StepDown Buck Question

[alex_nik]

Hello Everyone,

firstly I would have to thank you for your valuable inputs in similar topics that I used during now 1.5 years long project.
I am a hobby enthusiast, not from electrical engineering or programming field. With that being sad, you can conclude that I am a rookie but really into robotics :). I followed some tutorials on different sites that were incomplete regarding electrical components but I managed my robot to work. I was really proud.
Now to the problem, battery lasted really short and this is probably a good time to explain components.

1. 12 DOF robot with 12 x MG996R 10kg/cm .- each set to 6v and potentially max 2.5A in stall
2. 1x Teensy 5V
3. 1x WiFi Adapter 5v to 3.3v
4. 1x WiFi NRF24L01+ 2.4GHz which is operating on 3.3v
5. 4x 10A DC-DC 6.5-60v to 1.25-30v Adjustable Buck Converter to power Servos. 1 Per Leg set to 5v output each (because of the small battery pack).
6. 1x MPU-6050 Beschleunigungssensor 3 Achsen Gyroskop set to 5V
7. Wires are as short as possible - according to recommendation
8. For Signal Wires, I am using CAT5e Networking Cable (SFTP) to minimize interference
9. (Working Battery) Battery in Parallel 2x 7.2V, 1200mAh, NiMH giving a total of 7,2V 2400 mAh and 17.28W
10. Step Down Converted to power Teensy and connected components (Wifi, MPU)
    DC-DC Buck Voltage Converter 4.5-40V 12V To 5V/2A.

The above setup worked but the battery lasted really short - max 10 min and even shorter if movement was complex (yaw, pitch ....) .

So I thought I have a BlackandDecker Li-iOn battery 18V, 2A = 36W
Above components had a "declaration" that can sustain such voltage/current so I mounted this battery. As a consequence legs started twitching, jittering ..... but nothing burned (this is good right)
I measured component No 5. DC-DC Converters for Servos - even without servos connected and the output was simply fluctuating - 2v then 5 volts then 3v ..... after that I found on the internet that this component is not quite built correctly therefor the behavior.

I decided to buy - per servo buck converters and I decided for
5. Mini DC-DC Step-down Spannungsregler MP1584EN Buck Power Module Input 4.5V ~ 28V and Output 0,8~ 20V - set to 6V this time as I wanted my servos to move quicker with more powerful battery.

As a result two out of 6 regulators started smelling on burn, were extremely hot, legs were twitched and jittery once again and I disconnected all servos and observed the same behavior of fluctuating voltage on all converters.

At this point I started reading on the internet and on some forums it was written to use UBEC converter, they are built to do a step down for a professional and semi pro RC toys powering servos. I decided to replace my current No 5 solution and to potentially purchase 12 UBECS - one per servo.
5. UBEC Purchased - Hobbywing HW86010010 BEC 3A UBEC Regler for 2-6s LiPo.
with 5.5V-26V (2-6S Lipo or 5-18 cells NiMH /NiCd) and with the ouput 5V/3A oder 6V/3A.
18 cells nimh is around 21V and my current battery is 18V

I purchased only 3 to test a single leg but I mounted only one to test failed DC-DC converted.
Jumper was set to 6V output.

Now to the ultimate issue and the main questions:
It took only 1.5 sec there was no burn, smell, nothing, all servos just buzzed for a second and then silence. Ultimately I checked Teensy and it burned. Why damn?
Can someone explain me please:

1. Why UBEC caused a Teensy Burn. I assume that servo is now faulty and let voltage through the signal wire back to teensy causing the burn.
2. Any recommendations on which step - down - proven converter shall I use for described battery?
3. Can Li-ion battery itself be a problem?
4. How to prevent future burns of teensy - is there a way to isolate or place a fuse for a signal wire and how to do that....

Thank you once again for your help and I am really sorry for my long novel but I hope that it explained the situation at least a bit.

[mrburnette]

@alex_nik

Now to the ultimate issue and the main questions:
It took only 1.5 sec there was no burn, smell, nothing, all servos just buzzed for a second and then silence. Ultimately I checked Teensy and it burned.

Without a rather detailed schematic of your hookup, I going to venture an educated guess: The Teensy is 3.3 V logic and one or more of the servos (input) either was damaged by previous failures and overloaded the Teensy or the servo input "broke-down and back-fed" 6 Volts into the Teensy's output which destroyed the CMOS output driver inside the Teensy uC.

https://www.pjrc.com/teensy/td_libs_Servo.html#:~:text=Larger%20motors%20will%20need%20a%20separate%20power%20supply.,%2B5V%20motor%20power%20should%20%2Anot%2A%20connect%20to%20Teensy.

Larger motors will need a separate power supply. Usually power and ground wires should connect as directly as possible from your 5V power supply to the motors. A ground connecton from the motor to Teensy is required. If Teensy runs from USB power, usually +5V motor power should not connect to Teensy.

[alex_nik]

Hello, thanks for your answer. I did doublecheck and teensy v3.6 has input range Vin from 3.6V to 6V. Also robot was working until I started playing with batteries. I am attaching PDF with the logic and teensy "getting started" page to confirm. I am sorry if the logic is not according to standards but hopefully will give you more info on the cabling.

[mrburnette]

@alex_nik

teensy v3.6 has input range Vin from 3.6V to 6V.

DC input to regulator:

BUT...
The Teensy and all logic pins are 3.3V ...
Consider:

The "green wires" are outputs from Teensy, capable of sourcing < 3.3V and sinking down to approx 0.5 Volts.

Input/Output: The Teensy 3.6 has 62 GPIO pins with a current sink/source ability of 25mA. Pull-up resistors can be enabled on the pins.
Analog inputs: 25
Digital I/O pins: 62
Microcontroller: MK66FX1M0VMD18
Operating voltage: 3.3V

You have no protection in the "green" wiring ... Teensy will smoke if green goes higher than 3.3Volts or if the current sink exceeds rated max.

[alex_nik]

Hello, thanks for the clarification. This is then what happened.
The questions are

1. How to protect against that? Which fuse I have to use for green wires?
2. Why did UBEC fry servo and teensy through servo? Is that UBEC too powerful for servos?

Thanks a lot! Your input is highly appreciated.

[mrburnette]

@alex_nik

• How to protect against that? Which fuse I have to use for green wires?
• Why did UBEC fry servo and teensy through servo? Is that UBEC too powerful for servos?

MG996R_Tower-Pro.pdf (98.4 KB)

According to the servo datasheet, the signal (and Power) are suppose to be at the same voltage level. Your signal level is around 3 Volts from the Teensy. But Teensy online suggests that the Teensy can drive 5 Volt servos.

However, your design is out-of-spec. I cannot say if this was your problem or if some other issue occurred.

Honestly, maybe someone with lots of servo experience will be able to help. Maybe Paul's Teensy forum? We are dealing with a Frankenstein build, you need another Frankenstein who has had success.

Ray