Gentlemen,

I want to monitor current drawn from a car battery (12v lead-acid) in
such a way as when it drops below 200mA, a piezzo buzzer sounds. The
range of current the monitor will see will run from 15A to around
75mA. The 15A will last for only around 10s or so and once it settles
down will drop to 240mA for hours on end. This is just to monitor
quiescent current drain and there will be no cranking or anything
heavy to worry about.
What's the simplest way to do this?

- CD

On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:
> Gentlemen,
>
> I want to monitor current drawn from a car battery (12v lead-acid) in
> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
> range of current the monitor will see will run from 15A to around
> 75mA. The 15A will last for only around 10s or so and once it settles
> down will drop to 240mA for hours on end. This is just to monitor
> quiescent current drain and there will be no cranking or anything
> heavy to worry about.
> What's the simplest way to do this?
>
> - CD

Just measure the voltage drop from the battery to some point down from the main cable. If necessary, at a small resistor. But the cable itself might be enough to be detectable.

On Fri, 14 May 2021 15:46:49 -0700 (PDT), Ed Lee
<edward.ming.lee@gmail.com> wrote:

>On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:
>> Gentlemen,
>>
>> I want to monitor current drawn from a car battery (12v lead-acid) in
>> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>> range of current the monitor will see will run from 15A to around
>> 75mA. The 15A will last for only around 10s or so and once it settles
>> down will drop to 240mA for hours on end. This is just to monitor
>> quiescent current drain and there will be no cranking or anything
>> heavy to worry about.
>> What's the simplest way to do this?
>>
>> - CD
>
>Just measure the voltage drop from the battery to some point down from the main cable. If necessary, at a small resistor. But the cable itself might be enough to be detectable.

I can't see that working with a current source of such low internal
resistance - and *thick* cabling.

In article <090481af-215b-40ce-9bb6-b4cf282007e5n@googlegroups.com>,
edward.ming.lee@gmail.com says...
>
> Just measure the voltage drop from the battery to some point down from the main cable. If necessary, at a small resistor. But the cable itself might be enough to be detectable.
>
>

Yse that method to measure the current and one of the Arduino processors
to measure that drop and activate a buzzer, or just use a speaker and
let the Arduino generate the audio tone.

On Friday, May 14, 2021 at 3:56:59 PM UTC-7, Cursitor Doom wrote:
> On Fri, 14 May 2021 15:46:49 -0700 (PDT), Ed Lee
> <edward....@gmail.com> wrote:
>
> >On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:
> >> Gentlemen,
> >>
> >> I want to monitor current drawn from a car battery (12v lead-acid) in
> >> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
> >> range of current the monitor will see will run from 15A to around
> >> 75mA. The 15A will last for only around 10s or so and once it settles
> >> down will drop to 240mA for hours on end. This is just to monitor
> >> quiescent current drain and there will be no cranking or anything
> >> heavy to worry about.
> >> What's the simplest way to do this?
> >>
> >> - CD
> >
> >Just measure the voltage drop from the battery to some point down from the main cable. If necessary, at a small resistor. But the cable itself might be enough to be detectable.
> I can't see that working with a current source of such low internal
> resistance - and *thick* cabling.

There is still some voltage drop. You just need a very sensitive detection circuit.

On 2021-05-14, Cursitor Doom <cd@nowhere.com> wrote:
> Gentlemen,
>
> I want to monitor current drawn from a car battery (12v lead-acid) in
> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
> range of current the monitor will see will run from 15A to around
> 75mA. The 15A will last for only around 10s or so and once it settles
> down will drop to 240mA for hours on end. This is just to monitor
> quiescent current drain and there will be no cranking or anything
> heavy to worry about.
> What's the simplest way to do this?

wind some 15A magnet wire round a reed switch until you get the right
threshold.

--
Jasen.

