Re: programmable circuit breaker

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