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I have main propane heat but I also have some of the electric oil filled
radiators. I like the oil filled radiators for cold spots and to give
temperature adjustment to individual rooms.

The problem is that the heaters use up most of the power available on the
circuit. We quit using a heater that was plugged in to the same circuit as
the kitchen, every time we'd run the microwave, toaster, or other high
current appliance, it would kick the breaker out.

So it seems like a good project to use a current transformer to measure amp
draw on the breaker and automatically turn off the heater when the amps draw
is too high. I don't care much for the thermostats on the oil filled
radiators, it seems to more control the temperature of the heater, not so
much the room temperature. So I'm thinking an microcontroller, use the
analog inputs to read temperature and amps, and a digital output for a solid
state relay to the heater.

Anyone here mess with signal conditioning for current transformers? Seems
that I need a "precision rectifier" circuit and a capacitor to charge up to
the peak, then scale to RMS amps.

RogerN


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"RogerN" wrote in message
...

I have main propane heat but I also have some of the electric oil
filled radiators. I like the oil filled radiators for cold spots
and to give temperature adjustment to individual rooms.

The problem is that the heaters use up most of the power available
on the circuit. We quit using a heater that was plugged in to the
same circuit as the kitchen, every time we'd run the microwave,
toaster, or other high current appliance, it would kick the breaker
out.

So it seems like a good project to use a current transformer to
measure amp draw on the breaker and automatically turn off the
heater when the amps draw is too high. I don't care much for the
thermostats on the oil filled radiators, it seems to more control
the temperature of the heater, not so much the room temperature. So
I'm thinking an microcontroller, use the analog inputs to read
temperature and amps, and a digital output for a solid state relay
to the heater.

Anyone here mess with signal conditioning for current transformers?
Seems that I need a "precision rectifier" circuit and a capacitor to
charge up to the peak, then scale to RMS amps.

RogerN


I've been monitoring the backup electric heat in my house with a
clamp-on current probe, a TP4000ZC serial-output meter and a laptop.
The Windows datalogging program doesn't include a digital output but I
can read the meter in QBasic under DOS and could trigger a relay from
an LPT port control bit.

If you are only comparing the CT output to a trigger value the output
doesn't have to be linear.

I'll finish this later. You can't compete with Gwyneth on a Glee
rerun.


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RogerN wrote:

I have main propane heat but I also have some of the electric oil filled
radiators. I like the oil filled radiators for cold spots and to give
temperature adjustment to individual rooms.

The problem is that the heaters use up most of the power available on the
circuit. We quit using a heater that was plugged in to the same circuit as
the kitchen, every time we'd run the microwave, toaster, or other high
current appliance, it would kick the breaker out.

So it seems like a good project to use a current transformer to measure amp
draw on the breaker and automatically turn off the heater when the amps draw
is too high. I don't care much for the thermostats on the oil filled
radiators, it seems to more control the temperature of the heater, not so
much the room temperature. So I'm thinking an microcontroller, use the
analog inputs to read temperature and amps, and a digital output for a solid
state relay to the heater.

Anyone here mess with signal conditioning for current transformers? Seems
that I need a "precision rectifier" circuit and a capacitor to charge up to
the peak, then scale to RMS amps.

RogerN


Or just get a current sensing relay for a humidifier for a forced air
heating system. Have the output from the sense relay (which is monitoring
the branch circuit to your kitchen) turn up a NC relay that then cuts your
heater off. I can see there being problems with weird oscillations, but a
time delay relay to delay switch on of the heater may fix that.
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"Jim Wilkins" wrote in message
...
"RogerN" wrote in message
...

I have main propane heat but I also have some of the electric oil
filled radiators. I like the oil filled radiators for cold spots
and to give temperature adjustment to individual rooms.

The problem is that the heaters use up most of the power available
on the circuit. We quit using a heater that was plugged in to the
same circuit as the kitchen, every time we'd run the microwave,
toaster, or other high current appliance, it would kick the breaker
out.

