will this work

[default]

What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

A 12 Volt relay will carry current on it's normally open pins until
the circuit trips.

The question: can I run an op amp (or comparator) with a single supply
(the same 12V I'm monitoring) AND make use of the high voltage gain of
the amp to sense the voltage drop in the wire carrying current from
the relay to the load?

The non-inverting input would be closer to the power supply and
inverting input closer to the load.

If that won't work (input sensing close to the supply rail) plan B is
to just use a pair of voltage dividers to sense the same thing (drop
in a piece of wire) and sacrifice some gain.

 

[rickman]

default wrote on 11/1/2017 12:20 PM:

What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

A 12 Volt relay will carry current on it's normally open pins until
the circuit trips.

The question: can I run an op amp (or comparator) with a single supply
(the same 12V I'm monitoring) AND make use of the high voltage gain of
the amp to sense the voltage drop in the wire carrying current from
the relay to the load?

The non-inverting input would be closer to the power supply and
inverting input closer to the load.

If that won't work (input sensing close to the supply rail) plan B is
to just use a pair of voltage dividers to sense the same thing (drop
in a piece of wire) and sacrifice some gain.

Do you have an estimate for the current in the wire, the resistance of the
wire? You seem to be very concerned about the voltage across the circuit,
but not at all concerned about current used by the circuit. You can lower
the sense voltage with voltage dividers that only drop a small portion of
the voltage, say 10% and not sacrifice much in the way of gain.

Whether this circuit will work well depends on a lot of details. Sensing a
small DC value with an opamp is not so easy as the opamp has an
unpredictable DC offset (defined within a range though). With the small
voltages you are likely trying to detect this error will be significant.
You can use a multiplexer on the input to switch the same amplifier input
between the two points being measured to generate an AC signal which can be
amplified more easily and allow the opamp to be isolated from the DC level
of the signal with capacitor coupling.

On the other hand, there are commercial devices designed for high side
current sensing. They work much the way you describe but use techniques to
deal with the problems you will find designing your own. Have you looked
for any of those?

--

Rick C

Viewed the eclipse at Wintercrest Farms,
on the centerline of totality since 1998

 

[default]

On Wed, 1 Nov 2017 13:40:26 -0400, rickman <gnuarm@gmail.com> wrote:

default wrote on 11/1/2017 12:20 PM:
What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

A 12 Volt relay will carry current on it's normally open pins until
the circuit trips.

The question: can I run an op amp (or comparator) with a single supply
(the same 12V I'm monitoring) AND make use of the high voltage gain of
the amp to sense the voltage drop in the wire carrying current from
the relay to the load?

The non-inverting input would be closer to the power supply and
inverting input closer to the load.

If that won't work (input sensing close to the supply rail) plan B is
to just use a pair of voltage dividers to sense the same thing (drop
in a piece of wire) and sacrifice some gain.

Do you have an estimate for the current in the wire, the resistance of the
wire? You seem to be very concerned about the voltage across the circuit,
but not at all concerned about current used by the circuit. You can lower
the sense voltage with voltage dividers that only drop a small portion of
the voltage, say 10% and not sacrifice much in the way of gain.

I'm guessing that the current will be in the order to 1-4 amps. This
is for a stand-by super capacitor bank that runs hard drives, modem,
speakers etc.

Sounds like the voltage divider set to 1-2 volts under 12V may be the
way to do it. That also provides another benefit - I can incorporate
a pot into one leg of the differential divider and adjust for offset
and use it to set the tripping level.

Whether this circuit will work well depends on a lot of details. Sensing a
small DC value with an opamp is not so easy as the opamp has an
unpredictable DC offset (defined within a range though). With the small
voltages you are likely trying to detect this error will be significant.
You can use a multiplexer on the input to switch the same amplifier input
between the two points being measured to generate an AC signal which can be
amplified more easily and allow the opamp to be isolated from the DC level
of the signal with capacitor coupling.

If I were to get that complicated I could just as easily build an
isolated DC/DC converter, use it for the op amp, and an optical
coupler to run the relay side of things.

On the other hand, there are commercial devices designed for high side
current sensing. They work much the way you describe but use techniques to
deal with the problems you will find designing your own. Have you looked
for any of those?

I ruled that out already - I want to build it with parts I already
have laying around. A fuse would work too, but I like the idea of a
(simple, fast acting) resettable circuit breaker and pair of LEDs to
indicate the status.

