thermostat control of a heater

Thread Starter

denison

Joined Oct 13, 2018
236
Many heaters use a mechanical type thermostat which is a simple bi-metallic bar that bends with heat and makes or breaks a circuit. These are terrible for comfort because you are too hot when on and too cold when off. The digital control ones are not much better. They are either on or off.
So I made my own what I call 'temperature controlled power'. It works like this. You set a temp of say 21c. At 20c maximum power is applied to say a 1000 watt heater. At 20.5c power applied becomes 500 watt and so on. Proportional power is applied according to the temperature. If the temp reaches 21c the heater would turn off. Practically what happens is that the ambient temp in the heated space stays between 20c and 21c.
While it works perfectly I have ended up with a birds nest of wires. I want something more compact so I am looking for a commercial product which will do the same thing and I wont have a jumble of wires.
I think RS Components stock number 188-5165 temperature controller may do the job. This has an SSR output as well as a ordinary relay output. I have not been able to find out yet if the output of the SSR is zero crossing or phase control. I am guessing it would be zero crossing as most are. Then all you have to do is apply PWM to the SSR input .
So does anybody on this forum know of a good commercial product that will have 'temperature controlled power'. I guess its easy to see why mechanical thermostats dominate the market because of the cost of 188-5165 for example. My own circuit is very simple and should be able to be made commercially fairly cheaply.
 

Reloadron

Joined Jan 15, 2015
6,064
Temperature sensor? Temperature range? You make no mention of a choice of temperature sensor or actual range you wish to control within. You are describing PID (Proportional Integral Derivative) control. Start with a dimple Google of PID Temperature controllers. You can find inexpensive units like this one likely from China or more expensive versions. They offer a few output types like you mentioned and offer a wide range of input sensors all user programmable. You choose temperature control based on your needs like how tight of a band you wish to maintain.

Ron
 

Reloadron

Joined Jan 15, 2015
6,064
I never caught that this was just a room heater. My bad on that note. Go to store, buy thermostat, mount on wall. There done. :)

Ron
 

Thread Starter

denison

Joined Oct 13, 2018
236
Temperature sensor? Temperature range? You make no mention of a choice of temperature sensor or actual range you wish to control within. You are describing PID (Proportional Integral Derivative) control. Start with a dimple Google of PID Temperature controllers. You can find inexpensive units like this one likely from China or more expensive versions. They offer a few output types like you mentioned and offer a wide range of input sensors all user programmable. You choose temperature control based on your needs like how tight of a band you wish to maintain.

Ron
Hi Ron, I use a NTC thermistor in my design. I set to the comfortable temperature of 20c. This seems to be the best temp for human comfort. I use a range of approx 1c. Yes I had guessed I was looking for a PID type.
I will reiterate that there is no comparison between on/off thermostats and the type I have designed. With on/off types you feel alternatively hot and then cold. I still have a 1000/2000 watt fan heater with a thermostat. To avoid the thermostat working I set it to maximum and then just use the 1000/ 2000 setting as necessary.
The RS Components model I am looking at is very expensive. They are going to get back to me on the same questions you have posed. I also wanted to know if the control is zero voltage crossing or phase control of the mains. I use zero voltage crossing in my design as this avoids EMI.
 

MrSalts

Joined Apr 2, 2020
534
Get a Google Nest Learning Thermostat. Not the standard Google Nest, be sure to get the Learning version (-$250). This has timers to understand how much energy it takes to move the temp up or down one degree and, with that, it learned how quickly your home will lose or gain heat. It keeps checking how much time it takes for the temp change vs energy input to update its estimates. It can essentially tell you the outdoor windchill if below room temp outside or the solar load temp if your home needs cooling. It can maintain within a fraction of a degree through the day.
 

