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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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Current Transformer & heating controller.
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 |
#2
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Current Transformer & heating controller.
"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. |
#3
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Current Transformer & heating controller.
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. |
#4
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Current Transformer & heating controller.
"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 |
#5
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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 |
#6
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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 |
#7
Posted to rec.crafts.metalworking
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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 |
#8
Posted to rec.crafts.metalworking
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Current Transformer & heating controller.
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= |
#9
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Current Transformer & heating controller.
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 |
#10
Posted to rec.crafts.metalworking
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Current Transformer & heating controller.
"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 |
#11
Posted to rec.crafts.metalworking
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Current Transformer & heating controller.
"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 |
#12
Posted to rec.crafts.metalworking
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Current Transformer & heating controller.
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 |
#13
Posted to rec.crafts.metalworking
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Current Transformer & heating controller.
"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 |
#14
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Current Transformer & heating controller.
"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 |
#15
Posted to rec.crafts.metalworking
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Current Transformer & heating controller.
"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 |
#16
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Current Transformer & heating controller.
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. |
#17
Posted to rec.crafts.metalworking
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Current Transformer & heating controller.
"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 |
#18
Posted to rec.crafts.metalworking
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Current Transformer & heating controller.
"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 |
#19
Posted to rec.crafts.metalworking
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Current Transformer & heating controller.
"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/ |
#20
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Current Transformer & heating controller.
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. |
#21
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Current Transformer & heating controller.
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 |
#22
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Current Transformer & heating controller.
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. |
#23
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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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