There's a 4 amp PS constantly charging the caps and the caps can keep
the modem alive for >1/2 hour in a power failure. (they can also
start my truck, or a fire) The few milliamps the circuit and relay
will consume isn't a concern.

I remember doing something like this in a commercial supply we
designed for some point of sale cash registers in the 70's. We had
multiple supplies and all used high side sensing with resistors we
wound from phosphor bronze wire to get a repeatable current set point.
The op-amps were 747 (two to a package) and one half ran the regulator
and the other the current limiting, I think the power came from +/- 15
volt supplies which were part of the larger supply.

 

[Jim Thompson]

On Wed, 01 Nov 2017 12:20:39 -0400, default <default@defaulter.net>
wrote:

What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

A 12 Volt relay will carry current on it's normally open pins until
the circuit trips.

The question: can I run an op amp (or comparator) with a single supply
(the same 12V I'm monitoring) AND make use of the high voltage gain of
the amp to sense the voltage drop in the wire carrying current from
the relay to the load?

The non-inverting input would be closer to the power supply and
inverting input closer to the load.

If that won't work (input sensing close to the supply rail) plan B is
to just use a pair of voltage dividers to sense the same thing (drop
in a piece of wire) and sacrifice some gain.

Sensing the drop on a wire "with essentially no voltage drop" seems
risky to me. If you absolutely can't add a small series resistor I'd
go with a Hall-sensor to measure the current.

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

"Those [of us] who dream by day are cognizant of many things which
escape those who dream only by night" -Edgar Allan Poe

 

[rickman]

default wrote on 11/1/2017 2:23 PM:

On Wed, 1 Nov 2017 13:40:26 -0400, rickman <gnuarm@gmail.com> wrote:

default wrote on 11/1/2017 12:20 PM:
What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

A 12 Volt relay will carry current on it's normally open pins until
the circuit trips.

The question: can I run an op amp (or comparator) with a single supply
(the same 12V I'm monitoring) AND make use of the high voltage gain of
the amp to sense the voltage drop in the wire carrying current from
the relay to the load?

The non-inverting input would be closer to the power supply and
inverting input closer to the load.

If that won't work (input sensing close to the supply rail) plan B is
to just use a pair of voltage dividers to sense the same thing (drop
in a piece of wire) and sacrifice some gain.

Do you have an estimate for the current in the wire, the resistance of the
wire? You seem to be very concerned about the voltage across the circuit,
but not at all concerned about current used by the circuit. You can lower
the sense voltage with voltage dividers that only drop a small portion of
the voltage, say 10% and not sacrifice much in the way of gain.

I'm guessing that the current will be in the order to 1-4 amps. This
is for a stand-by super capacitor bank that runs hard drives, modem,
speakers etc.

Sounds like the voltage divider set to 1-2 volts under 12V may be the
way to do it. That also provides another benefit - I can incorporate
a pot into one leg of the differential divider and adjust for offset
and use it to set the tripping level.

Even if you are only building one of these, opamp offset will drift with
time and temperature. You need to have an idea of what voltage you will be
sensing so you can estimate the significance of the offset of your opamp.
You didn't complete that thought above.

Whether this circuit will work well depends on a lot of details. Sensing a
small DC value with an opamp is not so easy as the opamp has an
unpredictable DC offset (defined within a range though). With the small
voltages you are likely trying to detect this error will be significant.
You can use a multiplexer on the input to switch the same amplifier input
between the two points being measured to generate an AC signal which can be
amplified more easily and allow the opamp to be isolated from the DC level
of the signal with capacitor coupling.

If I were to get that complicated I could just as easily build an
isolated DC/DC converter, use it for the op amp, and an optical
coupler to run the relay side of things.

Complicated? What I described is a chopper amp and is the common way of
amplifying small DC levels. It only requires a small, 6 or 8 pin IC as a
mux and a capacitor to block the DC. If you want DC to drive a circuit it
only requires a diode and capacitor on the output. You think that is
complicated? I suppose you do need a 555 timer to drive the mux.

On the other hand, there are commercial devices designed for high side
current sensing. They work much the way you describe but use techniques to
deal with the problems you will find designing your own. Have you looked
for any of those?

I ruled that out already - I want to build it with parts I already
have laying around.

Oh, then please provide a list of your spare parts.

