[Dprglist] [Dprg_roborama_committee] Circuit Breakers

Murray Altheim murray18 at altheim.com
Fri Nov 20 10:41:51 PST 2020

Hi John,

It's with some irony that my robot is currently dead because I was in
the process of rewiring it to add two INA260 current sensors to the
motors so that I could sense a deadlocked motor, and I've somehow got
the 5v line showing up on my 3.3v line, so the I2C bus won't work. The
Pi initially wouldn't boot, but now it does but the I2C bus shows up
empty using i2cdetect.

So a simple fuse or a circuit breaker would have been easier to wire
up, but wouldn't have given me the information to more intelligently
react to an over-current situation. And I also agree with your point
about reaction time. You'll likely have blown all your delicate
circuits by the time the fuse or circuit breaker blows. [On the video
conference we were talking about the fuse/circuit breaker solution
more as a safety mechanism for short circuits, but the point is still

Sensors are the thing. You don't want to have to use your fingers to
find out if a wire is hot, which is apparently how Carl does it. :-)



On 21/11/20 6:43 am, John Swindle via DPRGlist wrote:
> Ron and Carl make important points: Fuses and circuit breakers limit the overload, but the fault has occurred, and the fault likely caused some damage somewhere, maybe in a place that you can't see. As Ron said, with more experience and 
> more care, there are fewer faults. Someone taught me long ago that if I felt that I needed a circuit breaker instead of a fuse, then maybe I was not being careful enough. And three more observations: 1) Some circuits, such as a LED light 
> bulb that failed when it fell into an aquarium: When I tore into it, I found a sacrificial component that blew out much faster than either a fuse or a circuit breaker. Cheaper and faster than fuses. 2) Fuses always blow, at some point even 
> when never passing more than rated current, since they work by heating and deforming the link. Circuit breakers that accumulate also trip even when there is not a fault. 3) As Carl said, active protection is always better, though more 
> expensive. Voltage regulators, zeners, crowbar circuits, sacrificial semiconductor junctions, are all faster and safer than fuses and circuit breakers.
> John Swindle

Murray Altheim <murray18 at altheim dot com>                       = =  ===
http://www.altheim.com/murray/                                     ===  ===
                                                                    = =  ===
     In the evening
     The rice leaves in the garden
     Rustle in the autumn wind
     That blows through my reed hut.
            -- Minamoto no Tsunenobu

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