Re: programmable circuit breaker

john larkin <jl@glen--canyon.com> wrote:

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.
>

 
It'd be interesting to know what the device does when exposed to a
burst of out of band AC. Figure 9 implies a well-behaved  rolloff,
for which 50ksamples/sec is likely to reveal an overcurrent trend
before saturation. If that's true the warning ought to be adequate.

>
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

8-)

If you're saying probe slips are beyond the scope of your design
it's understood and accepted. Are you anticipating a manual reset?

bob prohaska