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From: bp@www.zefox.net
Newsgroups: sci.electronics.design
Subject: Re: programmable circuit breaker
Date: Wed, 5 Mar 2025 15:46:43 -0000 (UTC)
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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