So it seems like a good project to use a current transformer to
measure amp draw on the breaker and automatically turn off the
heater when the amps draw is too high. I don't care much for the
thermostats on the oil filled radiators, it seems to more control
the temperature of the heater, not so much the room temperature.
So I'm thinking an microcontroller, use the analog inputs to read
temperature and amps, and a digital output for a solid state relay
to the heater.

Anyone here mess with signal conditioning for current transformers?
Seems that I need a "precision rectifier" circuit and a capacitor
to charge up to the peak, then scale to RMS amps.

RogerN


I've been monitoring the backup electric heat in my house with a
clamp-on current probe, a TP4000ZC serial-output meter and a laptop.
The Windows datalogging program doesn't include a digital output but
I can read the meter in QBasic under DOS and could control a relay
with an LPT port output bit.

If you are only comparing the CT output to a trigger value the
output doesn't have to be linear.

I'll finish this later. You can't compete with Gwyneth's tango on a
Glee rerun.


John Stamos can sing too.

Anyway, here is a cheap current transformer you might be able to
modify to turn on at a higher current, and detect the LED so you don't
have to worry about open-circuit fault voltage :
http://www.crmagnetics.com/Products/CR2550-P13.aspx

Another approach is to plug the heater into a remote-controlled switch
so you can shut it off -before- turning on the high-current load.

I don't know of a cheap current clamp for a DVM. Mine is an old Fluke
and IIRC came from an auction.

jsw


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Default Current Transformer & heating controller.

On 2014-02-01, RogerN wrote:

I have main propane heat but I also have some of the electric oil filled
radiators. I like the oil filled radiators for cold spots and to give
temperature adjustment to individual rooms.

The problem is that the heaters use up most of the power available on the
circuit. We quit using a heater that was plugged in to the same circuit as
the kitchen, every time we'd run the microwave, toaster, or other high
current appliance, it would kick the breaker out.

So it seems like a good project to use a current transformer to measure amp
draw on the breaker and automatically turn off the heater when the amps draw
is too high. I don't care much for the thermostats on the oil filled
radiators, it seems to more control the temperature of the heater, not so
much the room temperature. So I'm thinking an microcontroller, use the
analog inputs to read temperature and amps, and a digital output for a solid
state relay to the heater.

Anyone here mess with signal conditioning for current transformers? Seems
that I need a "precision rectifier" circuit and a capacitor to charge up to
the peak, then scale to RMS amps.

RogerN



I did exactly that for two sump pumps. Two are too big for one circuit
and there is no way to split them into separate circuits (they are
hooked up to the building that I do not own).

What you need is called a "current sensing relay" and they are
available. You need to use a current sensing relay and a contactor to
turn the heater no and off.

i


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Default Current Transformer & heating controller.

RogerN wrote:



Anyone here mess with signal conditioning for current transformers? Seems
that I need a "precision rectifier" circuit and a capacitor to charge up
to the peak, then scale to RMS amps.

No, you don't need such complexity. Put a small bridge rectifier directly
on the transformer winding, then a burden resistor sized to give you a
few Volts at the desired current. Add a capacitor and you have a
voltage approximately proportional to current. Then, rig a simple
comparator to trip at the desired point.

Now, one problem with this scheme is that under some loads
it could start cycling on and off, cutting the heater on and off
every second or so. So, you might need to add some hysteresis to
the comparator.

Jon
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Default Current Transformer & heating controller.

On Sat, 01 Feb 2014 17:19:56 -0600, RogerN wrote:

I have main propane heat but I also have some of the electric oil filled
radiators. I like the oil filled radiators for cold spots and to give
temperature adjustment to individual rooms.

The problem is that the heaters use up most of the power available on
the circuit. We quit using a heater that was plugged in to the same
circuit as the kitchen, every time we'd run the microwave, toaster, or
other high current appliance, it would kick the breaker out.

So it seems like a good project to use a current transformer to measure
amp draw on the breaker and automatically turn off the heater when the
amps draw is too high. I don't care much for the thermostats on the oil
filled radiators, it seems to more control the temperature of the
heater, not so much the room temperature. So I'm thinking an
microcontroller, use the analog inputs to read temperature and amps, and
a digital output for a solid state relay to the heater.

