I want to make a hotplate for controlling temperature of liquids. I
got this 2000W 220V - 50V AC SCR triac voltage regulator circuit (see photo attached), I can use it to manually control the AC input of a kettle heating element and all kinds of AC heating appliances.

Step 2 is to have a temperature probe which will detect when the liquid (or sand bath) is at a certain temperature, then cut off the heat in order to maintain that temperature. I'm new to electronics so not sure how to do this.

So I know that a thermocouple is used to measure temperature, I can submerge it in the liquid, and it will know when say 90C is reached. What do I connect the thermocouple to? I read about this MAX 6675 IC which interprets signals from thermocouple sensors, its a module for arduino I think.

I read about PID controllers but thats too advanced for what I need right now, a little later down the line Ill move onto PID controllers but for now all I need is a way to cut the current when the temperature reaches a certain point. Would I just plug the thermocouple into an arduino nano chip and write a program for arduino to turn a switch on and off at different points?

 

You do not need a thermocouple to control the temperature of a heater element, the SCR controller should be able to adjust the power into the heater element and thus adjust the amount of heat delivered. That part is simple and fairly easy to do. But the feedback for that sort of system will be the user adjusting the heat input while observing the temperature. The user is part of the feedback loop in that arrangement.
Using a sensor to hold a constant temperature without operator action is quite different. First, using a PID (Proportional +Integral +Derivative) controller is more than required in many cases. Proportional control will vary the heat input in proportion to the difference between the set temperature and the measured temperature. On/Off control can also be used to switch the heat on and off to hold the temperature within some range of desired temperatures. That is fairly simple to do electronicly, not requiring a lot more than the shown SCR controller. But it will usually not hold the temperature exactly, rather in a range that is close to the desired point. There are quite a few published circuits for both proportional and ON/Off controls. Usually they use a thermistor for sensing instead of a thermocouple.

 

Have a look on Ebay for something like this...

The controller you have is not really suitable for auto control.

 

REall

Have a look on Ebay for something like this...
View attachment 255968
The controller you have is not really suitable for auto control.

Really, the controller in that early post will be useful in adjusting the heat input and allowing this controller to work with much closer control. Temperature overshoot is caused partly by excess heater power, so reducing the power with the SCR controller will improve that aspect.

 

Yes, but he wants auto control and the one he has is only manual.

 

Yes, but he wants auto control and the one he has is only manual.

My response was for both devices to be used. The SCR controller to set the effective element power, NOT as part of the temperature holding loop. I was quite clear about that point, at least I thought that was made clear. Functionally, it is adjusting the loop gain.

 

Here's my suggestion .........
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Here's my suggestion .........
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View attachment 255994

How does the circuit shown relate to the controller that was posted? Would you expect the TS to construct this circuit?? The temperature sensor is a thermistor with a positive temperature coefficient. (Resistance rises with temperature.)
Will the form of the sensor shown be useful in the setup that the TS would be using?

 

How does the circuit shown relate to the controller that was posted? Would you expect the TS to construct this circuit??

It does exactly what he wants,
it's an easy project,
it provides visual indication of the Power-Level being applied,
and costs less than ~$50.oo in parts.
What's not to like ?
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How skilled at circuit building is the TS?? That will have an effect on what can easily be done.

 

Full set for temperature management:
----------------------------------------
XMT-800 PV SV Digits Display Alarm SSR Controller Temperature Control Meter + M6 K Type 2M Thermocouple + SSR-25DA Solid Relay
https://www.aliexpress.com/item/1500125138.html
US $14.99

ADDED:
Diagram below shows how my set is wired.

 

Full set for temperature management:
----------------------------------------
XMT-800 PV SV Digits Display Alarm SSR Controller Temperature Control Meter + M6 K Type 2M Thermocouple + SSR-25DA Solid Relay
https://www.aliexpress.com/item/1500125138.html
US $14.99

Considering that the last time I priced an SSR with those specs it was over $10, the package is quite a deal, even if it requires a separate DC supply to power the SSR. That price is hard to imagine.

 

Very hard to imagine !!
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But add a 5 volt power supply for the relay and it should do just what the TS requires. I still suggest keeping that SCR controller in series with the heater element to adjust the heater power. In that case, heater power equates directly to loop gain.

 

I want to make a hotplate for controlling temperature of liquids

How accurately do you need to control the temperature?