You can also make a mux with a few resistors and a pair of transistors. You
will need inverting drive for the two transistors.

A fuse would work too, but I like the idea of a
(simple, fast acting) resettable circuit breaker and pair of LEDs to
indicate the status.

There's a 4 amp PS constantly charging the caps and the caps can keep
the modem alive for >1/2 hour in a power failure. (they can also
start my truck, or a fire) The few milliamps the circuit and relay
will consume isn't a concern.

I remember doing something like this in a commercial supply we
designed for some point of sale cash registers in the 70's. We had
multiple supplies and all used high side sensing with resistors we
wound from phosphor bronze wire to get a repeatable current set point.
The op-amps were 747 (two to a package) and one half ran the regulator
and the other the current limiting, I think the power came from +/- 15
volt supplies which were part of the larger supply.

Do you have phosphor bronze wire on hand?

--

Rick C

Viewed the eclipse at Wintercrest Farms,
on the centerline of totality since 1998

 

[whit3rd]

On Wednesday, November 1, 2017 at 9:20:36 AM UTC-7, default wrote:

What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

As Jim T has said, a magnetic sensor makes a good match to the 'no voltage drop'
requirement. Hall sensors are one possibility, magnetic sensitive relays
another (with some electronics to latch it off). It's pretty easy to wind a
few turns of heavy wire around a reed switch. That scales up nicely,
there are 'protective relays' in power stations.

For really fine discrimination, a fluxgate magnetic sensor is good (if somewhat
complex).

 

[default]

On Wed, 1 Nov 2017 15:39:57 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Wednesday, November 1, 2017 at 9:20:36 AM UTC-7, default wrote:
What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

As Jim T has said, a magnetic sensor makes a good match to the 'no voltage drop'
requirement. Hall sensors are one possibility, magnetic sensitive relays
another (with some electronics to latch it off). It's pretty easy to wind a
few turns of heavy wire around a reed switch. That scales up nicely,
there are 'protective relays' in power stations.

For really fine discrimination, a fluxgate magnetic sensor is good (if somewhat
complex).

I'll give the reed relay thing a try. Hall effect is another good
one, I think there's an old keyboard around that has Hall sensors in
it.

Thanks

 

[default]

On Wed, 1 Nov 2017 14:58:49 -0400, rickman <gnuarm@gmail.com> wrote:

default wrote on 11/1/2017 2:23 PM:
On Wed, 1 Nov 2017 13:40:26 -0400, rickman <gnuarm@gmail.com> wrote:

default wrote on 11/1/2017 12:20 PM:
What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

A 12 Volt relay will carry current on it's normally open pins until
the circuit trips.

The question: can I run an op amp (or comparator) with a single supply
(the same 12V I'm monitoring) AND make use of the high voltage gain of
the amp to sense the voltage drop in the wire carrying current from
the relay to the load?

The non-inverting input would be closer to the power supply and
inverting input closer to the load.

If that won't work (input sensing close to the supply rail) plan B is
to just use a pair of voltage dividers to sense the same thing (drop
in a piece of wire) and sacrifice some gain.

Do you have an estimate for the current in the wire, the resistance of the
wire? You seem to be very concerned about the voltage across the circuit,
but not at all concerned about current used by the circuit. You can lower
the sense voltage with voltage dividers that only drop a small portion of
the voltage, say 10% and not sacrifice much in the way of gain.

I'm guessing that the current will be in the order to 1-4 amps. This
is for a stand-by super capacitor bank that runs hard drives, modem,
speakers etc.

Sounds like the voltage divider set to 1-2 volts under 12V may be the
way to do it. That also provides another benefit - I can incorporate
a pot into one leg of the differential divider and adjust for offset
and use it to set the tripping level.

Even if you are only building one of these, opamp offset will drift with
time and temperature. You need to have an idea of what voltage you will be
sensing so you can estimate the significance of the offset of your opamp.
You didn't complete that thought above.


Whether this circuit will work well depends on a lot of details. Sensing a
small DC value with an opamp is not so easy as the opamp has an
unpredictable DC offset (defined within a range though). With the small
voltages you are likely trying to detect this error will be significant.
You can use a multiplexer on the input to switch the same amplifier input
between the two points being measured to generate an AC signal which can be
amplified more easily and allow the opamp to be isolated from the DC level
of the signal with capacitor coupling.

