looking for AC current sensor

[Dan]

Can anyone suggest a device that I can use to detect current in an ac 110v
line? I was thinking
all I would need is a 'toroid' and to wrap some wire around it and then
convert the current to volts
then convert the voltage to digital using an adc. I want to measure the
power usage on my electric
service and log it to a spread sheet. I can do everything from the adc to
the PC, but I'm not up on
the best way to design the sensor. Would the toroid be like the transformer
core with the ac line
being the primary and the wire wraps being the 2ndary? I'm assuming the ac
current creates an
emf which is transferred to the the 2ndary and the more wraps would amplify
it more, but is there
another ac current induced in the 2ndary too? All my electronics hobby books
and magazines are
in storage. Any help would be welcome.

Dan

 

[Jim Thompson]

Can anyone suggest a device that I can use to detect current in an ac 110v
line? I was thinking
all I would need is a 'toroid' and to wrap some wire around it and then
convert the current to volts
then convert the voltage to digital using an adc. I want to measure the
power usage on my electric
service and log it to a spread sheet. I can do everything from the adc to
the PC, but I'm not up on
the best way to design the sensor. Would the toroid be like the transformer
core with the ac line
being the primary and the wire wraps being the 2ndary? I'm assuming the ac
current creates an
emf which is transferred to the the 2ndary and the more wraps would amplify
it more, but is there
another ac current induced in the 2ndary too? All my electronics hobby books
and magazines are
in storage. Any help would be welcome.

Dan

You can buy then ready-made on a small PCB card, 1 turn to many
turns... load the secondary with a low resistance and you have an
accurate current transformer *and* isolation.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| Jim-T@analog_innovations.com Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

For proper E-mail replies SWAP "-" and "_"

I love to cook with wine. Sometimes I even put it in the food.

 

[John Popelish]

Can anyone suggest a device that I can use to detect current in an ac 110v
line? I was thinking
all I would need is a 'toroid' and to wrap some wire around it and then
convert the current to volts
then convert the voltage to digital using an adc. I want to measure the
power usage on my electric
service and log it to a spread sheet. I can do everything from the adc to
the PC, but I'm not up on
the best way to design the sensor. Would the toroid be like the transformer
core with the ac line
being the primary and the wire wraps being the 2ndary? I'm assuming the ac
current creates an
emf which is transferred to the the 2ndary and the more wraps would amplify
it more, but is there
another ac current induced in the 2ndary too? All my electronics hobby books
and magazines are
in storage. Any help would be welcome.

Dan

You just reinvented the current transformer.

http://www.process-controls.com/Access_Control_Sales/Access_Current_Transformer.htm
http://leeh.ee.tut.fi/transformer/tra35.htm

These function just like any other transformer, with the secondary
current proportional to the primary current based on turns ratio.

For instance, if you have a 200 turn coil on a high permeability core,
with one load current carrying wire as a primary, the 200 turn coil
will produce a current that is 1/200thj of the load wire's current, as
long as the voltage developed by that coil does not saturate the
core. Loading the coil with a low resistance lowers the voltage
produced by the coil, improving the accuracy of the current
proportionality. If you connect a diode bridge to the coil, and then
load the bridge with the resistor, you get a very accurate full wave
rectified current waveform that you can average with an RC filter to
measure the average current with a high impedance meter or A/D
converter.

 

[Big John]

Hi Dan,

As others have said you are describing a current transformer. You can
make or buy one but make sure that you put a low value resistor across the
secondary (A/D converter side). The reason for this is a current
transformer drops current but boosts voltage. The voltage across the
secondary can become dangerously high without a shunt resistor.

You have a few options as to what to use. There are clamp on current
transformers that are usually used with DMMs. These are easy to use because
you don't have to break a connection, just clamp them on one of the leads.

There are also other options, try searching on "LEM module". You will
also have to write a few lines of code to convert the A/D data into an RMS
value of current. As others have also said you can also use a rectifier and
a small capacitor to get an average value of current. The average value of
current is 0.636 times the peak (full wave rectified current). But this
trick only works for sinewave currents so some loads like computers and
light dimmers won't read accurately. You also have to take the 0.7 volt
diode drop into account.

Hope this helps,
Big John

Dan wrote in message ...