On Fri, 14 May 2021 23:38:22 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

>Gentlemen,
>
>I want to monitor current drawn from a car battery (12v lead-acid) in
>such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>range of current the monitor will see will run from 15A to around
>75mA. The 15A will last for only around 10s or so and once it settles
>down will drop to 240mA for hours on end. This is just to monitor
>quiescent current drain and there will be no cranking or anything
>heavy to worry about.
>What's the simplest way to do this?
>
>- CD

A 10 mohm resistor, or equivalent wire, will drop 2 mV at 200 mA. You
can pick that off with a comparator or a rrio opamp used as a
comparator.

It drops 150 mV at 15A and dissipates about 2 watts.

Something like that.

A 15 amp schottky diode would give a better voltage range but get
hotter.

On Friday, May 14, 2021 at 4:49:06 PM UTC-7, John Larkin wrote:
> On Fri, 14 May 2021 23:38:22 +0100, Cursitor Doom <c...@nowhere.com>
> wrote:
> >Gentlemen,
> >
> >I want to monitor current drawn from a car battery (12v lead-acid) in
> >such a way as when it drops below 200mA, a piezzo buzzer sounds. The
> >range of current the monitor will see will run from 15A to around
> >75mA. The 15A will last for only around 10s or so and once it settles
> >down will drop to 240mA for hours on end. This is just to monitor
> >quiescent current drain and there will be no cranking or anything
> >heavy to worry about.
> >What's the simplest way to do this?
> >
> >- CD
> A 10 mohm resistor, or equivalent wire, will drop 2 mV at 200 mA. You
> can pick that off with a comparator or a rrio opamp used as a
> comparator.

FYI, gauge 1 wire, 1/3" diameter is around 100 mohm per foot. Small but measurable.

>
> It drops 150 mV at 15A and dissipates about 2 watts.
>
> Something like that.
>
> A 15 amp schottky diode would give a better voltage range but get
> hotter.

On Fri, 14 May 2021 16:56:21 -0700 (PDT), Ed Lee
<edward.ming.lee@gmail.com> wrote:

>On Friday, May 14, 2021 at 4:49:06 PM UTC-7, John Larkin wrote:
>> On Fri, 14 May 2021 23:38:22 +0100, Cursitor Doom <c...@nowhere.com>
>> wrote:
>> >Gentlemen,
>> >
>> >I want to monitor current drawn from a car battery (12v lead-acid) in
>> >such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>> >range of current the monitor will see will run from 15A to around
>> >75mA. The 15A will last for only around 10s or so and once it settles
>> >down will drop to 240mA for hours on end. This is just to monitor
>> >quiescent current drain and there will be no cranking or anything
>> >heavy to worry about.
>> >What's the simplest way to do this?
>> >
>> >- CD
>> A 10 mohm resistor, or equivalent wire, will drop 2 mV at 200 mA. You
>> can pick that off with a comparator or a rrio opamp used as a
>> comparator.
>
>FYI, gauge 1 wire, 1/3" diameter is around 100 mohm per foot. Small but measurable.

100 mohms per 1000 feet.

#10 is 1 mohm/foot.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

jlarkin@highlandsniptechnology.com wrote:
> On Fri, 14 May 2021 16:56:21 -0700 (PDT), Ed Lee
> <edward.ming.lee@gmail.com> wrote:
>
>> On Friday, May 14, 2021 at 4:49:06 PM UTC-7, John Larkin wrote:
>>> On Fri, 14 May 2021 23:38:22 +0100, Cursitor Doom <c...@nowhere.com>
>>> wrote:
>>>> Gentlemen,
>>>>
>>>> I want to monitor current drawn from a car battery (12v lead-acid) in
>>>> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>>>> range of current the monitor will see will run from 15A to around
>>>> 75mA. The 15A will last for only around 10s or so and once it settles
>>>> down will drop to 240mA for hours on end. This is just to monitor
>>>> quiescent current drain and there will be no cranking or anything
>>>> heavy to worry about.
>>>> What's the simplest way to do this?
>>>>
>>>> - CD
>>> A 10 mohm resistor, or equivalent wire, will drop 2 mV at 200 mA. You
>>> can pick that off with a comparator or a rrio opamp used as a
>>> comparator.
>>
>> FYI, gauge 1 wire, 1/3" diameter is around 100 mohm per foot. Small but measurable.
>
>
> 100 mohms per 1000 feet.
>
> #10 is 1 mohm/foot

