MDBT42 crashes with motor powered by its battery

Hi everyone,

I have a circuit with a PuckJS, a motor driver and a motor DC powered by the same CR2032 battery. I would like to replace the PuckJS with an MDBT42 but the MDBT42 crashes when the motor is powered. Everything works fine with PuckJS.

Does anyone know why ?

By the way the MDBT42 stop crashing if I add two big capacitors (1000uF 25V), one at board terminal, one at the battery terminal. But those capacitors are too big for the space I've got. I tried with smaller capacitors or just one but it doesn't work. I can't get why.

The motor draws 10-25mA.

Thanks a lot

2 Attachments

There is another thing :
with MDBT40, the motor is on by default, as if Digital Pins 28,29,30 were HIGH by default, while with Puck JS, motor is off by default, which seems to mean that Digital Pins 28,29,30 are LOW by default. Is it normal ?

The Vin pin of the MDBT42Q breakout goes to a regulator, and that regulator drops some voltage (don't want to look up right now, but probably couple hundred millivolts). If you use a battery (for example the CR2032) that's less than 3.6V, connect it to the 3.3 pin.

And not exactly what you asked, but a motor - I assume it's running not only for milliseconds - even at 10-20mA is huge for a CR2023.
You should try somewhat bigger battery. Watch out for it's voltage! If it's below 3.6V, connect to the 3.3 pin, if it's over that, connect to the Vin pin. For example, Li-ion, Li-poly must be connected to the Vin pin. LiFePo4, CR123A can be connected to the 3.3 pin.

Thanks Akos !

I connected battery to the 3.3 pin and it seems to be better indeed, 100uF capacitors is now enough to prevent crashing.

Any idea to improve this ? And why the puck doesn't have this issue ?

And yes you are right about the CR2032 max drain. Actually it drains 10-20mA just during 2 seconds and a very very few times. I will change battery to a special one allowing high drain but first I really would like to make the MDBT42 as stable as the Puck. I need the circuit to hold as much as possible with end-of-life batteries.

Sat 2021.05.15

from post #1
re: 'By the way the MDBT42 stop crashing if I add two big capacitors'

While I don't have that hardware to test, It is likely that:

As a motor is inductive and requires an initial massive draw to get going, that after the caps charged, there was sufficient current to start the motor turning. A coin cell isn't designed for high current spikes. see spec p.1 'Standard Continuous Discharge Current 0.3 mA'

re: 'but the MDBT42 crashes'
As E = I x R as current draw goes up, voltage goes down. It is likely that battery voltage fell too low for the MDBT42Q to operate.

I agree with @AkosLukacs at around ~260mAhr a CR2032 coin cell wont last that long

https://www.duracell.com/wp-content/uploads/2016/03/CR2032JCS0820.pdf

Sat 2021.05.15

re post #2: 'which seems to mean that Digital Pins 28,29,30 are LOW by default'

What is the pin mode set to?

http://www.espruino.com/Reference#l__global_pinMode
http://www.espruino.com/Reference#l__global_getPinMode

Sat 2021.05.15

re post #4: 'And why the puck doesn't have this issue ?'

Likely the regulator  U2   MCP1703T

See schematic link below heading 'Information'

http://www.espruino.com/MDBT42Q

See schematic ('Circuit Diagram') link(s) below heading 'Information'

http://www.espruino.com/Puck.js

That's better. I can only guess, but you are abusing that poor CR2032 battery, a couple of things come to my mind:
Battery itself. Do you test with the same battery? Or at least same brand, same state of charge? Might be simply the one with the MDBT42 breakout has less capacity, higher internal resistance, etc. Check the no-load battery voltage at least.
Battery holder and leads: Lead length can affect resistance and inductance. Might be enough for different behavior. Lead length from battery to the uC in case of the Puck is minimal, in case of the MDBT breakout I guess it's longer.

Do you have an oscilloscope to check voltages? Or if you don't have anything better, just sampling the voltage with any uC might help to get more info...

Edit: a capacitor with smaller voltage rating, that would be physically smaller. And maybe even a low-ESR small-ish capacity one could help...

Hi - I guess even if the motor draws 10-20mA normally, it's possible it takes a bit more that that for the first fraction of a second to get itself spinning?

I guess you're after something pretty small, but you can get pretty tiny LiPo batteries that would work great on the Breakout board's Vin pin (using the regulator). They'll also supply significant amounts of power so would easily handle your motor, and obviously they are rechargeable.

Also, as @AkosLukacs says, if you do want to use a capacitor, lower voltage ones tend to be smaller.

You could also look at surface mount capacitors. As you get to the bigger sizes they're pretty easy to hand solder. So for instance if you search for 4V surface mount capacitors there are a few around the 330uF size: https://www.mouser.co.uk/ProductDetail/Murata-Electronics/GRM32ER60G337ME05K?qs=qkDYIeTQ%252BEkpntsfMHJ5WA%3D%3D

That one's 3.2mm x 2.5mm - so you it's small enough to tuck away somewhere but big enough to solder without too much pain.