Looking for way to kill power to high current loads

[Mattjd]

The competition has a requirement of a kill switch.

Is there any specific rule wording around what a "kill switch" is required to do? It's not a matter of what you or anyone else unfamiliar with the specifics of the competition think it should do, it's a matter of complying with the rules - and proving/arguing how your implementation meets it.

(I've been involved with electric racing car design).

right, so from what I've gathered a kill switch is not even needed.

So what I've been working on in my free time is a switch to turn power on and off to the motors.

A draw up of my plan is here

I plan on using a power mosfet being driven by a gate driver, the input to the gate driver will be from a DAC, taking in a 5.5v (currently unknown frequency) PWM signal from the quadcopter receiver. This will allow us to turn the quadcopter on and off using the transceiver.

My parts list are

Power MOSFET

https://www.infineon.com/dgdl/irfh8202pbf.pdf?fileId=5546d462533600a40153561f7c351f0c

Gate Driver

https://www.diodes.com/assets/Datasheets/ZXGD3005E6.pdf

http://www.ti.com/lit/ds/symlink/dac081s101.pdf

I believe my design is sound/valid, I just need to find the values of Rsource/Rsink from the gate driver and determine if I need a gate-source resistor too. I believe we will be getting the signals for ~SYNC and SCLK from a raspberry pi that we are also using in another system (not entirely sure though, one of the more knowledgeable quadcopter members still needs to confirm).

In the end, I'll have 1 DAC and gate driver, driving 4 power mosfets (ideally, of course 4x everything is still doable).

I'd love any and all criticisms.

any thoughts on my implementation remotely turning power on and off?

[Marco]

I still think you're designing for a very strange failure mode. You are designing for the failure mode where the motor controllers get stuck on while ignoring their control input, while the receiver is still operating. Realistically the likelihood of failure modes where the motors get stuck on are receiver>flight controller>>>>>motor controller.

Just cut the inputs to the motor controllers, it's still not the likeliest failure mode ... but at least you're not putting more resistance in the power path.

[Mattjd]

Idk if I am not being clear, but I DO NOT NEED A KILLSWITCH, I WAS MISTAKEN.

What I am now working on, is just a method to wirelessly control the power to motors. I believe my method is valid, and parts selected, I am looking for some criticism on it.

[Mattjd]

 

[Ian.M]

Switching the motor power is non-trivial.   An optimised MOSFET design with have a comparable mass per motor to the ESC, and waste a similar amount of energy dissipated as heat.   I'd go with Marco and disconnect the ESC throttle inputs (after checking they shutdown on sustained LOS), and let the MOSFETs on the ESC cut the motor power. 

[Marco]

I didn't want to respond any more, because it just got too antagonistic and you seem to feel me ignorant. With a little help from Ian maybe I'll might get through. What I said had nothing to do with it being a proper kill switch or not, it just had to do with how to cut power to the motors. Apart from Ian two more people suggested just using the ESC themselves to cut off the power.

Mechatrommer said : "switch should be the mosfets in the ESC"
Belrmar said : "i would try to check if the ESC's have some kind of power down pin that you can feed from the receiver."

I checked and the ESCs turn down power after 1 second of control loss and go to very low power 2 seconds after. If that's not sufficient, generating a low duty cycle PWM input for the ESCs just takes a single CMOS inverter IC and will cut the power entirely as long as the ESCs are responding to input (they do have firmware which can bug out, but their complexity is far lower than the flight controller so it's not likely).

Regardless of whether you use the ESCs or a MOSFET you will have to decide whether you want to latch the signal or not (ie. if you cut the engines, should it take power cycling to turn them back on). Also need to decide how to decode the PWM signal, you can do it with CMOS logic, resistors, capacitors and diodes. Or throw a small microcontroller at it.

The DAC isn't necessary. The MOSFET you chose is underpowered (6x40>100) and even at 100A it would need heatsinking.

PS. you can probably buy some modules which turn a servo signal into a binary signal too if you're lazy ... also don't you have a spare binary signal on the receiver by any chance?

[Dubbie]

What sort of flight controller do you have? On my quad, I can cut all power with the flick of a switch. No extra hardware needed. It just disarms the motors. I use it when I land. I don’t see why you are going off reinventing wheels here.