Plus the AWG numbers correspond to amplitude in decibels--20 AWG is 10
mohm/foot.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Saturday, May 15, 2021 at 8:38:30 AM UTC+10, Cursitor Doom wrote:
> Gentlemen,
>
> I want to monitor current drawn from a car battery (12v lead-acid) in
> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
> range of current the monitor will see will run from 15A to around
> 75mA. The 15A will last for only around 10s or so and once it settles
> down will drop to 240mA for hours on end. This is just to monitor
> quiescent current drain and there will be no cranking or anything
> heavy to worry about.
> What's the simplest way to do this?

Probably a Hall effect sensor.

https://www.allegromicro.com/en/insights-and-innovations/technical-documents/hall-effect-sensor-ic-publications/allegro-hall-effect-sensor-ics

It may pay you to form the lead into a single loop, and wrap it around a chunk of ferrite so that you have a well-defined magnetic circuit with a place where you can stick the Hall effect sensor and detect a predictable magnetic field. This is much the same idea as using a reed-relay, but the hysteresis
will be a lot smaller.

Getting something like this to work under the hood of a car (where temperatures can get quite high) could be difficult. but this doesn't seem to be what Curistor Doom has in mind. Sounds more like something criminal (like brewing up illegal drugs).

--
Bill Sloman, Sydney

On Fri, 14 May 2021 20:50:01 -0700 (PDT), Bill Sloman
<bill.sloman@ieee.org> wrote:

>On Saturday, May 15, 2021 at 8:38:30 AM UTC+10, Cursitor Doom wrote:
>> Gentlemen,
>>
>> I want to monitor current drawn from a car battery (12v lead-acid) in
>> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>> range of current the monitor will see will run from 15A to around
>> 75mA. The 15A will last for only around 10s or so and once it settles
>> down will drop to 240mA for hours on end. This is just to monitor
>> quiescent current drain and there will be no cranking or anything
>> heavy to worry about.
>> What's the simplest way to do this?
>
>Probably a Hall effect sensor.
>
>https://www.allegromicro.com/en/insights-and-innovations/technical-documents/hall-effect-sensor-ic-publications/allegro-hall-effect-sensor-ics
>
>It may pay you to form the lead into a single loop, and wrap it around a chunk of ferrite so that you have a well-defined magnetic circuit with a place where you can stick the Hall effect sensor and detect a predictable magnetic field. This is much the same idea as using a reed-relay, but the hysteresis

Alternately, a C-clip of silicon steel sheet material,
with a hall sensor located between the butt ends will
give an indication. Threshold adjustible by varying
distance between clip ends (plastic screw).

.. . . . although I've only ever used this to trigger on
high current levels. It's surprisingly fast, if needs be.

RL

On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:

> I want to monitor current drawn from a car battery (12v lead-acid) in
> such a way as when it drops below 200mA, a piezzo buzzer sounds.

> What's the simplest way to do this?

It's called a sensitive relay. A few wraps of wire around a bar, and a
weak-spring set of points, with a fine adjust screw, can be set to trip
at the current of your choice.

This presumes that you can wire it in series with the battery, of course.
Possibly, you can adjust a permanent magnet and use a reed switch (like in
home security systems) to accomplish the same effect.

On Fri, 14 May 2021 22:17:31 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

>On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:
>
>> I want to monitor current drawn from a car battery (12v lead-acid) in
>> such a way as when it drops below 200mA, a piezzo buzzer sounds.
>
>> What's the simplest way to do this?
>
>It's called a sensitive relay. A few wraps of wire around a bar, and a
>weak-spring set of points, with a fine adjust screw, can be set to trip
>at the current of your choice.
>
>This presumes that you can wire it in series with the battery, of course.
>Possibly, you can adjust a permanent magnet and use a reed switch (like in
>home security systems) to accomplish the same effect.

