Help identify this mystery "S" pad on hall sensor PCB

[chuyskywalker]

So, I've got these 8.5" hoverboard motors and they're pretty bog standard as far as direct drive hub motors go; 3 phase, 3 halls, and a 5v pos/negative. However, there is mystery white wire coming off the motor as well.



Now I follow it inside, and it's hooked up to this "S" pad. Now, you might think "S for Speed" -- but that'd be kinda...dumb. This is a direct drive motor, each hall signal is a speed sensor if you know the pole count. So that doesn't make much sense.



The next, most common, reasonable guess would be that it's a temperature sensor. Dunno why you'd label that "S" (maybe "S"ensor?). So, you try to see what the PCB leads to and...



What in tarnation is going on here?

In every motor I've dealt with, temp is done with thermistors (PTC/NTC/whatever) which is just a heat driven resistor. If this is a temp sensor, how the heck does this work and what even is it doing?

Or, am I wildly off and it's neither speed nor temp but something else entirely I've never seen before?

What the heck am I looking at here?

(The trace that the white wire eventually connects to is ground)

 

[amberwolf]

Best guess from a google search for SMT SOT23 G1 is that those are transistors being used as diodes, or they're actually diodes, and since temperature can affect voltage drop. So the board in the controller probably puts a known current thru them, and watches for the voltage drop across them, measures that and calculates the temperature from that.

It might not actually measure temperature, but simply use them as an overheat switch; once voltage drop on them reaches some specific point it triggers an input to the MCU that shuts the controller down. There's easier ways to do that, but this is tiny and very cheap.


One of the datasheets here might be the right one:

Marking of electronic components, SMD Codes G1, G1**, G1***, G1-, G1-***, G11, G15N60, G1=**, G1=***, G1F, G1W, G1p, G1t. Datasheets BC847B, BFS20, DTB113ZK, EMG1, EUP7965-18VIR1, FMG1A, KST5551, MMBT5551, RT9161A-16PX, RT9166-33GVL, RT9166-33PVL, RT

Marking of electronic components, SMD Codes G1, G1**, G1***, G1-, G1-***, G11, G15N60, G1=**, G1=***, G1F, G1W, G1p, G1t. Datasheets BC847B, BFS20, DTB113ZK, EMG1, EUP7965-18VIR1, FMG1A, KST5551, MMBT5551, RT9161A-16PX, RT9166-33GVL, RT9166-33PVL, RT9715BGQW, SGB15N60, SGP15N60, SGW15N60...

www.s-manuals.com

A speed sensor would face magnets on the motor shell, not the windings, but a temperature sensor would face the windings, especially if it's an emergency shutdown limit.

 

[chuyskywalker]

I mean...if it works, it works. And I suppose if you can have the machines pick-n-place this for you, it save some manual assembly time of having to hand-place a thermistor into the motor. Still wild.

 

[AGS]

Reference diodes used a temperature sensors is my guess.