If I were to get that complicated I could just as easily build an
isolated DC/DC converter, use it for the op amp, and an optical
coupler to run the relay side of things.

Complicated? What I described is a chopper amp and is the common way of
amplifying small DC levels. It only requires a small, 6 or 8 pin IC as a
mux and a capacitor to block the DC. If you want DC to drive a circuit it
only requires a diode and capacitor on the output. You think that is
complicated? I suppose you do need a 555 timer to drive the mux.


On the other hand, there are commercial devices designed for high side
current sensing. They work much the way you describe but use techniques to
deal with the problems you will find designing your own. Have you looked
for any of those?

I ruled that out already - I want to build it with parts I already
have laying around.

Oh, then please provide a list of your spare parts.

A room of my house knee deep in tech junk. Most of the stuff in small
cardboard boxes with what project they went with written on the
outside. Cataloging isn't going to happen... Only the discrete parts
that I stock get cataloged.

You can also make a mux with a few resistors and a pair of transistors. You
will need inverting drive for the two transistors.


A fuse would work too, but I like the idea of a
(simple, fast acting) resettable circuit breaker and pair of LEDs to
indicate the status.

There's a 4 amp PS constantly charging the caps and the caps can keep
the modem alive for >1/2 hour in a power failure. (they can also
start my truck, or a fire) The few milliamps the circuit and relay
will consume isn't a concern.

I remember doing something like this in a commercial supply we
designed for some point of sale cash registers in the 70's. We had
multiple supplies and all used high side sensing with resistors we
wound from phosphor bronze wire to get a repeatable current set point.
The op-amps were 747 (two to a package) and one half ran the regulator
and the other the current limiting, I think the power came from +/- 15
volt supplies which were part of the larger supply.

Do you have phosphor bronze wire on hand?

No, that was back in the 70's. We needed some ultra low value
resistors for current shunts. I want to use the same technique since
it did work very well, but I don't remember how the power supplies
were arranged. Offset and temperature drift were not significant. I
don't think the client set a specification for the current limiting
just that all outputs could tolerate an indefinite short circuit.

 

[rickman]

default wrote on 11/1/2017 8:54 PM:

On Wed, 1 Nov 2017 14:58:49 -0400, rickman <gnuarm@gmail.com> wrote:

default wrote on 11/1/2017 2:23 PM:
On Wed, 1 Nov 2017 13:40:26 -0400, rickman <gnuarm@gmail.com> wrote:

default wrote on 11/1/2017 12:20 PM:
What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

A 12 Volt relay will carry current on it's normally open pins until
the circuit trips.

The question: can I run an op amp (or comparator) with a single supply
(the same 12V I'm monitoring) AND make use of the high voltage gain of
the amp to sense the voltage drop in the wire carrying current from
the relay to the load?

The non-inverting input would be closer to the power supply and
inverting input closer to the load.

If that won't work (input sensing close to the supply rail) plan B is
to just use a pair of voltage dividers to sense the same thing (drop
in a piece of wire) and sacrifice some gain.

Do you have an estimate for the current in the wire, the resistance of the
wire? You seem to be very concerned about the voltage across the circuit,
but not at all concerned about current used by the circuit. You can lower
the sense voltage with voltage dividers that only drop a small portion of
the voltage, say 10% and not sacrifice much in the way of gain.

I'm guessing that the current will be in the order to 1-4 amps. This
is for a stand-by super capacitor bank that runs hard drives, modem,
speakers etc.

Sounds like the voltage divider set to 1-2 volts under 12V may be the
way to do it. That also provides another benefit - I can incorporate
a pot into one leg of the differential divider and adjust for offset
and use it to set the tripping level.

Even if you are only building one of these, opamp offset will drift with
time and temperature. You need to have an idea of what voltage you will be
sensing so you can estimate the significance of the offset of your opamp.
You didn't complete that thought above.


Whether this circuit will work well depends on a lot of details. Sensing a
small DC value with an opamp is not so easy as the opamp has an
unpredictable DC offset (defined within a range though). With the small
voltages you are likely trying to detect this error will be significant.
You can use a multiplexer on the input to switch the same amplifier input
between the two points being measured to generate an AC signal which can be
amplified more easily and allow the opamp to be isolated from the DC level
of the signal with capacitor coupling.