Thread Starter

denison

Joined Oct 13, 2018
236
Hot then cold...repeat, sounds like the system has too much hysteresis and is cycling too slow.
Yes too much hysteresis. Mechanical thermostats are all like that. A digital control thermostat could be made with less hysteresis. But if you make it an on/off type you still have either maximum power or no power applied so you can't get away from a certain amount of hysteresis and slow cycling. And if you make the cycling too fast and are using a relay it is not going to last long.
That is why PID control is much better. It means proportional integral derivative control. A fancy name. I prefer 'proportional control' or 'temperature controlled power' because you are varying the power to an appliance according to the measured temperature.
The commercially available PID controllers are too bulky and expensive to replace on/off thermostats. My own design is much better. It doesn't for instance need a SSR (solid state relay). It can be made with a much smaller footprint.
 

kaindub

Joined Oct 28, 2019
88
The RS Components stock number 188-5165 is suitable for what you want.
I used lots of similar stuff on heater controls for plastics extruders.
You just connect your heater to the mechanical relay or SSR output. Because the time constant of heaters and air is so large PWM control is not necessary. nor does it work well
The controllers are full PID control which is essentially what you were describing.
BUT the success of all this is in the tuning of the PID.
 
Let's put temperature controllers into perspective. The good ones have all sort so configuration options, but lets;s look at the output: 0-5V, 1-5V, 0-20mA and 4-20mA. There are heat/cool models too. You have P, PI and PID control. With P-proportional control, the setpoint never matches the measured value. With PI it does. That adds integral, PID adds derivative and it helps with overshoot. those numbers above like 0-10V is 0-100%.

You also have bang/bang (the bimetalic thermostat), SCT (slow cycle triac), SCR (slow cycle relay), ZCP (zero crossin pulse) and PAF (Phase angle fired) except bang/bang and PAF you basically control with time or number of power cycles.

Phase angle fired gets you the finest control.
Now, you need to add a load type: e.g. tungsten, resistive, voltage or true power control
When the load is resistive, the box that take the 0-100% and does something with it makes the 0-100% proportional to V squared. Why V^2? Because power is (V^2)/R
So, the PID thingy takes a 0-10V signal and makes 100% proportional to V^2.

You add current limit and your basically there, PAF and current limit is REQUIRED when operating into an inductive load.

The resistive elements last longer if you use DC. Ask me how I know.

The controllers I used had two boxes, one did the control stuff and another did the power stuff.

PID is "cake" to do in software. I never did auto tuning.

I taught someone on ETO how to do PAF in software, but I don;t know how to implement current limit. He wanted proportional to V, because he was controlling speed.

A house thermostat could incorporate outdoor temperature and fuzzy logic because it has other constraints. e.g. be at 75F at 5:00 which is the programmed coming home time. The house thermostat I have knows the AC cooling and heating capacity , the CFM of the fan and the outside temperature. It also reports HVAC fault conditions.
 
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Thread Starter

denison

Joined Oct 13, 2018
236
The RS Components stock number 188-5165 is suitable for what you want.
I used lots of similar stuff on heater controls for plastics extruders.
You just connect your heater to the mechanical relay or SSR output. Because the time constant of heaters and air is so large PWM control is not necessary. nor does it work well
The controllers are full PID control which is essentially what you were describing.
BUT the success of all this is in the tuning of the PID.
My mistake in saying I use PWM. The method I use is to change the ratio of on pulses to off pulses ( the duty cycle). What is this called in technical language?
I have found the same thing as 188-5165 on e-bay for a fraction of the cost charged by RS. I will probably find that I don't get very good instructions for tuning.
I am using my heater control at the moment. Maximum power of the heater is 800 watts. I have set the temperature to 19c. My power meter is showing 500 watts. Comfort rating perfect. 13c outside temperature at the moment.
 
The method I use is to change the ratio of on pulses to off pulses ( the duty cycle).
I don;t think there is a standard term. Eurotherm would use zero crossing pulse or ZCP. the finest you can control is 1/2 a line cycle. Slow cycle triac, basically uses on and off time over a larger period. e.g. 5s on 1s off. not power cycles. 10 power half cycles on and 90 power 1/2 cycles off.
 

