Hi All,
I have just spent some time designing a small water heater controller and would like some help from those with more experience than I have. Mechanical thermostats have too much dead band and this design should provide very little hysteresis. PID controllers are way too expensive and complicated. So, I gave it a shot. I welcome suggestions, constructive criticism, and sage advice. I am sure I did not get it all right.
I am concerned about the wattage of the resistor on the AC side of the Triac. Any help is appreciated.
I calculated the current through an LED that will show the state of the heater. I shot for 5ma but am not sure I got it right.
I shot for 30ma for the IR LED in the optoisolator. Same question there. Did I get it right?
The Triac is a 12A job that I think should get ‘er done with ease. Maybe a heat sink is necessary?
The schematic is attached. I hope you will give it a look and tell me what you think.
Thanks
Moniker

 

Hello,

Looking at the schematic it looks almost OK.

Lower R4 to 2k7, this will assure that the transistor is saturated.
Enlarge R3 to 10K, this will be enough to cut the current of the transistor.
The 180Ω will be good to take a 1 Watt resistor as it is in a mains circuit.

The current for the MOC should be enough as the datasheet says a minimum of 15 mA and a maximum of 60 mA for the emmitor.

You can enlarge the current for the monitor led if it is to weak to a max of 20 mA.

Bertus

 

.... and this design should provide very little hysteresis.

Sure it does, it has no hysteresis.

While the absence of hysteresis would not harm your circuit element or the heater, the rapid switching ON/OFF of the heater at the set point will generate a lot of interference on the power supply line and causes problem to your other appliance. Flickering of lights and noise to radio etc...

However, adding hysteresis involves only a single resistor so you can wait to see if there is any problem before incorporating the hysteresis into your circuit.

 

Bertus,

Thanks for the review. I have not dealt with any of this stuff for over 45 years and it feels good that I was able to come close. It was fun, but a bit confusing, in reading over data sheets, figuring out what the devices do, and then trying to use them. Your help is appreciated.

BTW, I initially did ask the forum ware to set my time for the US Eastern time zone. If you could not find it, I guess the program had not updated yet. Also, I added a bit of personal stuff so that I am not a total black hole.

eblc,

Thanks for the reply. I guess all physical things in the universe have hysteresys to some degree or another. This circuit, however, should have very little as designed.

If I added the resistor you mention, it would have more. And the resistor could be adjusted for how much more. Indeed, the circuit can be used as is, and if excessive noise results, I will have to consider the feedback.

I am hopeing it will not have too much noise because the physical heater and water have too much hysteresys as it is. The mechanical thermostat had way too much, and added to the system over shoot and under shoot, the spread was too high. That is why I cooked up this design. I am trying to narrow the spread between the hottest and coldest water and do not want to add to the spread or go the PID route.

Someone mentioned that I might consider a zero-crossing version of the optoisolator. That would reduce noise without compromising low hysteresys. Any thoughts anyone?

Thank you both for your interest and help. I hope others will chime in as well.

Moniker

 

Hello,

As eblc1388 mentioned some hysteresys may reduce the amount of switching.
Here is explained how the hysteresys can be build into the circuit using only one resistor:
http://www.allaboutcircuits.com/vol_3/chpt_8/12.html
(read the part about midway the page).

Bertus

 

Besides hysteresis, your circuit will not work as it is now.

You have the IN and -IN input of the opamp connections reversed.

Right now the circuit will only work correctly with a positive temperature coefficient thermistor (PTC) but as you have indicated that you are using a negative temperature coefficient thermistor which decreases in resistance with rising temperature, then you will need to wire this to the (+) input of the opamp instead.

Just remember the opamp output should be high at room temperature so that the TRIAC & heater can be powered on to raise the water temperature.