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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: programmable circuit breaker
Date: Wed, 5 Mar 2025 11:01:48 -0000 (UTC)
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piglet <erichpwagner@hotmail.com> 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.
>> 
>> 
> 
> Aren’t fuses specified by minimum carrying current? So a 1A fuse is
> guaranteed to carry 1A forever and reacts I^2t only above that.
> 
> I guess your current sensor should remain monotonic upto the maximum
> possible current your supply can generate?
> 
> 

Nah, that’s way too normal and prudent. ;)

First off, obviously you want a conventional fuse sized to prevent the
thing catching fire if something fails.  Having an upstream switching
supply kick off at about the right point helps a lot too. 

You can certainly estimate the dissipation when the sensor rails, but it
requires making some assumptions about the current waveform.  

For a start, I might try identifying anomalous cases e. g. an overvolted
transformer saturating, and cut those off when the sensor rails, regardless
of I**2t. 

If the load is linear- looking, it may be reasonable to assume the current
is sinusoidal, and curve-fit the part that stays onscale. 

I’ll also put in a plug for my second-favorite method for fast temperature
measurement: cut out a little niche inside the thermal pour, right next to
the active device. Put an 0603 thermistor with one end on the pour and the
other on a skinny trace to whatever you’re measuring with. 

If you avoid the temptation to something more convenient that looks
similar, this trick will give you 100-ms thermal time constants, which is
good enough for many interesting things. (Which possibly includes
protecting power FETs from melting before the fuse blows.)

Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics