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Integral control (PWM) of AC voltage input to a heating element

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abe94

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Hi.
I'm designing an open loop voltage controller for my water heater element. I'm planning on using a triac and controlling it using PWM. There is no temperature sensor feedback , just plain AC voltage control. Now , could anyone point me in the right direction regarding this ? I researched AC voltage controllers from various books and have a general idea of the scheme , but get stumped on the finer points. How do I generate PWM trigger pulses ? I suppose it'll be using a 555 timer. If yes , then how ? Also , one of the books said that a capacitive filter was needed at the input to smooth out high switching frequency current drawn from the supply. Another book had exact calculations for the rms output voltage as per the duty cycle employed (Vorms = Vs√k)
where Vorms = output rms voltage
Vs = supply voltage
k = duty cycle
As you can see , my thoughts are pretty much scattered all over the place :-D
Somebody please help me streamline this.

p.s The supply voltage usually hovers around 200 V , give or take 10 or 20 V.
 

FvM

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Heater control will use full wave switching (as required by power quality regulations), in so far no high switching frequency would be involved.
 

R

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That means you need a zero-cross optoisolator triac driver (MOC3083) then you might use a simple 555 to generate a PWM signal with very low frequency (T=few seconds) to drive the optotriac.

This way, the voltage applied to your heating element will be half waves multiple thus there will be no stress on the mains power supply.
 

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Controlling heater power without temperature feedback refers to measuring the actual heater voltage with a true RMS circuit, or if you consider variation of the heater element, measuring real power.
 
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    abe94

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abe94

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Okay so I need an optoisolator/driver. Is it for synchronising the gate pulses to the zero crossing of the input voltage ?

Thanks for the answer.

Yes. Something along the lines. I'm going open loop because I'd like to get my feet wet before diving in fully. I plan on adding temperature sensors and feedback to the circuit. If that goes successfully , i'll add remote monitoring , logging and control to the system. Right now I just want to get over the base calculations and stuff.

The basic idea was to control the temperature of my water heater. If it gets really hot , I'll change the circuit parameters (the duty cycle of the gate pulse in this case) and decrease the voltage input to the heater coils. This would automatically reduce the heat output of the heater coils.

- - - Updated - - -

Okay so I need an optoisolator/driver. Is it for synchronising the gate pulses to the zero crossing of the input voltage ?

Controlling heater power without temperature feedback refers to measuring the actual heater voltage with a true RMS circuit, or if you consider variation of the heater element, measuring real power.

Yes. Something along the lines. I'm going open loop because I'd like to get my feet wet before diving in fully. I plan on adding temperature sensors and feedback to the circuit. If that goes successfully , i'll add remote monitoring , logging and control to the system. Right now I just want to get over the base calculations and stuff.

Thanks for the answer.

Yes. Something along the lines. I'm going open loop because I'd like to get my feet wet before diving in fully. I plan on adding temperature sensors and feedback to the circuit. If that goes successfully , i'll add remote monitoring , logging and control to the system. Right now I just want to get over the base calculations and stuff.

The basic idea was to control the temperature of my water heater. If it gets really hot , I'll change the circuit parameters (the duty cycle of the gate pulse in this case) and decrease the voltage input to the heater coils. This would automatically reduce the heat output of the heater coils.

That means you need a zero-cross optoisolator triac driver (MOC3083) then you might use a simple 555 to generate a PWM signal with very low frequency (T=few seconds) to drive the optotriac.

This way, the voltage applied to your heating element will be half waves multiple thus there will be no stress on the mains power supply.

Okay so I need an optoisolator/driver. Is it for synchronising the gate pulses to the zero crossing of the input voltage ?
 
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R

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Okay so I need an optoisolator/driver. Is it for synchronising the gate pulses to the zero crossing of the input voltage ?

Yes, switching at a zero-crossing greatly reduces radiated and conducted noise. Anyway, switching half-cycles can introduce a significant DC current in the power line so it's better (for the power company) to switch the whole cycles (one positive and one negative half wave). If you're using long trains of half-cycle pulses it won't be as noticeable though.
 

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Okay so I need an optoisolator/driver. Is it for synchronising the gate pulses to the zero crossing of the input voltage ?

Thanks for the answer.

Yes. Something along the lines. I'm going open loop because I'd like to get my feet wet before diving in fully. I plan on adding temperature sensors and feedback to the circuit. If that goes successfully , i'll add remote monitoring , logging and control to the system. Right now I just want to get over the base calculations and stuff.