Thread Starter

denison

Joined Oct 13, 2018
236
I don;t think there is a standard term. Eurotherm would use zero crossing pulse or ZCP. the finest you can control is 1/2 a line cycle. Slow cycle triac, basically uses on and off time over a larger period. e.g. 5s on 1s off. not power cycles. 10 power half cycles on and 90 power 1/2 cycles off.
I have designed my own heater because I wanted something that uses less power. I have 3 now. The 800w one I am using now in a small room. A 500w one in a small room. A 1000w heater in a large loungeroom.
I have a patent application for it sitting in the patent examiners office at the moment. It was prepared by a patent attorney.
See my answer to Kaindub.
 

Phil-S

Joined Dec 4, 2015
222
Microcontrollers are just perfect for a thermostat application.
For industrial stuff, the market has changed a lot since I was involved.
I would have looked at what Eurotherm had to offer, as well as Crouzet, Omron or many others in the process control market.
I'm not sure what the RS own brand stuff is like, but I haven't had any problems, and they will exchange rapidly any duff stuff.
I've also started to dabble with the "smart" stuff.
I have some Sonoff TH10 temperature modules that come with DS18B20 sensors.
So far, pleasantly surprised.
A water heater set to 50-degrees C switches at low 49.9 and high at 50.1.
An update in the firmware allows not just temperature control, but time/date scheduling.
I'm looking to replace the expensive and unreliable and complex Honeywell and Drayton room thermostats.
I'm tickled by the control from anywhere phone app, LAN control.
But the best bit is not having to wade through an instruction manual, holding a torch, to change the settings - so much easier on the phone, screen in front of you, somewhere comfortable.
As far as discrete sensors go, LM35 are good analogue devices, and DS18B20 are excellent digital devices.
 

Privatier

Joined Dec 10, 2008
4
I have forced air heating in my house and a thermostat on the wall. However the temperature always overshoots several degrees, primarily because the thermostat does not really sense the temperature in the room, but more like the temperature of the wall, which changes much slower. Also after the thermostat turns off the heater, the blower continues for about two minutes and delivers the heat stored in the ducts to the room. I first placed a thermometer next to the thermostat and one into the air return duct and recorded the measurements in a data logger, to understand what is going on. I then built a little controller with an Arduino, which stops the heating cycle after a predetermined amount of time, which is dependent on the current temperature, allowing for a longer cycle when temperature is low. The result is that there are somewhat more heat cycles now, but there are no more complaints about too high room temperature.
 
There are two methods used in HVAC systems for heat. One is time an the other is temperature to remove the residual heat.
In heat mode, the furnace controls the fan. In cool mode, the thermostat controls the fan.
Sometimes it can be set in the stat.
 

MisterBill2

Joined Jan 23, 2018
9,292
Get a Google Nest Learning Thermostat. Not the standard Google Nest, be sure to get the Learning version (-$250). This has timers to understand how much energy it takes to move the temp up or down one degree and, with that, it learned how quickly your home will lose or gain heat. It keeps checking how much time it takes for the temp change vs energy input to update its estimates. It can essentially tell you the outdoor windchill if below room temp outside or the solar load temp if your home needs cooling. It can maintain within a fraction of a degree through the day.
Certainly using a "Nest" brand of controller is by far the most costly approach you can find. And you will be limited to expensive NEST brand items to use with it.
 

MrSalts

Joined Apr 2, 2020
534
Certainly using a "Nest" brand of controller is by far the most costly approach you can find. And you will be limited to expensive NEST brand items to use with it.
"By far the most costly" depends on how one values their time. When I had young kids, my time was very valuable and I spent as much time with them as possible so I would gladly spend on a completed and safe device instead of starting with a handful of parts and at least several hours of code-writing, testing, debugging without any assurances that the end result would be better than what I was starting with.
Time is money.
 
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