Simple motor stalling failsafe?

[AleG]

I´m working on a little motor run timer for electric Free Flight models, and I thought about what would happen if the plane lands prematurely (not crashing, no sir, nothing of that sort! :p ) or gets stuck on a tree. Since there´s no radio control the motor would keep running until the timer ticks off, wether the prop is spinning or not...
A staled motor may be bad on the electronics, motor or even the battery, so anyone has a suggestion on how to make a small (read frigging light) simple failsafe circuit? I can´t think of anyting right now... :o

[Acetronics]

Hi,

Just use a 12v car fuse in series with the motor ... taking it off will secure when transport or handling.

Some other trick is to measure the voltage loss across the switching Mosfet ... and compare it to a preset value ... then, if needed, ground the gate signal.

Alain

[slipstick]

I've never yet had a problem. Since the timed run on most of my stuff is not much more than 30 seconds even if the motor is stalled for a fair amount of that time I seem to get away with it.

OTOH it depends what sort of timer you're making. If you already have a microcontroller in there you could perhaps do something with current sensing. A low ohm resistor doesn't weigh much or you might even be able to check the battery voltage drop.

I mainly use simple R/C/MOSFet timers and there's nothing I can think of you could do there.

Steve

[AleG]

Hi,

Just use a 12v car fuse in series with the motor ... taking it off will secure when transport or handling.

Some other trick is to measure the voltage loss across the switching Mosfet ... and compare it to a preset value ... then, if needed, ground the gate signal.

Alain

heh, a 12V car fuse would probably weight more than the motor itself... :D
The second one is more interesting, but as Slipstick says that probably would require some micro controller to make the measurement.

I've never yet had a problem. Since the timed run on most of my stuff is not much more than 30 seconds even if the motor is stalled for a fair amount of that time I seem to get away with it.

OTOH it depends what sort of timer you're making. If you already have a microcontroller in there you could perhaps do something with current sensing. A low ohm resistor doesn't weigh much or you might even be able to check the battery voltage drop.

I mainly use simple R/C/MOSFet timers and there's nothing I can think of you could do there.

Steve

The timer I´m working on uses the timeless (drum roll) 555, it actually works at 2.7V, go figure, so it´s perfect for a single cell electric plane. I guess that if I use a MOSFET with a higher amperage than nominally needed I shouldn´t have magic smoke releases...

*Sigh*... But I guess now I´ll have to get started on microcontrollers to put all the features I´d like on a timer. ;)

Off to start a new thread...

[Dan Baldwin]

You might be able to make your own fuse by using a single strand of a stranded small size wire. How much current does your system draw?

Dan

[Acetronics]

Hi, Aleg


heh, a 12V car fuse would probably weight more than the motor itself...
The second one is more interesting, but as Slipstick says that probably would require some micro controller to make the measurement.


The best I can offer you is a RAYCHEM " polyswitch " or equiv. : size of a cercap ( Thru hole model ! Lol ) and current goes to .9 to 5 Amps nominal for the RUE ( low resistance) series ...

once "triggered" keeps circuit open, if you cut the power will reset by itself ...

I use it for my gear retracts small e-motors ...

For Mosfet solution, It only needs a '393 comparator ( open collector output needed ...) and a couple of resistors ...

Alain

PS : When I began with, free flight meant "without RC" .... but size and weight were quite the same as R/C ....