 

Certainly the accuracy required is what we need to know, since even the simple SCR power controller can hold the temp within some range even without feedback. So the next move is to wait for the TS to let us know. And what can be achieved at a reasonable cost will depend a whole lot on what part of the world the TS is located in. Resources are more challenging in some parts of the world

 

Controlling the heat temperature can be dodgy. As @MisterBill2 said, there's a certain amount of thermal temperature overshoot:

Temperature overshoot is caused partly by excess heater power

I call this "Thermal Inertia" where the heating element, such as a kitchen oven or even a toaster oven, will cause the heat element to heat up and eventually glow red hot. When the desired temperature is reached power is cut off. However, because that heat element is still red hot the item being controlled continues to heat up and - you will have thermal overshoot as Bill describes. Then there's a thermal lag as well, when the temp has dropped below the set point and power is again restored to the element. It doesn't heat up instantly, rather, it takes time to heat up sufficiently to raise the temperature inside the oven.

While there is a certain amount of hysteresis involved in the standard toaster oven, the phenomena will manifest itself as shown in the diagram below:

With proportional control you can ramp up the temperature. As you approach the set temp the controller will cut back on the power so that the part is warmed up to the set temperature without the thermal inertia. Hysteresis shouldn't be a problem. It all depends on how tightly you want to control the heating.

I used to work as a micro section technician. I wanted to shorten the cure time of my samples. Using an oven would facilitate that issue. However, as I described the toaster oven, my boss wanted to save a few hundred bucks and brought in a toaster oven. It resulted in over heating my samples and burning the sample cups causing distortion and an unusable sample. All because the red hot element continued to heat after reaching temperature. So finding a lower set temperature should solve the melting issue. However, solving that issue meant I wasn't saving any time in the cure of samples.

I brought in an aluminum projects box and a light socket. I could control the temperature much easier using a light bulb as a heat source since it didn't approach 2,000˚F. Also, it heated when on and stopped heating when off. I cut a 2 hour cure time down to 15 minutes. Cheaper than a toaster oven and far more reliable. Setting one of those cheap timers to power the lamp for 10 minutes turned out to be the sweet-spot for my needs.

The whole point is to keep it simple as possible. Sure you can get a PID, but is that really necessary? For what I was doing - absolutely not. Still, for other oven applications where things had to be held precisely at a given temperature - sure. Remember to Keep It Stupidly Simple (the KISS principal). (I know, I know, others call it something else)

 

Controlling the heat temperature can be dodgy. As @MisterBill2 said, there's a certain amount of thermal temperature overshoot:

I call this "Thermal Inertia" where the heating element, such as a kitchen oven or even a toaster oven, will cause the heat element to heat up and eventually glow red hot. When the desired temperature is reached power is cut off. However, because that heat element is still red hot the item being controlled continues to heat up and - you will have thermal overshoot as Bill describes. Then there's a thermal lag as well, when the temp has dropped below the set point and power is again restored to the element. It doesn't heat up instantly, rather, it takes time to heat up sufficiently to raise the temperature inside the oven.

While there is a certain amount of hysteresis involved in the standard toaster oven, the phenomena will manifest itself as shown in the diagram below:
View attachment 256098
With proportional control you can ramp up the temperature. As you approach the set temp the controller will cut back on the power so that the part is warmed up to the set temperature without the thermal inertia. Hysteresis shouldn't be a problem. It all depends on how tightly you want to control the heating.

I used to work as a micro section technician. I wanted to shorten the cure time of my samples. Using an oven would facilitate that issue. However, as I described the toaster oven, my boss wanted to save a few hundred bucks and brought in a toaster oven. It resulted in over heating my samples and burning the sample cups causing distortion and an unusable sample. All because the red hot element continued to heat after reaching temperature. So finding a lower set temperature should solve the melting issue. However, solving that issue meant I wasn't saving any time in the cure of samples.

I brought in an aluminum projects box and a light socket. I could control the temperature much easier using a light bulb as a heat source since it didn't approach 2,000˚F. Also, it heated when on and stopped heating when off. I cut a 2 hour cure time down to 15 minutes. Cheaper than a toaster oven and far more reliable. Setting one of those cheap timers to power the lamp for 10 minutes turned out to be the sweet-spot for my needs.

The whole point is to keep it simple as possible. Sure you can get a PID, but is that really necessary? For what I was doing - absolutely not. Still, for other oven applications where things had to be held precisely at a given temperature - sure. Remember to Keep It Stupidly Simple (the KISS principal). (I know, I know, others call it something else)

The excess heating capacity of the toaster oven equates to excess loop gain, which tends to cause overshoot and instability. The heat continuing after the setpoint was reached equates to a forward path delay, almost certain to cause all sorts of problems. Really, no way an unmodified toaster oven could do that job correctly. The toaster oven came up to bat with at least three strikes to start with.
The much lower temperature and lower power, and much faster response of the light bulb were better in several different aspects, quickly demonstrated by solving the problem.