 

[John Popelish]

Big John wrote:
(snip)

There are also other options, try searching on "LEM module". You will
also have to write a few lines of code to convert the A/D data into an RMS
value of current. As others have also said you can also use a rectifier and
a small capacitor to get an average value of current. The average value of
current is 0.636 times the peak (full wave rectified current). But this
trick only works for sinewave currents so some loads like computers and
light dimmers won't read accurately. You also have to take the 0.7 volt
diode drop into account.

Actually, the diode drops are driven by the current output of the
transformer. So you do not have to correct the resistor voltage for
their drop. The diode distortion is almost all at the transformer
output voltage (which goes up).

 

[Jim Meyer]

Can anyone suggest a device that I can use to detect current in an ac 110v
line?

You can buy an F. W. Bell CLS-25 current sensor for $21 from
http://www.alliedelec.com. It measures 0-25 Amps, DC to 200KHz, and
gives you 1 mA out for each Amp in. Small, cheap, and really easy to
use.

Jim

 

[Active8]

You just reinvented the current transformer.

http://www.process-controls.com/Access_Control_Sales/Access_Current_Transformer.htm
http://leeh.ee.tut.fi/transformer/tra35.htm

These function just like any other transformer, with the secondary
current
inversely

proportional to the primary current based on turns ratio.

For instance, if you have a 200 turn coil on a high permeability core,
with one load current carrying wire as a primary, the 200 turn coil
will produce a current that is 1/200thj of the load wire's current, as
long as the voltage developed by that coil does not saturate the
core. Loading the coil with a low resistance lowers the voltage
produced by the coil, improving the accuracy of the current
proportionality. If you connect a diode bridge to the coil, and then
load the bridge with the resistor, you get a very accurate full wave
rectified current waveform that you can average with an RC filter to
measure the average current with a high impedance meter or A/D
converter.

 

[Dana Raymond, a minor God]

Here's a question...
You wish to log your "power" usage, but I've only heard mention of current.
I believe the utilities' power meter measures both current and voltage to
record power.

Someone else should be able to tell us if these meter's perform power factor
correction in their measurement. For some reason I doubt it. If not, then a
motor (in your fridge, say) would cost you as if it was running at its
heaviest mechanical load, no matter what.

If you are trying to measure power consumption in an identical manner to the
power meter, then I would look for standards governing such meters'
behavior. Response time, response to non-sinusoid current, and
voltage/current relationship should be governed by some standards body, me
thinks.

Dana Raymond

 

[Paul Hovnanian P.E.]

Here's a question...
You wish to log your "power" usage, but I've only heard mention of current.
I believe the utilities' power meter measures both current and voltage to
record power.
Yes.

Someone else should be able to tell us if these meter's perform power factor
correction in their measurement. For some reason I doubt it.

They do. Its inherent in their design. They are a kind of induction
motor where the torque on the meter disk is proportional to the product
of the in-phase components of the current and voltage inputs.

If not, then a
motor (in your fridge, say) would cost you as if it was running at its
heaviest mechanical load, no matter what.

If you are trying to measure power consumption in an identical manner to the
power meter, then I would look for standards governing such meters'
behavior. Response time, response to non-sinusoid current, and
voltage/current relationship should be governed by some standards body, me
thinks.

Whatever you do with a couple of A/D converters and a microprocessor, it
will be difficult (and pointless) to try to model the behavior of a
mechanical meter as far as its transient response.

The important things to get right are handling the power factor (phase
angle between voltage and current) and the rejection of harmonics.

Multiplying the instantaneous voltage and current wave forms with an
analog multiplier is probably the best approach, since a harmonic
current wave form multiplied by the fundamental voltage wave form will
result in a net power signal of zero. Its easier than digitizing the
voltage and current wave forms and then doing the math in software.

 

[Big John]

Hi John,

I agree with you - but it depends on the circuit configuration. If the
shunt resistor, which also converts your current to a voltage, is after the
rectifier(s) then the diode drops aren't much of a factor. I don't like
this configuration only because if one of the rectifiers fails and opens up
(unlikely - but possible) the transformer loses it's shunt resistance and
very bad things will happen.

I prefer (and it's only my opinion) to put the shunt resistor (which has a
higher then needed power rating) right at the transformer's terminals. I'd
let the A/D converter see the AC signal and do the math in software, but if
I needed to get an average current signal I'd set the A/D converter to the
highest scale and use Schottkey diodes (0.2 volt drop) to minimize
distortion.