Sounds similar to Jason's idea, which had an elegant simplicity about
it. This will just be for a one-off quick and dirty work-around, so
doesn't warrant the investment of much development.

On Fri, 14 May 2021 20:50:01 -0700 (PDT), Bill Sloman
<bill.sloman@ieee.org> wrote:

>On Saturday, May 15, 2021 at 8:38:30 AM UTC+10, Cursitor Doom wrote:
>> Gentlemen,
>>
>> I want to monitor current drawn from a car battery (12v lead-acid) in
>> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>> range of current the monitor will see will run from 15A to around
>> 75mA. The 15A will last for only around 10s or so and once it settles
>> down will drop to 240mA for hours on end. This is just to monitor
>> quiescent current drain and there will be no cranking or anything
>> heavy to worry about.
>> What's the simplest way to do this?
>
>Probably a Hall effect sensor.
>
>https://www.allegromicro.com/en/insights-and-innovations/technical-documents/hall-effect-sensor-ic-publications/allegro-hall-effect-sensor-ics
>
>It may pay you to form the lead into a single loop, and wrap it around a chunk of ferrite so that you have a well-defined magnetic circuit with a place where you can stick the Hall effect sensor and detect a predictable magnetic field. This is much the same idea as using a reed-relay, but the hysteresis
> will be a lot smaller.

Which is basically Jason Bett's idea re-badged as your own.

>
>Getting something like this to work under the hood of a car (where temperatures can get quite high) could be difficult. but this doesn't seem to be what Curistor Doom has in mind. Sounds more like something criminal (like brewing up illegal drugs).

That earns you another .sig append.

--

Britain: do the right thing and hand Australia back to China.

On Saturday, May 15, 2021 at 9:19:04 PM UTC+10, Cursitor Doom wrote:
> On Fri, 14 May 2021 20:50:01 -0700 (PDT), Bill Sloman
> <bill....@ieee.org> wrote:
> >On Saturday, May 15, 2021 at 8:38:30 AM UTC+10, Cursitor Doom wrote:
> >> Gentlemen,
> >>
> >> I want to monitor current drawn from a car battery (12v lead-acid) in
> >> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
> >> range of current the monitor will see will run from 15A to around
> >> 75mA. The 15A will last for only around 10s or so and once it settles
> >> down will drop to 240mA for hours on end. This is just to monitor
> >> quiescent current drain and there will be no cranking or anything
> >> heavy to worry about.
> >> What's the simplest way to do this?
> >
> >Probably a Hall effect sensor.
> >
> >https://www.allegromicro.com/en/insights-and-innovations/technical-documents/hall-effect-sensor-ic-publications/allegro-hall-effect-sensor-ics
> >
> >It may pay you to form the lead into a single loop, and wrap it around a chunk of ferrite so that you have a well-defined magnetic circuit with a place where you can stick the Hall effect sensor and detect a predictable magnetic field. This is much the same idea as using a reed-relay, but the hysteresis
> > will be a lot smaller.
>
> Which is basically Jason Bett's idea re-badged as your own.

Cursitor Doom seems to be in there competing with John Doe and Flyguy to post the most idiotic misapprehension.

An Allegro hall effect sensor is a rather different kind of magnetic field sensor from a reed relay, and a much less cranky one.
> >Getting something like this to work under the hood of a car (where temperatures can get quite high) could be difficult. but this doesn't seem to be what Curistor Doom has in mind. Sounds more like something criminal (like brewing up illegal drugs).
>
> That earns you another .sig append.

So who cares why a psychotic idiot ventilates his idiocies? All we know is that you like to be obnoxious, and it does seem to be only area where you demonstrate any competence at all.