Anyone here mess with signal conditioning for current transformers?
Seems that I need a "precision rectifier" circuit and a capacitor to
charge up to the peak, then scale to RMS amps.


Do your A to D conversion at the 3rd or higher harmonic of line frequency
(higher really is better), square, and average. Then take the square
root, and you have RMS.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
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On 2/2/2014 1:19, RogerN wrote:

I have main propane heat but I also have some of the electric oil filled
radiators. I like the oil filled radiators for cold spots and to give
temperature adjustment to individual rooms.

The problem is that the heaters use up most of the power available on the
circuit. We quit using a heater that was plugged in to the same circuit as
the kitchen, every time we'd run the microwave, toaster, or other high
current appliance, it would kick the breaker out.

So it seems like a good project to use a current transformer to measure amp
draw on the breaker and automatically turn off the heater when the amps draw
is too high. I don't care much for the thermostats on the oil filled
radiators, it seems to more control the temperature of the heater, not so
much the room temperature. So I'm thinking an microcontroller, use the
analog inputs to read temperature and amps, and a digital output for a solid
state relay to the heater.

Anyone here mess with signal conditioning for current transformers? Seems
that I need a "precision rectifier" circuit and a capacitor to charge up to
the peak, then scale to RMS amps.


Or just buy the ready-made product, current sensing relay.. For example:
http://www.grainger.com/product/SCHN...&sgAttributes=


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Thanks all for a lot of good ideas. The reason for my over engineered
project using an Arduino microcontroller is to have a programmable
thermostat with programmable current sensing and hysteresis, plus time
delays, etc. Also, there are arduino libraries for communicating X-10 over
the power lines. I'm not sure if X-10 modules will be powerful enough for
my heaters but I can use them to switch on a solid state relay.

If I can use the current sensing relay idea, or the comparator idea, I can
have my current sensing on digital input pins, saving some analog pins for
temperature sensing.

My current idea is have the current transformers and Arduino at the breaker
box. When the current goes too high (or temperature setpoint), the
Arduino sends an X-10 command to shut off the heater on that circuit.
In short, it will be a multi-zone programmable thermostat that tries to
prevent breaker tripping.

This project came about because of the price of propane, I want to have an
alternative if the propane runs out while the price is outrageous.

RogerN


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"Ignoramus23944" wrote in message
...

I did exactly that for two sump pumps. Two are too big for one circuit
and there is no way to split them into separate circuits (they are
hooked up to the building that I do not own).

What you need is called a "current sensing relay" and they are
available. You need to use a current sensing relay and a contactor to
turn the heater no and off.

i


That might work, it's kind of a gray area. I need to be able to adjust the
amps where it kicks OFF and the amps where it kicks ON. For example, if my
heater takes 10A then I don't want to turn it ON until the line current
drops to 4A or less, but it doesn't need to kick out until the amps get up
around 13-15A.

The thermo cube thing you mentioned is working great in the utility room, I
hadn't ever heard of them until you mentioned it. Thanks!

RogerN




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"Jon Elson" wrote in message
...

RogerN wrote:

Anyone here mess with signal conditioning for current transformers?
Seems
that I need a "precision rectifier" circuit and a capacitor to charge up
to the peak, then scale to RMS amps.

No, you don't need such complexity. Put a small bridge rectifier directly
on the transformer winding, then a burden resistor sized to give you a
few Volts at the desired current. Add a capacitor and you have a
voltage approximately proportional to current. Then, rig a simple
comparator to trip at the desired point.

Now, one problem with this scheme is that under some loads
it could start cycling on and off, cutting the heater on and off
every second or so. So, you might need to add some hysteresis to
the comparator.

Jon


I was thinking about the burden resistor on the DC side of a full wave
bridge rectifier, so the voltage drop across the rectifiers doesn't affect
the reading, but I'm not sure how much voltage I should get out of the CT
without causing problems. I was planning to have a small value of burden
resistor to keep the voltage output low.

