Looking for way to kill power to high current loads - Page 1
Forum section: electronics / projects

author: Mattjd
author_rank: Regular Contributor
timestamp: February 16, 2018, 10:10:43 pm
content: Hi, working on a quadcopter project at school. We have four motors right now, am thinking of going to six and their controllers need to be supplied with 25 volts and are said to supply a continuous 40 ampsthe controller in question ishttp://www.helipal.com/hobbywing-flyfun-40a-opto-esc.htmlSo the total supplied current is going be pretty high.The competition has a requirement of a kill switch. We're thinking of hooking up a relay to each controller and having 1 signal to kill all of them at once. I've been looking at automotive relays, and am looking for opinions.

author: Benta
author_rank: Super Contributor
timestamp: February 16, 2018, 10:18:58 pm
content: A "Kill Switch" for this kind of thing is not done with a relay. It normally needs to be a mechanical way to cut off power. On race cars, it can be a short cable with banana plugs on the outside that anyone (eg, resque personel) can pull.Something controlled over the RC equipment I wouldn't call a kill switch.

author: Mattjd
author_rank: Regular Contributor
timestamp: February 16, 2018, 10:29:04 pm
content: Understood, however I'm trying to explore a way to rc power to the quad copter bc in past we had to manually unplug the battery while rotors are still running and it's quite dangerous

author: Benta
author_rank: Super Contributor
timestamp: February 16, 2018, 10:43:36 pm
content: The idea of a kill switch is that it needs to work when everything else fails.This means it can't be RC controlled, because if you have massive interference it won't work.Infrared, perhaps?The switch itself need to be preloaded, so a weak impulse or a crash will release it.Lots to think about

author: Marco
author_rank: Super Contributor
timestamp: February 16, 2018, 10:56:29 pm
content: Maybe they don't trust their software not to lock up?The motors are brushless though, it's not like a shorted MOSFET will make them run continuously. Just disconnect the ESC's control wires, easier than trying to disconnect the power.

author: Benta
author_rank: Super Contributor
timestamp: February 16, 2018, 11:16:32 pm
content: Marco, it's akill switch.Nobody cares about the technology behind, it's there to allow a layman make the thing inoperable.Thatyouknow it's a brushless motor doesn't help, OK?

author: Marco
author_rank: Super Contributor
timestamp: February 16, 2018, 11:51:35 pm
content: Quote from: Benta on February 16, 2018, 11:16:32 pmHeh, internet tough guyYou might be having fun brain storming how to reliably get the kill switch signal in there, but I'd suggest not trying to command me what do discuss. It's antagonistic and futile, I only obey mods who do that.The laymen pressing the kill switch just wants the rotors to stop, whether internally disconnecting a couple of signal wires accomplishes that or trying to block a couple 100 Amperes is irrelevant to him.

author: Mattjd
author_rank: Regular Contributor
timestamp: February 17, 2018, 04:14:16 am
content: As I mentioned, putting the killswitch aside, does cutting the power to the fan controllers using automotive relays seem feasible? We'd like a way to turn on and off the copter through rc.

author: BradC
author_rank: Super Contributor
timestamp: February 17, 2018, 04:17:12 am
content: Quote from: Mattjd on February 17, 2018, 04:14:16 amFeasible but heavy.

author: Marco
author_rank: Super Contributor
timestamp: February 17, 2018, 04:53:11 am
content: A starter solenoid weighs well over a lb and takes amps of coil current to close. They aren't designed to run continuously, so no, it's not an option. You need a 300A+ MOSFET based switch if you really want to kill that power in a reasonable way.Just disconnecting the ESC control wires is much simpler though and the ESC continuing to drive the motor without control input is an unlikely failure mode.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 17, 2018, 04:56:29 am
content: Quote from: Benta on February 16, 2018, 10:43:36 pmhence activate "in-circuit" kill switch when there is no transmission detected for sometime. and then put another short cable on the out side in case anybody can manage to reach it while the copter is tumbling upside down with full power motor.this relaysare the lightest we can get we can parallel 5 of them for each ESC, but i dont like this mechanical idea in a portable and lightweight prioritized item like a quadcopter, it can take up many space, draw more current through their coils everytime. i would say the kill switch should be the mosfets in the ESC, make them as robust as they can, double the capacity etc and make it right in the copter's FW. a short cable on the outside as mechanical kill switch only for the organizer's entertainment.

author: Marco
author_rank: Super Contributor
timestamp: February 17, 2018, 05:01:29 am
content: Quote from: Mechatrommer on February 17, 2018, 04:56:29 amEven if they fail short the motor will stop, so their robustness isn't really relevant to their ability to kill the motor.