Just my opinion,
Big John

 

[Don Lancaster]

Can anyone suggest a device that I can use to detect current in an ac 110v
line? I was thinking
all I would need is a 'toroid' and to wrap some wire around it and then
convert the current to volts
then convert the voltage to digital using an adc. I want to measure the
power usage on my electric
service and log it to a spread sheet. I can do everything from the adc to
the PC, but I'm not up on
the best way to design the sensor. Would the toroid be like the transformer
core with the ac line
being the primary and the wire wraps being the 2ndary? I'm assuming the ac
current creates an
emf which is transferred to the the 2ndary and the more wraps would amplify
it more, but is there
another ac current induced in the 2ndary too? All my electronics hobby books
and magazines are
in storage. Any help would be welcome.

Dan

These are called "current transformers" and are a standard item.
We have a few in stock and offer them on eBay from time to time. As
"ABEJA".

The interesting question is whether the Radio Shack snap on ferrite
noise filter can be used to make a decent current transformer. The
crucial issue is having to break the primary wire for installation.
The material is dead wrong, but it just may work.

For years I've envisioned an ISOPOD (search http://www.tinaja.com )
energy reporter that is a tennis ball shaped object that you open and
then snap onto your incoming power mains. This wireless device would be
a killer ap for home energy management. Should even be self-powerable.


--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected] fax 847-574-1462

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Vlad]

Dan
I just prepared a long description of my power sampling and plotting
system but for some mysterious reason the message has disappeared.

In short what I did was to modify a couple of small transformers by
removing the center "-" of the "E" and articulate the "I" (the crazy
things we have to do because we can't post binaries files on some
groups). I can post pictures on a different group if you so desire.
I then installed the units around the main power wires that come into
the house and run about 50 feet of wire to the area where I have the
computers plotting the results.

To convert from AC current to the RS232 I use a Radio Shack
multimeter.

I am measuring and plotting currents up to 100 Amps .
I am sensing the two "phases" of the main power circuit and adding
them up in order to get total power.

This is not R.M.S . If you want to measure R.M.S I would suggest that
you get and adapt a surplus current meter of the type use on AM radio
stations .

The most difficult thing on this project is the installation of the
sampling units with the power on. A short on that are and the
consequences can be lethal.

I can give you more details on the complete contraption if you so
desired.

Vlad

 

[Dana Raymond, a minor God]

Very interesting Paul. Your comment about cancellation of harmonic currents
was new to me and very enlightening. I have one question:

If utility power meter's correct for power factor, then why the market for
power factor correction devices in the first place? I believe these devices
were originally developed by NASA for power savings, and work by lowering
the operating voltage of an AC motor until it's current and voltage
waveforms are in phase (PF=1). They are normally seen in industrial settings
as the power costs saved can be quite substantial. Of course, the correction
device must be placed before the power meter.

Make sense?
Dana Raymond

 

[John Popelish]

Very interesting Paul. Your comment about cancellation of harmonic currents
was new to me and very enlightening. I have one question:

If utility power meter's correct for power factor, then why the market for
power factor correction devices in the first place? I believe these devices
were originally developed by NASA for power savings, and work by lowering
the operating voltage of an AC motor until it's current and voltage
waveforms are in phase (PF=1). They are normally seen in industrial settings
as the power costs saved can be quite substantial. Of course, the correction
device must be placed before the power meter.

Those devices for refrigerators save actual power, by lowering the
copper and iron losses in refrigeration motors. Power factor current
heats motor windings and excess voltage causes iron losses. Adding a
capacitor in parallel with the motor will cancel the inductive
current, but will not reduce the motor losses. The big energy savings
is not just the energy that does not disappear into motor losses, but
is also the reduced load on the refrigeration system that does not
have to pump those losses out of the condenser along with all the
other heat load. The refrigerator will run fewer hours a day.

This multiplier effect (and the fact that refrigerators run so many
hours a day) makes the power factor correctors much more cost
effective for refrigeration systems than other home motor loads. For
instance, the waste heat produced in a furnace blower reduces the time
the furnace must run, since it adds to the furnace heat output.