--
Bill Sloman, Sydney

On Sat, 15 May 2021 12:18:59 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

>On Fri, 14 May 2021 20:50:01 -0700 (PDT), Bill Sloman
><bill.sloman@ieee.org> wrote:
>
>>On Saturday, May 15, 2021 at 8:38:30 AM UTC+10, Cursitor Doom wrote:
>>> Gentlemen,
>>>
>>> I want to monitor current drawn from a car battery (12v lead-acid) in
>>> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>>> range of current the monitor will see will run from 15A to around
>>> 75mA. The 15A will last for only around 10s or so and once it settles
>>> down will drop to 240mA for hours on end. This is just to monitor
>>> quiescent current drain and there will be no cranking or anything
>>> heavy to worry about.
>>> What's the simplest way to do this?
>>
>>Probably a Hall effect sensor.
>>
>>https://www.allegromicro.com/en/insights-and-innovations/technical-documents/hall-effect-sensor-ic-publications/allegro-hall-effect-sensor-ics
>>
>>It may pay you to form the lead into a single loop, and wrap it around a chunk of ferrite so that you have a well-defined magnetic circuit with a place where you can stick the Hall effect sensor and detect a predictable magnetic field. This is much the same idea as using a reed-relay, but the hysteresis
>> will be a lot smaller.
>
>Which is basically Jason Bett's idea re-badged as your own.

The trick is to get a reasonably accurate trip point that is about 1%
of the maximum current.

Hall sensors are not very temperature stable and have tiny outputs. 15
amps will heat it up. The magnetic circuit could easily be magnetized
by the big current and shift the threshold.

Wrapping 15 amps worth of wire around a reed, and tripping at 200 mA,
would be interesting too. Reed relays typically get warm at operating
current, much less at 100x operating current. The 15 amps could
magnetize the reeds so that they never open.

The series schottky diode takes care of the dynamic range problem, at
the cost of maybe half a volt at 15 amps. Or just use a shunt resistor
and some good electronics.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On Sat, 15 May 2021 12:15:57 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

>On Fri, 14 May 2021 22:17:31 -0700 (PDT), whit3rd <whit3rd@gmail.com>
>wrote:
>
>>On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:
>>
>>> I want to monitor current drawn from a car battery (12v lead-acid) in
>>> such a way as when it drops below 200mA, a piezzo buzzer sounds.
>>
>>> What's the simplest way to do this?
>>
>>It's called a sensitive relay. A few wraps of wire around a bar, and a
>>weak-spring set of points, with a fine adjust screw, can be set to trip
>>at the current of your choice.
>>
>>This presumes that you can wire it in series with the battery, of course.
>>Possibly, you can adjust a permanent magnet and use a reed switch (like in
>>home security systems) to accomplish the same effect.
>
>Sounds similar to Jason's idea, which had an elegant simplicity about
>it. This will just be for a one-off quick and dirty work-around, so
>doesn't warrant the investment of much development.

Ideas are fine, but eventually you have to make the numbers work.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On Sat, 15 May 2021 07:10:39 -0700, jlarkin@highlandsniptechnology.com
wrote:

>On Sat, 15 May 2021 12:15:57 +0100, Cursitor Doom <cd@nowhere.com>
>wrote:
>
>>On Fri, 14 May 2021 22:17:31 -0700 (PDT), whit3rd <whit3rd@gmail.com>
>>wrote:
>>
>>>On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:
>>>
>>>> I want to monitor current drawn from a car battery (12v lead-acid) in
>>>> such a way as when it drops below 200mA, a piezzo buzzer sounds.
>>>
>>>> What's the simplest way to do this?
>>>
>>>It's called a sensitive relay. A few wraps of wire around a bar, and a
>>>weak-spring set of points, with a fine adjust screw, can be set to trip
>>>at the current of your choice.
>>>
>>>This presumes that you can wire it in series with the battery, of course.
>>>Possibly, you can adjust a permanent magnet and use a reed switch (like in
>>>home security systems) to accomplish the same effect.
>>
>>Sounds similar to Jason's idea, which had an elegant simplicity about
>>it. This will just be for a one-off quick and dirty work-around, so
>>doesn't warrant the investment of much development.
>
>Ideas are fine, but eventually you have to make the numbers work.