author: Mattjd
author_rank: Regular Contributor
timestamp: February 17, 2018, 04:13:26 pm
content: im looking into power mosfets to act as a switch, but am having trouble thinking of how to actually press the button.

author: Benta
author_rank: Super Contributor
timestamp: February 17, 2018, 10:15:35 pm
content: QuoteStay cool, it's nicer for everyone.

author: nctnico
author_rank: Super Contributor
timestamp: February 18, 2018, 12:50:08 am
content: Put a fuse in series with the battery (if it ain't there already then it is a good addition) and use a thyristor to simply short the power (and thus blow the fuse). Thyristors are very rugged devices and this way you can be sure the entire device is powered down.

author: Teledog
author_rank: Regular Contributor
timestamp: February 18, 2018, 01:59:37 am
content: Perhaps this unit..and haveitswitch a HD contact relay?https://hobbyking.com/en_us/turnigy-receiver-controlled-switch-1.html

author: Belrmar
author_rank: Regular Contributor
timestamp: February 18, 2018, 11:04:42 pm
content: Quote from: nctnico on February 18, 2018, 12:50:08 amWell having to replace a fuse each ime you land is going to get a bit annoying and expensive, i would try to check if the ESC's have some kind of power down pin that you can feed from the receiver... just brainstorming. If we dont want to get heavy i would try to go to less of a kill switch to more or less a forced shutdown

author: nctnico
author_rank: Super Contributor
timestamp: February 19, 2018, 12:26:54 am
content: Quote from: Belrmar on February 18, 2018, 11:04:42 pmI'd expect a kill switch to be an emergency device and not a regular shutdown (for that you can use a couple of fat MOSFETs).

author: Phoenix
author_rank: Frequent Contributor
timestamp: February 19, 2018, 01:13:07 am
content: Quote from: Mattjd on February 16, 2018, 10:10:43 pmIs 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).

author: Mattjd
author_rank: Regular Contributor
timestamp: March 06, 2018, 03:35:04 am
content: Quote from: Phoenix on February 19, 2018, 01:13:07 amright, 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 hereI 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 arePower MOSFEThttps://www.infineon.com/dgdl/irfh8202pbf.pdf?fileId=5546d462533600a40153561f7c351f0cGate Driverhttps://www.diodes.com/assets/Datasheets/ZXGD3005E6.pdfhttp://www.ti.com/lit/ds/symlink/dac081s101.pdfI 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.

author: xani
author_rank: Frequent Contributor
timestamp: March 06, 2018, 08:35:02 am
content: Quote from: Benta on February 16, 2018, 10:43:36 pmIt sure can, just not in usual way.Just make it react to two inputs. One is "kill switch signal". Other is "RF is down for more than ~2 seconds".Then if you lose RF for whatever reason, it will shut itself down, it ensures that there will be no case where engines have power but there is nothing to control them.Of course, then any interference will cause your quad to drop from the sky...

author: Rerouter
author_rank: Super Contributor
timestamp: March 06, 2018, 10:15:00 am
content: For most quadcopters, this would be called the failsafe state, e.g. loss of communication, or other weirdness, most recievers can be programmed for a failsafe state, so you could use an output to trip your kill switch and disconnect power from the motors, wire this in series with a physical kill switch of some kind on the quad (probably on the belly, my experience is its the safest place to grab a hovering quad)

author: Kalvin
author_rank: Super Contributor
timestamp: March 06, 2018, 10:50:44 am
content: Losing RF-signal and communication link due to interference or some reason should deactivate the power supply. Yes, you will be losing the copter as it falls down and may hit someone. Solenoid driven contactor ("relay") would be a simple solution. Pressing a kill-switch or losing the communication-based watchdog will de-energize the contactor and the power supply will be turned off. In order to energize the contactor one has to press a start button so the circuit may not energize itself after it has been shut down.