As others have pointed out, it’s impossible for the ESCs to spin the motors if they are not receiving control signals. Just cut that. It seems ludicrous to mess with switching the main supply.

[Mattjd]

I didn't want to respond any more, because it just got too antagonistic and you seem to feel me ignorant. With a little help from Ian maybe I'll might get through. What I said had nothing to do with it being a proper kill switch or not, it just had to do with how to cut power to the motors. Apart from Ian two more people suggested just using the ESC themselves to cut off the power.

Mechatrommer said : "switch should be the mosfets in the ESC"
Belrmar said : "i would try to check if the ESC's have some kind of power down pin that you can feed from the receiver."

I checked and the ESCs turn down power after 1 second of control loss and go to very low power 2 seconds after. If that's not sufficient, generating a low duty cycle PWM input for the ESCs just takes a single CMOS inverter IC and will cut the power entirely as long as the ESCs are responding to input (they do have firmware which can bug out, but their complexity is far lower than the flight controller so it's not likely).

Regardless of whether you use the ESCs or a MOSFET you will have to decide whether you want to latch the signal or not (ie. if you cut the engines, should it take power cycling to turn them back on). Also need to decide how to decode the PWM signal, you can do it with CMOS logic, resistors, capacitors and diodes. Or throw a small microcontroller at it.

The DAC isn't necessary. The MOSFET you chose is underpowered (6x40>100) and even at 100A it would need heatsinking.

PS. you can probably buy some modules which turn a servo signal into a binary signal too if you're lazy ... also don't you have a spare binary signal on the receiver by any chance?

First off, I'd like to thank everyone who is attempting to help me. Next, I wasn't accusing anyone of ignorance; if anyone is ignorant of the topic it is me. With that being said I felt like the discussion was digressing into what constitutes a valid kill switch, even after I claimed to have been mistaken on the requirements. 

I read the suggestions about the mosfet and looked into that; the design I posted is what i came up with from the mosfet suggestion.

I had planned on building 4 of those circuits (minus the dac), one for each motor/controller. I know the powet fets have multiple G/S but I am ignoring that.

Just a quick question, why wouldn't I have to run the PWM signal from the receiver into a DAC before feeding it into the gate driver?

Also, the ESC is

http://www.helipal.com/hobbywing-flyfun-40a-opto-esc.html

I am having trouble finding a digital copy of the user manual to post, but will keep looking.

found the user manual

https://p11.secure.hostingprod.com/@hobbypartz.com/ssl/ibuyrc/manual/07E-FLYFUN.pdf

The receiver is

https://hobbyking.com/en_us/turnigy-9x-2-4ghz-8ch-receiver-v2.html?gclid=Cj0KCQiAuP7UBRDiARIsAFpxiRKTz0vqq_E8koTLubu6loL47vah3CeMN-Xi_KSlmCT31VauqbNYUicaAjcJEALw_wcB&gclsrc=aw.ds&___store=en_us

User manual

https://hobbyking.com/media/file/725056143X2037269X20.pdf


We are using channel 1-4 for movement and need one of the additional channels for arm/disarm.

So if I am understanding the user manual correctly for the receiver, there are 9 distribution channels total and therefore 4 left after fulfilling the above requirements.

[Mattjd]

What sort of flight controller do you have? On my quad, I can cut all power with the flick of a switch. No extra hardware needed. It just disarms the motors. I use it when I land. I don’t see why you are going off reinventing wheels here.

As others have pointed out, it’s impossible for the ESCs to spin the motors if they are not receiving control signals. Just cut that. It seems ludicrous to mess with switching the main supply.

You're speaking of arming/disarming correct? I've spoke with my teammates, about this. They still want something to cut mains manually.

[Dubbie]

Do you mean physically without the radio connection working?

Or just over radio control?

I know I wouldn't approach an out of control drone that still had the motors turning under any circumstances.
and if the radio IS working, then what is wrong with the disarm command?

[Mattjd]

Do you mean physically without the radio connection working?

Or just over radio control?

I know I wouldn't approach an out of control drone that still had the motors turning under any circumstances.
and if the radio IS working, then what is wrong with the disarm command?

There is nothing wrong with the disarm command, but the teammates want a way to cut mains for some reason.