Re: programmable circuit breaker

On Wed, 5 Mar 2025 15:46:43 -0000 (UTC), bp@www.zefox.net wrote:

john larkin <jl@650pot.com> wrote:

 
We're designing a modular power system and figured we should have a
relay module, and my PCB layout guy is fast so I decided to whip out a
simple module.
 
Featuritus kicks in. We normally measure voltages and currents, so the
next logical step is to make it a programmable circuit breaker too.
That also protects my relays and PCB traces, to some extent.
 
So how might a user program a circuit breaker? Just RMS current with
some time constant? Allow fast and slow trips?
 
Fuses are usually specified to trip at some I^2*T, but that can't be
the whole story, because 1 mA is a lot of I^2*T in ten years.
 
And my current sensor saturates. If the module is specified for 7.5
amps, and the 10-amp Hall sensor saturates a bit past 12 amps, so a
zillion amps looks like 12 so the I^2*T math doesn't work at, say, 30
amps.
 
Sine waves sort of work if they don't clip too hard. Luckily, sine
waves are kinda flat on top.
 
So I need a trip algorithm. That will be executed in an FPGA that sees
a fast ADC that is digitizing the Hall sensor output.
 

>
It seems essential to have enough headroom in  Hall sensor(s) to see
past the setpoint. Maybe two sensors, one for precise, long-duration
control and a second for transients with lower resolution. If the first
goes to zero, look at the second. If it's not zero too, trip.
>
One could also use dI/dT to anticipate things going wrong and using
that as a sort of "pre-warning" signal. This invites nuisance trips,
so it would require some amount of filtering. 
>
bob prohaska

I'm using

MONOLITHIC POWER    MCS1802GS-10-Z

partly because we have them in stock. It's officially a 10 amp part
and clips a bit above +-12 amps, so should be OK to protect a relay
channel rated for 7.5 amps.

I'm thinking of doing two RMS current calculations. A fast, maybe 1 ms
calc, with a fixed 10 amp trip. And a user-programmable trip, 1 amp to
8 amps, selectably fast or slow. Something like that.