author: Ian.M
author_rank: Super Contributor
timestamp: March 06, 2018, 11:50:45 am
content: Why would you do that for the loss of signal failsafe state? If you kill the power with the craft out of ground effect, its certain damage and at any significant altitude or velocity, it will probably be destroyed.You should *NEVER* be flying the craft close enough to anyone (except the operator team), that unexpectedly cutting power so it drops out of the sky is the least worst choice to minimise the risk of injury. I'm not saying it shouldn't have the capability to disarm/shutdown on command in flight mode, just that it should be explicitly commanded as pilot judgement is required.If the LOS in flight is due to interference, the intelligent thing to do would be to initially auto-stabilise and adjust throttles to closely approximate hover power. If its got an altimeter enter hover mode, If its got a GPS (and its got lock), enter position hold mode. If you've got enough sensors to support auto-land, sustained LOS should probably trigger that with an option to trigger RTB mode for operating over water or other hostile terrain.OTOH if it is landed, LOS should cut the power + disarm.The problem with killing power is how to do it without making the craft much much larger and heavier, so making it more dangerous. If your ESCs have an accessible reset pin on their MCUs, and the ESC firmware hasn't disabled the reset pin, grounding all the reset pins will shut down all the ESCs, even if they've got a firmware bug. Unfortunately I don't think that's common.Many ESCs will cut power on prolonged loss of throttle signal, but that's relying on firmware so its not a true killswitch.Probably the best option is to route the main power through a pluggable link, with fairly low retention force and an accessible loop of cable sticking up, and have a 4' stick with a strong hook on the end to yank it out. There is some risk while arming it, but that can be mitigated if the landing gear has loops the stick can be passed through - put the stick right through the gear and stand on it and the craft isn't going anywhere while you plug the link in, then withdraw the stick.

author: Rerouter
author_rank: Super Contributor
timestamp: March 06, 2018, 12:09:50 pm
content: Ian.M It comes down to your local regulations somewhat, for Australia if the device lacks a return to home function, or position hold. it its intended to fall out of the sky on signal loss rather than continue in whatever direction its flying in, the assumption being that a small drone with stopped propellers is generally less dangerous than one flying forward at what could be maximum speed.For the DJI crowd, gps is baked in, and the hold or return options are met, for the racing group, a GPS reciever is weight that is generally ditched, with the controlled drop being preferred. not a landing, but a drop, if a propeller is partially damaged the drone can shoot off in 1 direction while trying to reach level without ever quite reaching it. leaving a motor kill / stall the only reliably safe optionAnd finally, racing drones due to there light weight mostly bounce on grassy areas undamaged, even on concrete you would rarely break a frame, maybe bend a motor and break some props, but you carry spares. I can say my own crash counter is in excess of 150+ on a 1.3kg drone some from almost 60m up, and have yet to break a frame of the core electronics, many props damaged, but still quickly repairable, the only crash i struggled to walk away from was top speed into a light pole (~135kmph). that bent a motor, trashed a camera, and broke a number of screws.

author: Mattjd
author_rank: Regular Contributor
timestamp: March 06, 2018, 02:56:34 pm
content: Quote from: Mattjd on March 06, 2018, 03:35:04 amany thoughts on my implementation remotely turning power on and off?

author: Marco
author_rank: Super Contributor
timestamp: March 06, 2018, 04:36:24 pm
content: 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.

author: Mattjd
author_rank: Regular Contributor
timestamp: March 06, 2018, 08:03:25 pm
content: 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.

author: Ian.M
author_rank: Super Contributor
timestamp: March 07, 2018, 01:26:43 am
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: March 07, 2018, 10:07:17 am
content: 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?

author: Dubbie
author_rank: Unknown
timestamp: March 07, 2018, 10:24:12 am
content: 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.

author: Mattjd
author_rank: Regular Contributor
timestamp: March 07, 2018, 03:52:47 pm
content: Quote from: Marco on March 07, 2018, 10:07:17 amFirst 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 ishttp://www.helipal.com/hobbywing-flyfun-40a-opto-esc.htmlI am having trouble finding a digital copy of the user manual to post, but will keep looking.found the user manualhttps://p11.secure.hostingprod.com/@hobbypartz.com/ssl/ibuyrc/manual/07E-FLYFUN.pdfThe receiver ishttps://hobbyking.com/en_us/turnigy-9x-2-4ghz-8ch-receiver-v2.html?gclid=Cj0KCQiAuP7UBRDiARIsAFpxiRKTz0vqq_E8koTLubu6loL47vah3CeMN-Xi_KSlmCT31VauqbNYUicaAjcJEALw_wcB&gclsrc=aw.ds&___store=en_usUser manualhttps://hobbyking.com/media/file/725056143X2037269X20.pdfWe 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.

author: Mattjd
author_rank: Regular Contributor
timestamp: March 08, 2018, 10:35:55 pm
content: Quote from: Dubbie on March 07, 2018, 10:24:12 amYou're speaking of arming/disarming correct? I've spoke with my teammates, about this. They still want something to cut mains manually.

author: Dubbie
author_rank: Unknown
timestamp: March 08, 2018, 11:10:37 pm
content: 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?

author: Mattjd
author_rank: Regular Contributor
timestamp: March 09, 2018, 01:26:07 am
content: Quote from: Dubbie on March 08, 2018, 11:10:37 pmThere is nothing wrong with the disarm command, but the teammates want a way to cut mains for some reason.