If I were to get that complicated I could just as easily build an
isolated DC/DC converter, use it for the op amp, and an optical
coupler to run the relay side of things.

Complicated? What I described is a chopper amp and is the common way of
amplifying small DC levels. It only requires a small, 6 or 8 pin IC as a
mux and a capacitor to block the DC. If you want DC to drive a circuit it
only requires a diode and capacitor on the output. You think that is
complicated? I suppose you do need a 555 timer to drive the mux.


On the other hand, there are commercial devices designed for high side
current sensing. They work much the way you describe but use techniques to
deal with the problems you will find designing your own. Have you looked
for any of those?

I ruled that out already - I want to build it with parts I already
have laying around.

Oh, then please provide a list of your spare parts.

A room of my house knee deep in tech junk. Most of the stuff in small
cardboard boxes with what project they went with written on the
outside. Cataloging isn't going to happen... Only the discrete parts
that I stock get cataloged.

Let me know when you get that catalog together and I'll give you suggestions
based on what you have on hand. Oh wait, you must know what you have on
hand as you have already said you don't have any micro switches.

You can also make a mux with a few resistors and a pair of transistors. You
will need inverting drive for the two transistors.


A fuse would work too, but I like the idea of a
(simple, fast acting) resettable circuit breaker and pair of LEDs to
indicate the status.

There's a 4 amp PS constantly charging the caps and the caps can keep
the modem alive for >1/2 hour in a power failure. (they can also
start my truck, or a fire) The few milliamps the circuit and relay
will consume isn't a concern.

I remember doing something like this in a commercial supply we
designed for some point of sale cash registers in the 70's. We had
multiple supplies and all used high side sensing with resistors we
wound from phosphor bronze wire to get a repeatable current set point.
The op-amps were 747 (two to a package) and one half ran the regulator
and the other the current limiting, I think the power came from +/- 15
volt supplies which were part of the larger supply.

Do you have phosphor bronze wire on hand?

No, that was back in the 70's. We needed some ultra low value
resistors for current shunts. I want to use the same technique since
it did work very well, but I don't remember how the power supplies
were arranged. Offset and temperature drift were not significant. I
don't think the client set a specification for the current limiting
just that all outputs could tolerate an indefinite short circuit.

Ok, I can't help you if you only want to use what you have on hand and you
don't know what that is. I don't see how you can get anything done this way.

--

Rick C

Viewed the eclipse at Wintercrest Farms,
on the centerline of totality since 1998

 

[Ian Field]

"default" <default@defaulter.net> wrote in message
news:n0qkvc1nj22s03r3ff52c2gpgg77kq34is@4ax.com...

On Wed, 1 Nov 2017 15:39:57 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Wednesday, November 1, 2017 at 9:20:36 AM UTC-7, default wrote:
What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

As Jim T has said, a magnetic sensor makes a good match to the 'no voltage
drop'
requirement. Hall sensors are one possibility, magnetic sensitive relays
another (with some electronics to latch it off). It's pretty easy to
wind a
few turns of heavy wire around a reed switch. That scales up nicely,
there are 'protective relays' in power stations.

For really fine discrimination, a fluxgate magnetic sensor is good (if
somewhat
complex).


I'll give the reed relay thing a try. Hall effect is another good
one, I think there's an old keyboard around that has Hall sensors in
it.

You need quite a few turns around a reed relay unless there's a fair bit of
current. back in the days of dial up modems, a magazine article was
published to thwart rogue dialler software. The fraud drops your connection
and dials a server on a premium rate number. The reed had about 200t of wire
round it and both were in series carrying the line current. Dropping the
line released the reed and you had to manually hold a bypass button to
establish a new connection.

someone suggested a Hall effect current sensor, its simply an assembly that
places a hall effect semiconductor device in fixed proximity to a wire
you're passing the current through.

If its AC - a current transformer might be cheaper.

 

[default]

On Mon, 6 Nov 2017 20:30:52 -0000, "Ian Field"
<gangprobing.alien1@virginmedia.com> wrote:

"default" <default@defaulter.net> wrote in message
news:n0qkvc1nj22s03r3ff52c2gpgg77kq34is@4ax.com...
On Wed, 1 Nov 2017 15:39:57 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Wednesday, November 1, 2017 at 9:20:36 AM UTC-7, default wrote:
What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

As Jim T has said, a magnetic sensor makes a good match to the 'no voltage
drop'
requirement. Hall sensors are one possibility, magnetic sensitive relays
another (with some electronics to latch it off). It's pretty easy to
wind a
few turns of heavy wire around a reed switch. That scales up nicely,
there are 'protective relays' in power stations.