 

[Vlad]

Those devices for refrigerators save actual power, by lowering the
copper and iron losses in refrigeration motors. Power factor current
heats motor windings and excess voltage causes iron losses. Adding a
capacitor in parallel with the motor will cancel the inductive
current, but will not reduce the motor losses. The big energy savings
is not just the energy that does not disappear into motor losses, but
is also the reduced load on the refrigeration system that does not
have to pump those losses out of the condenser along with all the
other heat load. The refrigerator will run fewer hours a day.

This multiplier effect (and the fact that refrigerators run so many
hours a day) makes the power factor correctors much more cost
effective for refrigeration systems than other home motor loads. For
instance, the waste heat produced in a furnace blower reduces the time
the furnace must run, since it adds to the furnace heat output.

But during the heating season, the furnace will run longer because the
heat generated by the blower is decreased. Right ?

Vlad

 

[John Popelish]

But during the heating season, the furnace will run longer because the
heat generated by the blower is decreased. Right ?

If you had a power factor corrector on it, yes. Thus, for this
application, the operating time, energy saving multiplier is less than
1.

 

[Vlad]

John
First, please be patient. english is a foresight language to me.

I don't see the total efficiency been less then 1.
I understand that you lose the advantage you have during the
refrigeration time but during the heating season, since all the
electrical energy is converted into heat inside of the house , isn't
the end result 1 or very very close to 100% efficiency ?

Vlad

 

[Paul Hovnanian P.E.]

Very interesting Paul. Your comment about cancellation of harmonic currents
was new to me and very enlightening. I have one question:

If utility power meter's correct for power factor, then why the market for
power factor correction devices in the first place? I believe these devices
were originally developed by NASA for power savings, and work by lowering
the operating voltage of an AC motor until it's current and voltage
waveforms are in phase (PF=1). They are normally seen in industrial settings
as the power costs saved can be quite substantial. Of course, the correction
device must be placed before the power meter.

Power factor is important to industrial customers because they are
billed for their reactive power as well as their real power. Residential
customers are not, so power factor correction is of no value to the
average consumer.

Harmonic currents are not quite the same as reactive power, although
they share similar characteristics from the point of view of being a
detriment to power distribution systems. Reactive currents are at the
power system fundamental frequency but are out of phase with the system
voltage so they transmit no net real power. Harmonic currents are at
multiples of the system voltage and also transmit no net real power.
However, in both cases, they cause system voltage drops and I^2R losses.

Harmonic currents present additional problems to digital metering
systems if not properly accounted for. Particularly for systems which
sample the current sinusoidal peak and then just do math under the
assumption of a pure fundamental sine wave. What with all the
electronics power supplies, solid state dimmers and motor controllers,
the load currents are becoming more and more loaded with harmonics.

 

[John Popelish]

John
First, please be patient. english is a foresight language to me.

I don't see the total efficiency been less then 1.
I understand that you lose the advantage you have during the
refrigeration time but during the heating season, since all the
electrical energy is converted into heat inside of the house , isn't
the end result 1 or very very close to 100% efficiency ?

Vlad

The power factor corrector circuit lowers the effective voltage, to
not waste power overexciting the induction motor. For motors used in
refrigeration (and cooled by the refrigeration process) You get an
extra savings because the waste heat you eliminate also lowers the
load on the refrigeration system allowing it to run with less torque
or for less time, or both. So you save more total energy that just
the reduced motor losses. And if the refrigerator dumps its heat into
an air conditioned space, that heat energy gets handles a second time,
so reducing it saves even more. This is the multiplier effect I
mentioned.

If you put one of these correctors on a furnace motor, it may save
about as much motor waste heat as it does on the refrigerator motor,
but this drop in waste heat actually requires some other source of
heat (the furnace fuel) to make up the loss, since heat is what the
furnace is trying to produce. So the electrical savings you get
operating this sort of motor is offset (at least partly) by some other
fuel cost increase, so it is not usually cost effective to put the
power factor correctors on any motors in the home except hermetically
sealed refrigeration motors.

 

[Dan]

I may be only interested in how long one certain circuit is on for how many
hours in a day.
I have a house and an apartment on one meter and I want to be able to either
rent them
both as 'utilities paid' or charge them for usage of just the air
conditioning/heating which
account for the lion's share of the bill. I could have a few wires around
the line for the ac
on both the house and the apt and have a comparator turn on a digital signal
and record
it to PC and that would work for now. But thanks for all the great info.