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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: programmable circuit breaker
Date: Wed, 05 Mar 2025 09:49:44 -0800
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On Wed, 5 Mar 2025 17:07:43 -0000 (UTC), bp@www.zefox.net wrote:

>john larkin <jl@650pot.com> wrote:
>> 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.
>
>The question seems to be how fast the current can rise to saturate
>the sensor. That would depend on the entire circuit.  It might
>be predictable in your case and so adequate for your situation.

I assume that a current overload will have zero risetime. A relay
contact closing can actually generate picosecond edges.

I'll be digitizing each Hall sensor current at 50k samples/second, and
probably doing some FPGA lowpass filtering. An RMS calculation
inherently lowpass filters.

>
>I'm not sure it'll recognize a fault induced by somebody probing
>the circuit and accidentally shorting something. That's when a 
>dI/dT or other independent sensor would be helpful. 
>
>bob prohaska
>

Don't do that.

https://www.dropbox.com/scl/fi/trdug87h0w498h94znv3x/Probe_Slips.jpg?rlkey=r5a22cx09twhsn9edq4c6es6f&raw=1