For really fine discrimination, a fluxgate magnetic sensor is good (if
somewhat
complex).


I'll give the reed relay thing a try. Hall effect is another good
one, I think there's an old keyboard around that has Hall sensors in
it.

You need quite a few turns around a reed relay unless there's a fair bit of
current. back in the days of dial up modems, a magazine article was
published to thwart rogue dialler software. The fraud drops your connection
and dials a server on a premium rate number. The reed had about 200t of wire
round it and both were in series carrying the line current. Dropping the
line released the reed and you had to manually hold a bypass button to
establish a new connection.

I found a reed switch that gave the specs as 30 turns of wire at one
amp should actuate. I want it to trip at 4 amps so that should be
less than 10 turns.

I'm still playing with op amps and haven't tried the reed or hall
switches yet.

someone suggested a Hall effect current sensor, its simply an assembly that
places a hall effect semiconductor device in fixed proximity to a wire
you're passing the current through.

If its AC - a current transformer might be cheaper.
DC

 

[default]

On Mon, 6 Nov 2017 20:30:52 -0000, "Ian Field"
<gangprobing.alien1@virginmedia.com> wrote:

"default" <default@defaulter.net> wrote in message
news:n0qkvc1nj22s03r3ff52c2gpgg77kq34is@4ax.com...
On Wed, 1 Nov 2017 15:39:57 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Wednesday, November 1, 2017 at 9:20:36 AM UTC-7, default wrote:
What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

As Jim T has said, a magnetic sensor makes a good match to the 'no voltage
drop'
requirement. Hall sensors are one possibility, magnetic sensitive relays
another (with some electronics to latch it off). It's pretty easy to
wind a
few turns of heavy wire around a reed switch. That scales up nicely,
there are 'protective relays' in power stations.

For really fine discrimination, a fluxgate magnetic sensor is good (if
somewhat
complex).


I'll give the reed relay thing a try. Hall effect is another good
one, I think there's an old keyboard around that has Hall sensors in
it.

You need quite a few turns around a reed relay unless there's a fair bit of
current. back in the days of dial up modems, a magazine article was
published to thwart rogue dialler software. The fraud drops your connection
and dials a server on a premium rate number. The reed had about 200t of wire
round it and both were in series carrying the line current. Dropping the
line released the reed and you had to manually hold a bypass button to
establish a new connection.

someone suggested a Hall effect current sensor, its simply an assembly that
places a hall effect semiconductor device in fixed proximity to a wire
you're passing the current through.

If its AC - a current transformer might be cheaper.

Ok this looks like a winner to me (for it's simplicity and hopefully
reliability).

I tried a bare reed switch I happened to have and wound 20 turns of 22
AWG magnet wire around it in two layers. It works at .992 amps. So
I'll cut that to 5 turns of wire and try again.

I also have a number of reed relays around and plan to cut the fine
wire off and use the bobbin and connection pins to pretty it up.

 

[default]

On Wed, 1 Nov 2017 15:39:57 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Wednesday, November 1, 2017 at 9:20:36 AM UTC-7, default wrote:
What I want to do is make a 12vdc electronic circuit breaker, with
essentially no voltage drop for current sensing.

As Jim T has said, a magnetic sensor makes a good match to the 'no voltage drop'
requirement. Hall sensors are one possibility, magnetic sensitive relays
another (with some electronics to latch it off). It's pretty easy to wind a
few turns of heavy wire around a reed switch. That scales up nicely,
there are 'protective relays' in power stations.

For really fine discrimination, a fluxgate magnetic sensor is good (if somewhat
complex).

I'm going to use a reed since that seems to be simple, easy to
calculate. 20 turns of 22 AWG magnet wire around it in two layers.
worked at .992 amps. I can up the wire to 14 AWG and lower the turns.
The diameter of the reed is small enough so that the wire length is
only a few inches and will have very low resistance.

I'll latch the switching relay 'on' with a transistor and just use the
reed to ground the base so I'll only be switching a 2 milliamp
resistive load. A couple of pushbuttons for on/reset and off/tripped,
some LEDs and it should be good to go.