but it has a 500mA continuous current rating and thus has an appropriate current rating for the powering the APM2 which is ~200mA.

That is not to say the circuit is perfect.

We see three common failure modes with the APM2:

1. Unlike the APM1, the APM2 requires an external fuse when it is powered via the output rail.  The failure mode we see is that people are off by one pin when they connect something to the input rail and they short the +5V rail to the ground rail.    Because there is no fuse, diode D1 blows.
2. Overvoltage on the +5V input damages the serial multiplexer.  There is good evidence to suggest that transients (perhaps from the motors and the ESCs) are causing an overvoltage situation and damaging the TSA23157, U7.
3. Connecting the power supply to the APM2 backwards also damages the APM2, and generally it is the multiplexer that blows.

We have made some changes to the input circuit and will be incorporating those changes in the next revision of the board, however in the meantime I recommend you reference the

Alternative ways to power your board

section of

http://code.google.com/p/arducopter/wiki/APM2board

What are we doing about it before the next release?

1. As of last week, the MBR0520LT1G has been replaced with a MBR120VLSFT1 Schottky  diode which has a 1.0A  current rating.
2. Starting this week, APM2s are shipping with an external 500mA Pico fuse to install between the power distribution board and the APM2, and also a 6.0V Zener diode to be installed in a reverse bias arrangement across the +5V rail to prevent damage in the case of over voltage or reverse voltage.
3. The Pico fuse and the Zener diode will be made available from to anybody with a current APM2 who asks for them.

If you want to save yourself some time and get these components yourself, you can find them here:

http://www.digikey.com/product-detail/en/0251.500MRT1L/F3167TR-ND/7...

and here

http://www.digikey.com/product-detail/en/1N5233B-T/1N5233BDITR-ND/616

Cheers,

Craig

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Permalink Reply by R. D. Starwalt on July 4, 2012 at 6:36am

Three weeks ago I assembled my un-assembled APM2. No Diodes, repeat.. there were no diodes in the kit.

I did receive *jumpers* for header pins that are used if you choose power options for on/off the APM2 board.

I did receive single pins for the mounting the onboard GPS module.

Alex, you say "but with it came two diodes (i think) one of the being a zener diode"

It is true that there is not much assembly instructions for the 'kit'. Here is what I received:

Is what you received the same as the picture above?

 

Regards,

Doug

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DistributorPermalink Reply by UnmannedTechShop.co.uk on July 6, 2012 at 5:34am

Here are instructions on how to solder the new diode and fuse onto your Ardupilot Mega 2 board.

 

Firstly you will get 2 components with your board.  A pico fuse (top), and a zeren diode (bottom)

 

The fuse you need to solder onto your power input to your APM (either your ESC wire, or from your PDB if you are flying an arducopter)

 

Its a good idea to put some heat shrink over that to avoid shots.

 

Next you must solder the zener diode onto your ardupilot mega board. 

The diode is soldered onto your 5V and GND of the APM2 OUTPUT rail as shown above.  Make sure its connected the correct way around

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Permalink Reply by Justin on August 6, 2012 at 4:52am

Instead of connecting the Zeiner diode across the APM output rail, can I instead install it between the +5 and GND rails on my PDB that is supplying the power to the output rail?

Regards,
-J

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Permalink Reply by Justin on August 8, 2012 at 4:29pm

Anyone have any thought on this? Circuit analysis was a long time agom for me, but it seems to me it would have the same effect.

Thanks,
-J

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Permalink Reply by Vernon Barry on August 8, 2012 at 6:17pm

The entire point of the diode is to prevent over voltage from people who seem to not be able to read the directions that say only apply 5 volts to power the APM (there is at least 1 question a week about max voltage-sorry for the rant) and the second function is to also protect the APM against reverse voltage (when somebody plugs the connector into the APM wrong(it's easy enough to do). The DIODE is NOTHING if the fuse isn't in series, as it will just smoke, burn out and then still let the APM burn out.

So if you put the diode on the power distro board, it doesn't protect against reverse voltage, incorrect wiring or other issues at the APM. Further, if the Fuse is NOT in series BEFORE the diode, all it will do is burn out, likely burn out the regulator on the ESC, smoke some wiring in the process, and still let the APM burn up.

Therefore, if you don't understand any of that, don't even bother with the diode, you will only cause more problems. The directions above with the picture are about as clear as it gets. Do not deviate from that as again, it seems it's more likely you will do harm than any good.

Zener diodes are not like regular diodes. A regular diode is basically a one way valve, but a zener has a special trick in that in the backwards (reversebiased) mode, it conducts at a certain voltage but doesn't conduct below that voltage (AKA crude shunt regulator, think shunt=short circuit). This is why the fuse is critical as without it, the diode is a dead short to the power source when 2 conditions occur (over 5 volts and reverse polarity). Both cases should blow the fuse protecting both the source (likely your ESC or BEC), and the load (the APM, receiver, sensors, and anything else plugged in). THe key here is that the fuse must be in between the power source and the diode and the load or else=you let the magic smoke out. I need to also clarify, do not put the fuse in series with the diode across the load, as the fuse will blow, but since it's not in series with the load, the APM will still burn too.

 

Again, sorry for the rant but because this gets asked so often,  I feel our education system is failing us to teach the basics. I'm not trying to insult and hence, the rather lengthy explanation on the how and why as an attempt to remind folks who forgot.

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