I can do the comparator and hysteresis, that would be great for the basic ON
and OFF but I was also wanting to use temperature input and am considering
using X-10 to control power to the heater. There are low voltage/contact
modules for X-10 that I could use with a thermostat in series with the
comparator output to shut heat off when the temperature was reached or the
current was above the max.

RogerN


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On 2014-02-02, RogerN wrote:
"Ignoramus23944" wrote in message
...

I did exactly that for two sump pumps. Two are too big for one circuit
and there is no way to split them into separate circuits (they are
hooked up to the building that I do not own).

What you need is called a "current sensing relay" and they are
available. You need to use a current sensing relay and a contactor to
turn the heater no and off.

i


That might work, it's kind of a gray area. I need to be able to adjust the
amps where it kicks OFF and the amps where it kicks ON. For example, if my
heater takes 10A then I don't want to turn it ON until the line current
drops to 4A or less, but it doesn't need to kick out until the amps get up
around 13-15A.


Do not overthink this, do it with a particular current limit like 4A
and enjoy a working system.

The thermo cube thing you mentioned is working great in the utility room, I
hadn't ever heard of them until you mentioned it. Thanks!


Good to hear!

i
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"RogerN" wrote in message
m...
Thanks all for a lot of good ideas. The reason for my over
engineered project using an Arduino microcontroller is to have a
programmable thermostat with programmable current sensing and
hysteresis, plus time delays, etc. Also, there are arduino
libraries for communicating X-10 over the power lines. I'm not sure
if X-10 modules will be powerful enough for my heaters but I can use
them to switch on a solid state relay.

If I can use the current sensing relay idea, or the comparator idea,
I can have my current sensing on digital input pins, saving some
analog pins for temperature sensing.

My current idea is have the current transformers and Arduino at the
breaker box. When the current goes too high (or temperature
setpoint), the Arduino sends an X-10 command to shut off the heater
on that circuit.
In short, it will be a multi-zone programmable thermostat that tries
to prevent breaker tripping.

This project came about because of the price of propane, I want to
have an alternative if the propane runs out while the price is
outrageous.

RogerN


Don't forget to allow for the faults the Arduino can cause, like
shutting off all the heat.
jsw


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"Jim Wilkins" wrote in message ...

"RogerN" wrote in message

snip

This project came about because of the price of propane, I want to have
an alternative if the propane runs out while the price is outrageous.

RogerN


Don't forget to allow for the faults the Arduino can cause, like shutting
off all the heat.
jsw


If all else fails I'll just plug the heaters in and adjust the thermostats
manually, and if the breaker trips, I'll have to reset it, at least for the
10 minutes it takes me to program and swap out the Arduino! If the crash if
from a software glitch, I may have to run in manual mode a bit longer :-)

RogerN


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"Tim Wescott" wrote in message
...

On Sat, 01 Feb 2014 17:19:56 -0600, RogerN wrote:

snip
Anyone here mess with signal conditioning for current transformers?
Seems that I need a "precision rectifier" circuit and a capacitor to
charge up to the peak, then scale to RMS amps.


Do your A to D conversion at the 3rd or higher harmonic of line frequency
(higher really is better), square, and average. Then take the square
root, and you have RMS.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com


I did some checking, the Arduino's AnalogRead() function is supposed to take
around 100uS. I'm thinking I can take a A/D reading ~641uS apart and get 26
readings in 16.666mS. Then do the RMS calculations from the readings. This
should be great because I should be able to take the voltage from the burden
resistor to provide 2V P-P at full scale. Note I'm putting a 100A CT on a
15 A circuit so I should have around 1/10 amp resolution per count (10 bit).
If this doesn't read well, I'll get a lower amperage CT.

So thanks for the idea! That saves an Op Amp rectifier and filter! I never
thought of doing the calculations right from the AC signal.

RogerN




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On 02/03/2014 04:24 PM, RogerN wrote:
"Tim Wescott" wrote in message
...

On Sat, 01 Feb 2014 17:19:56 -0600, RogerN wrote:

snip
Anyone here mess with signal conditioning for current transformers?
Seems that I need a "precision rectifier" circuit and a capacitor to
charge up to the peak, then scale to RMS amps.