The basic idea was to control the temperature of my water heater. If it gets really hot , I'll change the circuit parameters (the duty cycle of the gate pulse in this case) and decrease the voltage input to the heater coils. This would automatically reduce the heat output of the heater coils.

- - - Updated - - -





Okay so I need an optoisolator/driver. Is it for synchronising the gate pulses to the zero crossing of the input voltage ?

Yes, switching at a zero-crossing greatly reduces radiated and conducted noise. Anyway, switching half-cycles can introduce a significant DC current in the power line so it's better (for the power company) to switch the whole cycles (one positive and one negative half wave). If you're using long trains of half-cycle pulses it won't be as noticeable though.

OK I've selected a chip for this. Its the MOC3041. It has a zero crossing detector built on chip. Now all I need to do is to feed a PWM pulse through the cathode of the optocoupler. One more question thats bugging me : Do I have to power the gate drive and signal generation ic's using a seperate DC supply ? Cant I just feed the rectified and regulated voltage from the power supply ? I'm considering this because they need to be working only when there's AC power fed to the circuit. This way I wouldn't need to change batteries every now and then. Is this method safe ? Or would it introduce noise in the control and drive signals ?
 

R

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You could use a transformerless mains-derived power supply (some design tips are presented here or here):

xformerless_PS.gif
 

SunnySkyguy

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The proper way is to define the expectations then design to meet them, not cozy up with an inadequate solution.

Reliability is key so the sensor connections must never fail or open sensor would force OFF condition.. by careful planning and connections.

Since water heater is rather slow, PWM is not a needed, only ON / OFF control with a Triac or Relay Switch and Thermistor epoxied to tank using reliable connections. Some noise is expected so hysteresis of a degrees or so is OK. I just used a thermistor bridge to comparator to drive a relay for waterbed in the 70's. Some filtering for AC on sensor is necessary. Tons of circuits will do this. Pick any

I dont know why you wouldn't just take any home digital thermostat that has the range, and put thermistor external with reed relay drive to power relay. ( CHeap and dirty)
 

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The proper way is to define the expectations then design to meet them, not cozy up with an inadequate solution.

Reliability is key so the sensor connections must never fail or open sensor would force OFF condition.. by careful planning and connections.

Since water heater is rather slow, PWM is not a needed, only ON / OFF control with a Triac or Relay Switch and Thermistor epoxied to tank using reliable connections. Some noise is expected so hysteresis of a degrees or so is OK. I just used a thermistor bridge to comparator to drive a relay for waterbed in the 70's. Some filtering for AC on sensor is necessary. Tons of circuits will do this. Pick any

I dont know why you wouldn't just take any home digital thermostat that has the range, and put thermistor external with reed relay drive to power relay. ( CHeap and dirty)


Yeah I could use a digital controller. It would make the task dirty and cheap. But where's the fun in that.. My actual intention was to make it a learning project and a plus point would be that I would be able to control the heater using it. I wasn't exactly dying for a robust temp control system , I just liked what I read in class and decided to create a practical implementation for fun and learning. I could have used a simpler technique , like the one you mentioned , but I was actually interested in AC voltage controllers and just happened to have a heater to use it on :-D
 

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You could use a transformerless mains-derived power supply (some design tips are presented here or here):

View attachment 118682

WOW this is awesome !! Thanks for you help man ! Really appreciate it ! A quick question though.. wouldn't using a transformer provide isolation from the main circuit ? Or is it overkill for this small application ? I don't want to over-design it.. If there is such a thing. Thanks again !
 

R

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Of course, you could use a transformer but I thought you were looking for a simple solution.

If you're going to use a 555 timer to generate that PWM signal then you don't need a galvanic isolation.
 

SunnySkyguy

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Yeah I could use a digital controller. It would make the task dirty and cheap. But where's the fun in that.. My actual intention was to make it a learning project and a plus point would be that I would be able to control the heater using it. I wasn't exactly dying for a robust temp control system , I just liked what I read in class and decided to create a practical implementation for fun and learning. I could have used a simpler technique , like the one you mentioned , but I was actually interested in AC voltage controllers and just happened to have a heater to use it on :-D

A perfect project then is to make a hot water cooking tank for dinner called Sous Vide which requires that you cooking for hours at low temp ( usually precisely controlled below 50'C to prevent killing of nutrients. ) https://anovaculinary.com/pages/sous-vide-precision-cooking/

Then implement a PID feedback to control the phase of the triac. An XOR gate or full wave bridge easily creates a pulse for every zero crossing, then adjust delay after this in s/w or VCO timer or Voltage controlled one-shot time delay up to 1/2T for OFF
 

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