StepDown Buck Question - Caused Teensy Burn

[anik]

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 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)....

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.

I posted the very same question in Arduino forum (link below) but there is a open question where teensy forum is better suitable:
https://forum.arduino.cc/t/stepdown-buck-question/937143

 

[MarkT]

1. 12 DOF robot with 12 x MG996R 10kg/cm .- each set to 6v and potentially max 2.5A in stall

So you need upto 30A potentially on the 6V rail?

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).

Should be able to handle this.

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

No, not 17.28W - 2.4Ah is the capacity, not the rated current. You have 17.28Wh of energy in the batteries, ie 62kJ.
The power the batteries can source is unknown - it probably isn't anything like enough for your servos all at stall, and
2.4Ah for something that might be using 10's of amps isn't very much.

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 ....) .

Indeed, very little battery capacity.

So I thought I have a BlackandDecker Li-iOn battery 18V, 2A = 36W

Do you mean 2Ah and 36Wh again? That might have better current handling though (being LiIon)

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 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?

Did you test the output voltage before use?
Did you connect to just the servos (no Teensy), and use a servo-tester - I would have done this, one careful
step at a time when commissioning a new setup. Especially with RC components which have very lax quality
control as they are made down to the lowest possible cost. I've never trusted a BEC built into an ESC for instance
as I've seen several ESCs explode into flames.

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.

Cheap RC hardware is not my first choice for supplying an expensive microcontroller. Adding series resistors in signal
lines can be useful extra protection (opto-isolation is the fool-proof method however).

2. Any recommendations on which step - down - proven converter shall I use for described battery?

I'd consider using a buck converter to 7V or so and then a linear regulator down to 5V for the electronics - this
provides the best of both, efficiency in the step down from 18V, then the extra protection and reliability of the
linear regulator for the logic rail (no startup hiccups?, less noise).

Careful testing during commisioning with a 'scope to ensure power voltages are behaving themselves - you don't
want to use the Teensy as a glitch-sensor, that gets expensive.

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)....

as mentioned a series resistor of a few k can help isolate from rogue voltage should stuff go wrong, or opto-isolation for
complete safety (though for high-bandwidth signals you'll need the right type of isolator that's fast enough).

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.

I posted the very same question in Arduino forum (link below) but there is a open question where teensy forum is better suitable:
https://forum.arduino.cc/t/stepdown-buck-question/937143

 

[anik]

Do you mean 2Ah and 36Wh again? That might have better current handling though (being LiIon)

Yes I meant 2Ah and 36Wh. This is where the trouble started.

Did you test the output voltage before use?

Yes, after each hardware change of the component No 5 (step down converter) I tested output voltage (with multimeter) and it was according to specs. 6V jumper delivered 6V to servos.

Did you connect to just the servos (no Teensy), and use a servo-tester - I would have done this, one careful

well, no I haven't done that.... Well we learn from mistakes ...

Cheap RC hardware is not my first choice for supplying an expensive microcontroller.

Oh, well the first two components No 5 (step down converter) i bought were really cheap. 1.5 EUR pp. UBEC was 12 EUR pp. Also these UBECs are part of the RC Cars which cost 300+ EUR which should make them more secure then some 1.5 EUR chip. This was my reasoning and this is why I decided to purchase them. But if you have another experience (UBECs burning up ...) I really appreciate your input. Please do let me know which exactly step down converter would you use for servos?

opto-isolation

Thank you very much, this sounds really good. I will try to find some tutorials on that one - how to connect it.... Do you have any recommendation for my usage? I would use it only for signal wires to make sure if Servo is malfunctioning that it does not deliver 6V or more to Teensy ever again.

I'd consider using a buck converter to 7V or so and then a linear regulator down to 5V for the electronics

Excellent point, thanks will implement it.
Info.
I thought that my UBEC fried some servos, forcing more voltage through signal line. But I measured today all servos (without teensy) and ubec delivers 6V and on signal line is 0.1 or 0.2v is that supposed to be like that? I measured as well current step down converter and it is delivering 5V to (late) teensy.
It is still not clear to me why would teensy burn if the voltage is everywhere fine. - Maybe i am not measuring correctly something....

Thanks once again for your reply. I highly appreciate it.

 

[MarkT]

Transients don't show up on a meter, you need a 'scope to see them - the best protection is keep logic power and motor power
systems separate so that surges/spikes/drop-outs on the motor supply don't damage or reset the logic chips.

Do you have a diagram of all the power and ground connections ?

 

[anik]

Transients don't show up on a meter, you need a 'scope to see them - the best protection is keep logic power and motor power
systems separate so that surges/spikes/drop-outs on the motor supply don't damage or reset the logic chips.

Do you have a diagram of all the power and ground connections ?

The only thing what I have i in the first post. Is this diagram that you are asking for? There I draw all power/ground and signal connections.

Thanks a lot.

 

[MarkT]

Do you have a better resolution version with enough pixels to read the writing?

 

[anik]

Do you have a better resolution version with enough pixels to read the writing?

Hopefully this is better:

 

[MarkT]

So no ground returns on the servo data lines? Not something to miss out with all this high current stuff around, you
have a bunch of loop antennas connected directly to the Teensy pins in effect.

Series resistors on those lines are a very wise idea given the layout, but all logic signals need a ground return in the same cable.

Can you clarify the full info on this part:

 

[anik]

So no ground returns on the servo data lines?

One can see that I am complete rookie

Not something to miss out with all this high current stuff around, you
have a bunch of loop antennas connected directly to the Teensy pins in effect.

Series resistors on those lines are a very wise idea given the layout

Can you please tell me which resistors do I have to use?

but all logic signals need a ground return in the same cable.

Can you please draw how do I have to connect single signal wire (I assume each signal wire will be connected in the same way) in order to achieve what you proposed? I really hope it is ok with you. This will make it simpler for me to do that as fast as possible without searching for a correct way - again really rookie.

Can you clarify the full info on this part:

This is component No 10 from the list. DC-DC Buck Voltage Converter 4.5-40V 12V To 5V/2A.
https://www.ebay.de/itm/122204389197?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649

It will take input voltage up to 40V and convert it to 5v. I tested it with the Multimeter and it seems ok. Then I connect that to Teensy to power it and its components. I will in the future that two buck converters one lowering to 7V and another (above one) lowering to 5V for teensy - unless you have better proposal.

Best regards

 

[anik]

Hello and Happy New Year. I just wanted to check if you can give me an info about opto isolator that I need to use for teensy? There are quite a lot there and I would really appreciate if you can point me which one to use? If this is too complex, can you tell me what do I need to pay attention to?
Best