Smoke Stopper

  • todo

    • cheaper than COTS efuse board for initial power up

  • use case

    • ground validation

  • why

    • maybe cheaper than COTS

    • fun

  • who

    • unassigned

  • timeline

    • no efs effort

    • deprioritized for now

 

OTS Smokestopper:

image-20240529-221459.png
image-20240529-221551.png

Features (both present in OTS and this project):

  • Customizable current threshold (1A, 2A and 3A - not in OTS)

  • Customizable current shut-off delay (not really clear if these are exact or maximum delays in the OTS device)

  • The delay is set using a pushbutton,

    • where holding it puts the device in a configuration mode, and the LED blinks accordingly to its currently configured delay mode (1 blink = quickest delay, …, 3 blinks = slowest) ,

    • then the amount of consecutive button clicks that follow configure the mode (one click sets it to smallest delay),

    • holding the button again takes it out of this configuration mode, and the mode is saved even after power cycling.

  • My guess is that they had to use some kind of digital logic circuits, non-volatile memory, and comparator:

    • A CPLD, with a clock, probably listens to the button presses, and programs an EEPROM chip based on the user input.

    • The CPLD also, from the EEPROM configuration, determines how long to respond after receiving a signal from a comparator that signifies an overcurrent event.

    • The CPLD responds to the overcurrent event (after the delay) by outputting a signal to the over-current protection circuitry (e.g. switch driver).

    • Since CPLDs are cheap in bulk (programmers are fairly expensive - one time purchase), this could be done fairly inexpensively.

  • Fault indicator LEDs

  • Manual ON/OFF/RESET switches

  • $12.99 VIFLY, >$40 on Amazon

Block Diagram:

Current Switch:

Current Measurement/Sense:

  • High-side current sense IC (needs to be supplied by battery; max common mode input is VCC), 360uA supply current. $3.77 MPQ8112AGJ-AEC1-Z Monolithic Power Systems Inc. | Integrated Circuits (ICs) | DigiKey

  • But we can also use low-side current sense. A flaw with low-side sense is that ground faults cannot be detected. However, in our configuration, they seem highly unlikely. The smoke-stopper’s current switch will be in between drone circuitry and the battery’s ground, so for a ground fault to occur, the circuitry will need to bypass the entire smokestopper, which is unlikely:

  • Additionally, in this architecture, adding a small resistance between the load’s ground and supply ground has no worrisome effect (unlike with a load on a PCB, where adding a resistance between its ground pins and the ground plane can cause EMI problems and/or issues with how components, that are connected directly to supply ground, interface with this load - because their grounds have a potential difference). This doc mentions these problems of low-side sense, which aren’t present in this scenario.

  • Current Sense IC, 20x Gain, Built-in comparator, latching output functionality, 10us fault response, $0.89 INA381A1IDSGR Texas Instruments | Integrated Circuits (ICs) | DigiKey

    • Because its common mode voltage rating is maxed out at 26V, in the case that the load is a complete short, it’s likely beneficial to add a zener diode to clamp the voltage at the sense pins to prevent damage. This zener diode will also protect the shunt resistor.

    • Latching output:

  • 50mOhm shunt resistor WSLP1206R0500FEA Vishay Dale | Resistors | DigiKey

    • Worst case, will dissipate 0.45W, so I chose 1W; high availability; nice value of 50mOhms

    • Current sense amp will ouput 1V/1A, which works great if we power the circuit with 5V.

Reverse Voltage Protection:

Low Voltage Power Supply:

  • Will use a 5V supply, mainly due to the current sense circuitry (nominal supply voltage of the amp is 5V)

  • Linear Regulator (instead of switching because low noise - for sense circuitry, and circuit consumes barely any power, will show in power analysis here) (query)
    Might cascade regulators to reduce power dissipated per regulator (if heating is an issue). Could also put diodes (or power resistor) in series with regulator input to reduce drop-out voltage, and consequently power dissipated by regulator.

