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OpenServoCAN (...for hobby standard size Servos)

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TrickyNekro:

--- Quote from: mvs on January 12, 2019, 06:34:31 am ---In backwards parallel connection one LED will limit reverse voltage applied to another led to its forward voltage and vice versa. For blue and white LEDs it will be around 3.0-3.2V at nominal current, well below typicaly allowed 5V maximum.
I have made a simulation in LTspice for better understanding. Blue line is output voltage of V1 (+/- 15V), green line is resulting voltage across LEDs (+/- 3.085V).

--- End quote ---
Hey, thanks for the effort but really my brain had some f@rting problems, it needed some extra time to open itself to the world again.
Of course, of course, you are absolutely right.

But, I will ridiculously double down a little bit and say that actually they can protect against open faults, or wrong placement faults.
And there is always the option to just short them out.

So I will leave them in place for the moment and note in the schematic that they can be either placed or shorted. Gives you more options.
Never the less, thank you for your contribution mate!

Cheers!

Poe:
PWR_EN appears to control T3.  Dis/Connecting PWR_V+ to V+. 
D5 appears to limit T3's Vgs.
When PWR_EN is high, T5 is off, T3 and T4 are on.
When PWR_EN goes low, T5 turns on just long enough to short C1, then stays off.  Once C1 discharges T3 and T4 are off. 
PWR_VIN is still supplied to PWR_V+ through T4's body diode.

Is this correct?
What is the purpose of T6?

TrickyNekro:

--- Quote from: Poe on January 18, 2019, 02:35:39 am ---PWR_EN appears to control T3.  Dis/Connecting PWR_V+ to V+. 
D5 appears to limit T3's Vgs.
When PWR_EN is high, T5 is off, T3 and T4 are on.
When PWR_EN goes low, T5 turns on just long enough to short C1, then stays off.  Once C1 discharges T3 and T4 are off. 
PWR_VIN is still supplied to PWR_V+ through T4's body diode.

Is this correct?
What is the purpose of T6?

--- End quote ---

The whole T5, T6 and T7 along with R12, R13 and R14 circuit is a sightly modified version of the twin-NPN totem pole circuit with the inclusion of R13 both for soft-start and current limiting for the D5.

T5 acts always, when it´s in its ON state, as a voltage follower. When T7 is enabled by the microcontroller it pulls the base of T5 low so it deactivates it and pulls the "load" low through T6.
T6 is there to "isolate" (not galvanically by any means) the output from the base drive of the T5. Because in this circuit R13 is present, it might have been possible to omit T6, but I kept it there for reasons
of convention. T6 can also be any kind of diode, but I have found that generally using the same kind of BJT works better and easier (as you don´t have to calculate capacitances to limit shoot-through).
The same circuit you can use also with Darlington transistors, or a "reverse" circuit you could also use with PNP transistors.

It´s really often that you use said circuit as a MOSFET driver, did it myself many times in fact. Of course not with R13 in place.
It is always inverting and its high-z state is always high. You can limit the output voltage of a normal totem pole circuit with just a Zener from the base of the high side NPN to ground.

I have attached a picture of the normal version of the circuit below for comparison. Resistor values are not painted or measured to spec.

TrickyNekro:
I set up everything so far in GitHub : https://github.com/TrickyNekro/OpenServoCAN for anyone interested.

I am also thinking of starting a gofundme campaign for buying the first materials that I need.
Already for 10 boards it will cost me around 135€ with some extra parts and without some others, probably will have to go out fishing in Microchip Direct.

But If I ever wanted to produce this thing, it would most probably cost less than 20€ per piece to the end buyer, so that´s something!

TrickyNekro:
I know it´s been a time since last update but I have been busy, work and all...

But guess what work does? It gives you money for your crazy projects! :-DD

Well I have this plus another two boards assembled, so that I can begin testing soon.
Unfortunately it will be now weekend priority project so until I get a very long leave, it will be some time until I make serious progress  :-[

But project will be completed never the less, just in a longer time. Meanwhile I´m also ordering servos from ebay for testing.

If anyone has some broken servos (good motor and gears - burnt PCB) please PM me. It could really help if you wish to send them my way instead of the trash bin!

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