Do your A to D conversion at the 3rd or higher harmonic of line frequency
(higher really is better), square, and average. Then take the square
root, and you have RMS.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com


I did some checking, the Arduino's AnalogRead() function is supposed to take
around 100uS. I'm thinking I can take a A/D reading ~641uS apart and get 26
readings in 16.666mS. Then do the RMS calculations from the readings. This
should be great because I should be able to take the voltage from the burden
resistor to provide 2V P-P at full scale. Note I'm putting a 100A CT on a
15 A circuit so I should have around 1/10 amp resolution per count (10 bit).
If this doesn't read well, I'll get a lower amperage CT.

So thanks for the idea! That saves an Op Amp rectifier and filter! I never
thought of doing the calculations right from the AC signal.

RogerN


I'd be interested in the actual USEFUL resolution you can get in this
noisy environment. I tried to do 10 bits on a PIC in a quiet
environment and was very disappointed.

Might be interesting to use an X-10 appliance module to control the
heater. Don't know about Arduino. PIC Basic has library support for
X-10 commands. Or just take apart a controller and wire to a button.
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"Ignoramus24227" wrote in message
...

On 2014-02-02, RogerN wrote:

That might work, it's kind of a gray area. I need to be able to adjust
the
amps where it kicks OFF and the amps where it kicks ON. For example, if
my
heater takes 10A then I don't want to turn it ON until the line current
drops to 4A or less, but it doesn't need to kick out until the amps get
up
around 13-15A.


Do not overthink this, do it with a particular current limit like 4A
and enjoy a working system.


For that I'd have to get to downstream wires, either get the current relay
in the wall or splice wires out to an electrical box. If I tried it at the
breaker, the heater kicking ON would cause the relay to turn ON, causing the
heater to go off, and back and forth. At the breaker, I can set it to turn
the heater OFF at 14A and not kick back on until it's below 4A, the 10A that
would allow my heater to run on reduced power, around 8A.

The advantage of combining current limiting and a programmable thermostat
seem worth the little extra to me. Either a thermostat or over current cut
off requires turning the heater ON and OFF(the high current expensive part),
add a temperature sensor ($2) and a current sensor ($5) does it all in one.

RogerN


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"RogerN" wrote in message
...
"Jim Wilkins" wrote in message ...

"RogerN" wrote in message

snip

This project came about because of the price of propane, I want to
have an alternative if the propane runs out while the price is
outrageous.

RogerN


Don't forget to allow for the faults the Arduino can cause, like
shutting off all the heat.
jsw


If all else fails I'll just plug the heaters in and adjust the
thermostats manually, and if the breaker trips, I'll have to reset
it, at least for the 10 minutes it takes me to program and swap out
the Arduino! If the crash if from a software glitch, I may have to
run in manual mode a bit longer :-)

RogerN


I've been testing what temperature one ~700 W electric baseboard
radiator with a low-temp thermostat can maintain, to address my
insurance company's concern about reliable automatic heat as backup
for my wood stove. Apparently it plus heat lost from the basement
water heater will maintain at least 45 F during a cold spell. It shut
off on a 28F sunny day.

The preliminary test runs I made during the first Polar Vortex added
less than $2 per day to my electric bill.
jsw


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"RogerN" wrote in message
m...
"Tim Wescott" wrote in message
...

On Sat, 01 Feb 2014 17:19:56 -0600, RogerN wrote:

snip
Anyone here mess with signal conditioning for current
transformers?
Seems that I need a "precision rectifier" circuit and a capacitor
to
charge up to the peak, then scale to RMS amps.


Do your A to D conversion at the 3rd or higher harmonic of line
frequency
(higher really is better), square, and average. Then take the
square
root, and you have RMS.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com


I did some checking, the Arduino's AnalogRead() function is supposed
to take around 100uS. I'm thinking I can take a A/D reading ~641uS
apart and get 26 readings in 16.666mS. Then do the RMS calculations
from the readings. This should be great because I should be able to
take the voltage from the burden resistor to provide 2V P-P at full
scale. Note I'm putting a 100A CT on a 15 A circuit so I should
have around 1/10 amp resolution per count (10 bit). If this doesn't
read well, I'll get a lower amperage CT.