True, but for a one-off at least I don't need to worry about device
parameter spread or monte carlo analysis. I can taylor it to work with
the devices I pluck from my junk box and toss it away once it's done
it's job. Selecting the best suggestion here is the tricky bit AFAIC.
I can see some merit in all of them - except the one Bill Sloman put
forward (as usual).

On Sat, 15 May 2021 16:47:34 +0100, Cursitor Doom <cd@nowhere.com>
wrote:

>On Sat, 15 May 2021 07:10:39 -0700, jlarkin@highlandsniptechnology.com
>wrote:
>
>>On Sat, 15 May 2021 12:15:57 +0100, Cursitor Doom <cd@nowhere.com>
>>wrote:
>>
>>>On Fri, 14 May 2021 22:17:31 -0700 (PDT), whit3rd <whit3rd@gmail.com>
>>>wrote:
>>>
>>>>On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:
>>>>
>>>>> I want to monitor current drawn from a car battery (12v lead-acid) in
>>>>> such a way as when it drops below 200mA, a piezzo buzzer sounds.
>>>>
>>>>> What's the simplest way to do this?
>>>>
>>>>It's called a sensitive relay. A few wraps of wire around a bar, and a
>>>>weak-spring set of points, with a fine adjust screw, can be set to trip
>>>>at the current of your choice.
>>>>
>>>>This presumes that you can wire it in series with the battery, of course.
>>>>Possibly, you can adjust a permanent magnet and use a reed switch (like in
>>>>home security systems) to accomplish the same effect.
>>>
>>>Sounds similar to Jason's idea, which had an elegant simplicity about
>>>it. This will just be for a one-off quick and dirty work-around, so
>>>doesn't warrant the investment of much development.
>>
>>Ideas are fine, but eventually you have to make the numbers work.
>
>True, but for a one-off at least I don't need to worry about device
>parameter spread or monte carlo analysis. I can taylor it to work with
>the devices I pluck from my junk box and toss it away once it's done
>it's job. Selecting the best suggestion here is the tricky bit AFAIC.
>I can see some merit in all of them - except the one Bill Sloman put
>forward (as usual).

I doubt that a reed relay that can tolerate 15 amps will trip around
200 mA.

Or that a Hall sensor would be stable enough to work over that current
range.

Even a one-off has to work.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On Fri, 14 May 2021 16:48:55 -0700, John Larkin
<jlarkin@highland_atwork_technology.com> wrote:

>On Fri, 14 May 2021 23:38:22 +0100, Cursitor Doom <cd@nowhere.com>
>wrote:
>
>>Gentlemen,
>>
>>I want to monitor current drawn from a car battery (12v lead-acid) in
>>such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>>range of current the monitor will see will run from 15A to around
>>75mA. The 15A will last for only around 10s or so and once it settles
>>down will drop to 240mA for hours on end. This is just to monitor
>>quiescent current drain and there will be no cranking or anything
>>heavy to worry about.
>>What's the simplest way to do this?
>>
>>- CD
>
>A 10 mohm resistor, or equivalent wire, will drop 2 mV at 200 mA. You
>can pick that off with a comparator or a rrio opamp used as a
>comparator.
>
>It drops 150 mV at 15A and dissipates about 2 watts.
>
>Something like that.
>
>A 15 amp schottky diode would give a better voltage range but get
>hotter.

If you use an op-amp as a comparator, 200uV is purrfectly practical.

Eg.
http://ww1.microchip.com/downloads/en/DeviceDoc/MCP6V66-Family-Data-Sheet-DS20006266A.pdf

So a foot/30cm of AWG10.

Much lower than that and I'd start to worry about thermal EMFs. It
might still be possible to make it not work with some pathological
arrangement.

