I ran into the following post at RCGroups which describes a potentially critical error condition with the 3DR power module.
I cannot verify the claims but at least they appear well founded so I'll leave it to the tech guys to comment on that.
Post URL: http://www.rcgroups.com/forums/showpost.php?p=24505558&postcount=2048
Post by jabram:
These folks over at DIY Drones did not mess with the pins
http://diydrones.com/forum/topics/ap...ment%3A1044651
They did experience a spike when disconnecting Lipo while performing ESC calibration of all ESC at once.
How does one assemble a multicopter without messing with pins ? CANT BE DONE -
Disconnect Lipo BEFORE messing with any wiring.
Check, double check, triple check you wiring modifications carefully, VERY CAREFULLY before connecting Lipo
DIY Drones Power Module that you have faith in is a primitive amateur design.
I do not recommend anyone use the DIY Drones Power Module.
The switching regulator chip 3DR use in the simplistic amateur design Power Module is only rated at a Maximum input voltage of 18V, The manufacturer of the chip ONLY recommends it for use with a 12V input supply, hence my "amateur" comment, they did NOT read or did not comprehend the datasheet..
It is extremely easy for a Lipo spike when connecting or disconnecting it to exceed 18V, specially when using a 4S Lipo and can also easily occur with 3S Lipo
Comments
I would like to use it with a 6S battery, will it be possible to disable the switching regulator and to provide the energy for the APM from the BESC?
How can i do it? I just want this module to provide me with current/tension readings...
Wayne - dont use it with 6S. How long have you been using it that way, out of interest?
Aurelio, this would be a common thing if this were a brushed DC motor, connected directly to the battery. But a brushless ESC, I think it won't happen. The inducted current would be automatically shorted through the body diodes of the FET's in the ESC.
I am not commenting on the design. I haven't looked at it really. But if you are looking for a voltage transient ('spike') with an oscilloscope, you are probably looking for something that happens in a 1ns time frame. Testing it with the scope on 20mS and 50mS per division would quite likely not show it. Set the scope to trigger at 15 volts at 1nS per division and see if it happens.
I wonder if the context where any failures could happen (if in fact they are possible...?) is unplugging a battery *while current is drawn*, that is, while the motors are running. In this case there would be an inductive voltage spike, which the shunt diode is then expected to absorb.
So much for an "amateur" design.:-)
Regards,
TCIII
Yes, that is a nice ramp. And the sparks are just the capacitors on the ESCs charging up, and that is charged directly from the battery (or trough a very small shunt resistor). So let's move on and start discussing the next interesting topic :).
This entire case is about nothing really. If you read the datasheet for the TI PTH08080W design, it has an internal soft-start circuit to prevent output spikes and in-rush current from the input source. The example figure 11. in the datasheet show pretty much exactly the same results as the scope images posted by Arthur.
So I guess the lesson here is, always remember to read the entire datasheet.
I have to agree with Nick Arsov. Designing a good SMPS or a RF board can take a lot of iterations, so it can get very cost ineffective for small batches like the ones 3DR makes.
A friend gave me his but now I get strange current readings in Mission Planner:
http://www.diydrones.com/forum/topics/wrong-amps-and-battery-percen...
When the short circuit happend, the ESC Power was connected to the output rail.
Could that have somehow damaged my APM2.5 which now causes the strange ampere readings?
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