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Failed to connect to MySQL: (1203) User howardkn already has more than 'max_user_connections' active connectionsPath: ...!weretis.net!feeder9.news.weretis.net!news.quux.org!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail From: john larkin Newsgroups: sci.electronics.design Subject: Re: programmable circuit breaker Date: Wed, 05 Mar 2025 09:49:44 -0800 Organization: A noiseless patient Spider Lines: 80 Message-ID: <2b3hsj9vg2fu8ll1dk1827a6ce3rb7mni0@4ax.com> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Injection-Date: Wed, 05 Mar 2025 18:44:27 +0100 (CET) Injection-Info: dont-email.me; posting-host="45ad0d1f52f6e0482080b2035546685f"; logging-data="2656079"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/SU6jslIwzh+bYT+FQwec9" User-Agent: ForteAgent/8.00.32.1272 Cancel-Lock: sha1:ank4dSc0MOt+5SXM+eGk46SAGnc= Bytes: 4344 On Wed, 5 Mar 2025 17:07:43 -0000 (UTC), bp@www.zefox.net wrote: >john larkin wrote: >> On Wed, 5 Mar 2025 15:46:43 -0000 (UTC), bp@www.zefox.net wrote: >> >>>john larkin 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