--
Best regards,
Spehro Pefhany

On Sat, 15 May 2021 15:26:36 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Fri, 14 May 2021 16:48:55 -0700, John Larkin
><jlarkin@highland_atwork_technology.com> wrote:
>
>>On Fri, 14 May 2021 23:38:22 +0100, Cursitor Doom <cd@nowhere.com>
>>wrote:
>>
>>>Gentlemen,
>>>
>>>I want to monitor current drawn from a car battery (12v lead-acid) in
>>>such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>>>range of current the monitor will see will run from 15A to around
>>>75mA. The 15A will last for only around 10s or so and once it settles
>>>down will drop to 240mA for hours on end. This is just to monitor
>>>quiescent current drain and there will be no cranking or anything
>>>heavy to worry about.
>>>What's the simplest way to do this?
>>>
>>>- CD
>>
>>A 10 mohm resistor, or equivalent wire, will drop 2 mV at 200 mA. You
>>can pick that off with a comparator or a rrio opamp used as a
>>comparator.
>>
>>It drops 150 mV at 15A and dissipates about 2 watts.
>>
>>Something like that.
>>
>>A 15 amp schottky diode would give a better voltage range but get
>>hotter.
>
>If you use an op-amp as a comparator, 200uV is purrfectly practical.

Don't get catty with us.

>
>Eg.
>http://ww1.microchip.com/downloads/en/DeviceDoc/MCP6V66-Family-Data-Sheet-DS20006266A.pdf
>
>So a foot/30cm of AWG10.
>
>Much lower than that and I'd start to worry about thermal EMFs. It
>might still be possible to make it not work with some pathological
>arrangement.

That comparator is only good for 5 volts, but that's actually good. A
resistor/zener thing will let one hang it on the 12v rail, and provide
transient protection. And a regulated threshold.

With 25 uV offset, the shunt can be pretty small, like a fat PCB trace
or a few 1206 resistors in parallel.

A less exotic amp or comparator might be OK with a trimpot to set the
trip point.

--

John Larkin Highland Technology, Inc

The best designs are necessarily accidental.

On 5/15/2021 9:09 AM, jlarkin@highlandsniptechnology.com wrote:
> On Sat, 15 May 2021 12:18:59 +0100, Cursitor Doom <cd@nowhere.com>
> wrote:
>
>> On Fri, 14 May 2021 20:50:01 -0700 (PDT), Bill Sloman
>> <bill.sloman@ieee.org> wrote:
>>
>>> On Saturday, May 15, 2021 at 8:38:30 AM UTC+10, Cursitor Doom wrote:
>>>> Gentlemen,
>>>>
>>>> I want to monitor current drawn from a car battery (12v lead-acid) in
>>>> such a way as when it drops below 200mA, a piezzo buzzer sounds. The
>>>> range of current the monitor will see will run from 15A to around
>>>> 75mA. The 15A will last for only around 10s or so and once it settles
>>>> down will drop to 240mA for hours on end. This is just to monitor
>>>> quiescent current drain and there will be no cranking or anything
>>>> heavy to worry about.
>>>> What's the simplest way to do this?
>>>
>>> Probably a Hall effect sensor.
>>>
>>> https://www.allegromicro.com/en/insights-and-innovations/technical-documents/hall-effect-sensor-ic-publications/allegro-hall-effect-sensor-ics
>>>
>>> It may pay you to form the lead into a single loop, and wrap it around a chunk of ferrite so that you have a well-defined magnetic circuit with a place where you can stick the Hall effect sensor and detect a predictable magnetic field. This is much the same idea as using a reed-relay, but the hysteresis
>>> will be a lot smaller.
>>
>> Which is basically Jason Bett's idea re-badged as your own.
>
> The trick is to get a reasonably accurate trip point that is about 1%
> of the maximum current.
>
> Hall sensors are not very temperature stable and have tiny outputs. 15
> amps will heat it up. The magnetic circuit could easily be magnetized
> by the big current and shift the threshold.
>
> Wrapping 15 amps worth of wire around a reed, and tripping at 200 mA,
> would be interesting too. Reed relays typically get warm at operating
> current, much less at 100x operating current. The 15 amps could
> magnetize the reeds so that they never open.

They get much less warm at 100x operating current?!