So thanks for the idea! That saves an Op Amp rectifier and filter!
I never thought of doing the calculations right from the AC signal.

RogerN


This gives a hint of how the Kill-A-Watt measures power:
http://www.ladyada.net/make/tweetawatt/



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On 2/4/2014 5:21, RogerN wrote:
For that I'd have to get to downstream wires, either get the current relay
in the wall or splice wires out to an electrical box. If I tried it at the
breaker, the heater kicking ON would cause the relay to turn ON, causing the
heater to go off, and back and forth. At the breaker, I can set it to turn
the heater OFF at 14A and not kick back on until it's below 4A, the 10A that
would allow my heater to run on reduced power, around 8A.


Use a timer relay triggered by the current sensing relay.
If current is too much, switch off heating for XX minutes.
Problem solved.


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On 2014-02-04, Kristian Ukkonen wrote:
On 2/4/2014 5:21, RogerN wrote:
For that I'd have to get to downstream wires, either get the current relay
in the wall or splice wires out to an electrical box. If I tried it at the
breaker, the heater kicking ON would cause the relay to turn ON, causing the
heater to go off, and back and forth. At the breaker, I can set it to turn
the heater OFF at 14A and not kick back on until it's below 4A, the 10A that
would allow my heater to run on reduced power, around 8A.


Use a timer relay triggered by the current sensing relay.
If current is too much, switch off heating for XX minutes.
Problem solved.


You got it exactly wrong (in my opinion, and with due respect).

The relay, to be used with a timer, should turn on the heater on XX
seconds after the other devices have been turned off.

i
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On 2/5/2014 5:16, Ignoramus13005 wrote:
On 2014-02-04, Kristian Ukkonen wrote:
On 2/4/2014 5:21, RogerN wrote:
For that I'd have to get to downstream wires, either get the current relay
in the wall or splice wires out to an electrical box. If I tried it at the
breaker, the heater kicking ON would cause the relay to turn ON, causing the
heater to go off, and back and forth. At the breaker, I can set it to turn
the heater OFF at 14A and not kick back on until it's below 4A, the 10A that
would allow my heater to run on reduced power, around 8A.


Use a timer relay triggered by the current sensing relay.
If current is too much, switch off heating for XX minutes.
Problem solved.


You got it exactly wrong (in my opinion, and with due respect).

The relay, to be used with a timer, should turn on the heater on XX
seconds after the other devices have been turned off.


We can agree to disagree.

In the OP case I would do it so that if the microwave etc. is
used and the current (microwave + heater) comes too big, the heater
will be switched off for 5 minutes (or such time). After 5 minutes,
if current demand is small enough, heater continues to heat. If
there is again too much current demand, the heater is off another
5 minutes. etc..

That is quite easy to do. Current sensing relay controlling timer relay
controlling heater contactor/relay. Also, the heater doesn't really
mind being off 5 minutes or 10 minutes. The thermal mass of the room is
big enough. It will lowpass-filter any short stopping of heating.

That's just how I would do it.
Many ways to do the same end effect, I agree.

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Default Current Transformer & heating controller.

"Kristian Ukkonen" wrote in message
...

In the OP case I would do it so that if the microwave etc. is
used and the current (microwave + heater) comes too big, the heater
will be switched off for 5 minutes (or such time). After 5 minutes,
if current demand is small enough, heater continues to heat. If
there is again too much current demand, the heater is off another
5 minutes. etc..

That is quite easy to do. Current sensing relay controlling timer
relay
controlling heater contactor/relay. Also, the heater doesn't really
mind being off 5 minutes or 10 minutes. The thermal mass of the room
is
big enough. It will lowpass-filter any short stopping of heating.

That's just how I would do it.
Many ways to do the same end effect, I agree.


If I had the problem I'd add a remote time-delay-relay control that
shuts off the heater during meal prep to avoid the race between the
current-sensing relay and the overloaded breaker.
http://www.pexsupply.com/Fantech-FD6...FW3NOgodlzkA3w
jsw


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