Response Delay Circuitry:

  • RC circuit at INA381’s current sense output line

  • Counter + Oscillator circuit (counter query, osc query) - Probably overkill, too expensive, and too power consuming for a linear regulator.

    • when certain counter value is obtained (which counts only when amp output reaches some threshold value → need an external comparator for this), comparator is tripped (after a fixed amount of time, which can be either programmed, or comparator looks at a different counter bit for a different delay: i.e. less significant bit for smaller delay, more significant for longer delay)

Status Notifier LEDs:

  • Fault LED (RED)

    • Red when RST switch is closed and when overcurrent fault is detected

  • ON LED (GREEN)

    • Green when RST switch is open and when no overcurrent fault is present

  • Both of the LEDs above will only be on if the 5V regulator is working, so they can be used as power indicators.

  • APHB1608LCGKSURKC Kingbright | Optoelectronics | DigiKey (Red and Green LEDs in one package)

  • Separate red & green LEDs.

  • The two-in-one comparator can drive them in the following configuration

Switches

 

 

 

POWER ANALYSIS

As explained in the power-supply section, the 5V linear regulator should output a maximum of 5.9mA.

  • INA381 (current sense chip): 450uA

  • Threshold Current Dividers and pull-up resistors: max 2mA

  • Comparator Quiescent Current: max 50uA

  • LEDs: 5.9-(the above) = 4.4mA max

    • As only 2 LEDs are needed, and only one should be powered on at once, the 2mA LEDs should work great being powered off of the 5V regulator.

 

COST ANALYSIS

For buying every component (not including the majority of passives or the XT connectors) at the per-unit price, the cost is $7.22 per board. For reference, OTS is $12.99/board. This seems pretty reasonable.

 

P/N
Part
Purpose
Qty
Buying Qty
Link
 $/unit
$/10
$/25
$/50
$/100
Total $
Main Components
BUK9M42-60EX
MOSFET N-CH 60V 22A LFPAK33
Current Switch
1
1
Digikey
0.87
0.748

 

 

0.5177
0.87
INA381A1IDSGR
IC CURRENT SENSE 1% 8WSON
Current Sense
1
1
Digikey
0.63
0.552
0.5192

 

0.4236
0.63
WSLP1206R0500FEA
RES 0.05 OHM 1% 1W 1206
Current Sense Res
1
1
Digikey
0.6
0.479
0.4328
0.3898
0.322
0.6
GSFD500P10
MOSFET, P-CH, SINGLE, -30.00A
Reverse-Voltage
1
1
Digikey
0.98
0.855

 

 

0.5914
0.98
MM5Z12VT1G
DIODE ZENER 12V 500MW SOD523
Rev-Voltage Zener
1
1
Digikey
0.32
0.217

 

 

0.106
0.32
ZXTR2005Z-13
IC REG LINEAR 5V 38MA SOT89
Lin Regulator
1
1
Digikey
0.82
0.7
0.6536

 

0.5232
0.82
AZV3002S-13
IC COMPARATOR 2 GEN PUR 8SO
Comparator
1
1
Digikey
0.86
0.735
0.6868

 

0.5493
0.86
APHB1608LCGKSURKC
LED GREEN/RED CLEAR 4SMD
Status LEDs
1
1
Digikey
0.71
0.436

 

 

0.2507
0.71
SLW-826947-5A-RA-D
SWITCH SLIDE DPDT 0.3A 50V
ON/RESET Switch
1
1
Digikey
0.65
0.63
0.6072

 

0.5258
0.65
SLW-1516255-6A-RA-D
SWITCH SLIDE SP3T 0.3A 50V
Delay Switch
1
1
Digikey
0.78
0.749
0.7212

 

0.6244
0.78
Other Passives

 

 

5k Overvoltage Pulldown
1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Total
 
 
 
 
 
 
 
 
 
 
 
7.22