> The series schottky diode takes care of the dynamic range problem, at
> the cost of maybe half a volt at 15 amps. Or just use a shunt resistor
> and some good electronics.
>
>
>

On Sat, 15 May 2021 09:01:37 -0700, jlarkin@highlandsniptechnology.com
wrote:

>On Sat, 15 May 2021 16:47:34 +0100, Cursitor Doom <cd@nowhere.com>
>wrote:
>
>>On Sat, 15 May 2021 07:10:39 -0700, jlarkin@highlandsniptechnology.com
>>wrote:
>>
>>>On Sat, 15 May 2021 12:15:57 +0100, Cursitor Doom <cd@nowhere.com>
>>>wrote:
>>>
>>>>On Fri, 14 May 2021 22:17:31 -0700 (PDT), whit3rd <whit3rd@gmail.com>
>>>>wrote:
>>>>
>>>>>On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:
>>>>>
>>>>>> I want to monitor current drawn from a car battery (12v lead-acid) in
>>>>>> such a way as when it drops below 200mA, a piezzo buzzer sounds.
>>>>>
>>>>>> What's the simplest way to do this?
>>>>>
>>>>>It's called a sensitive relay. A few wraps of wire around a bar, and a
>>>>>weak-spring set of points, with a fine adjust screw, can be set to trip
>>>>>at the current of your choice.
>>>>>
>>>>>This presumes that you can wire it in series with the battery, of course.
>>>>>Possibly, you can adjust a permanent magnet and use a reed switch (like in
>>>>>home security systems) to accomplish the same effect.
>>>>
>>>>Sounds similar to Jason's idea, which had an elegant simplicity about
>>>>it. This will just be for a one-off quick and dirty work-around, so
>>>>doesn't warrant the investment of much development.
>>>
>>>Ideas are fine, but eventually you have to make the numbers work.
>>
>>True, but for a one-off at least I don't need to worry about device
>>parameter spread or monte carlo analysis. I can taylor it to work with
>>the devices I pluck from my junk box and toss it away once it's done
>>it's job. Selecting the best suggestion here is the tricky bit AFAIC.
>>I can see some merit in all of them - except the one Bill Sloman put
>>forward (as usual).
>
>I doubt that a reed relay that can tolerate 15 amps will trip around
>200 mA.
>
>Or that a Hall sensor would be stable enough to work over that current
>range.
>
>Even a one-off has to work

Yes, it does. Anyway, you're one of the Top Blokes here nowadays, what
in your opinion is the best approach to use?

On Sunday, May 16, 2021 at 2:01:46 AM UTC+10, jla...@highlandsniptechnology..com wrote:
> On Sat, 15 May 2021 16:47:34 +0100, Cursitor Doom <c...@nowhere.com> wrote:
> >On Sat, 15 May 2021 07:10:39 -0700, jla...@highlandsniptechnology.com wrote:
> >>On Sat, 15 May 2021 12:15:57 +0100, Cursitor Doom <c...@nowhere.com> wrote:
> >>>On Fri, 14 May 2021 22:17:31 -0700 (PDT), whit3rd <whi...@gmail.com> wrote:
> >>>>On Friday, May 14, 2021 at 3:38:30 PM UTC-7, Cursitor Doom wrote:

<snip>
> Or that a Hall sensor would be stable enough to work over that current range.

It doesn't have to work over that current range. Cursitor Doom just wants to detect when the current falls below 200mA.

A seventy-five time higher magnetic field isn't going to damage a Hall effect sensor. It will probably drive it into saturation, but that isn't going to be a problem.

http://www.farnell.com/datasheets/2354215.pdf

The magnetic field at the centre of single turn current loop is

B=μ0.I/2R(at center of loop) Telsa

where u0 is 4.pi.0^-7 newton per ampere^-2 ( Henry/metre).

For a 1cm radius single turn loop carrying 200mA that's about 12.6uT, or about 0.126 Gauss (which isn't much).

The A1324 has a 10 Gauss worst case shift across it's temperature range, so you'd need more turns and a soft-ferromagnet core to get it up into the range where it would be easy to get a useful output.
> Even a one-off has to work.

The chance that a one-off engineered by Cursitor Doom is ever going to work isn't high.

--
Bill Sloman, Sydney