StepDown Buck Question

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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 open source tutorials that were incomplete regarding electrical scheme and 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 3.6 @ 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 @ 5v output each (because of the small battery pack).
6. 1x MPU-6050 Beschleunigungssensor 3 Achsen Gyroskop set @ 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 (for 1.5 years long) 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 (stepd down converters) 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 a 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 of these 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 (only teensy connected via USB) - no reaction on PC, no lcd, it started to be extremely hot so I unplugged it. 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 (e.g which fuse to purchase)....
 
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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.
 
A couple of thoughts:

Most amazon/ebay etc. advertisers list the lnstantaneous maximum current for devices, not the continuous current. Always used a buck regulator rated 2 - 3 times higher than you need, for it to have a decent chance of working properly.

I tried some similar to that MP1584EN module and one died at barely half an amp, destroying the device it was powering.

The LM2596 ones like these seem to be OK, so far:

I've used a few of these; they seem OK, but I'd consider them for 5A max:

These may be OK for 10A+


Buck converters often only work properly and give a stable output with a load connected - you need to set the voltage with a resistor across the output initially then re-adjust with the final load connected.

With high current circuits, be VERY careful with all the ground/0V interconnections. Use heavy wire so the voltage drops due to current stay down to fractions of a volt at worse.

Ground voltage differences due to voltage drops at high current can cause erratic problems or damage connected small signal electronics such as the Teeny.


ps. Is it a Spot Mini robot?
 
thank you for your suggestion
 
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