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I need to control the temperature with high precision, and for this task would be good a temperature controller with pt-100 sensor, something like this one PID Temperature Controller ITC-100. As I see that, it can be very simple: there is an input (pt100 sensor), an output - to the heater element (my heating element needs 12V supply). However, I still have some things which are unclear for me:

  1. Is it generally supposed that there is a power source for the heater element which I also put in the circuit or usually the temperature controller gives the voltage at the output ? Or is that voltages supposed for the external SSR, not my heater?
  2. In many sources I have seen circuits with external SSR. However, such temperature controllers already have internal relay. Should it be possible without any problem just wire output to my heater (because in all schemes output goes to the external relays)?
  3. Wires soldered to the pt sensor shouldn't influence much to the measurement, right?
Antonio
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  • What are the electrical specification for the heating element. Looking at the electrical specifications for the controller it appears the max output is 3W at (12vdc-24vdc) Please update the post with this additional infomation. – Mahendra Gunawardena Jun 03 '16 at 00:54

3 Answers3

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  1. Wires soldered to the PT sensor shouldn't influence much to the measurement, right?

These PT sensors are based on electrical resistance, so extending the leads will cause a slight change in the readings. You can minimise this with good connections and thick wire. (Thermocouples are different, you would need to use the right sort of wire for those.)

Andy
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1.Is it generally supposed that there is a power source for the heater element which I also put in the circuit or usually the temperature controller gives the voltage at the output ? Or is that voltages supposed for the external SSR, not my heater?

2.In many sources I have seen circuits with external SSR. However, such temperature controllers already have internal relay. Should it be possible without any problem just wire output to my heater (because in all schemes output goes to the external relays)?

Typically, the decision to use the controller's internal power source, internal relay or an external relay would be based on the requirements for the heater. If switching the heater's load is within the capacity of the internal power or relay, it's simpler to just use that. (Remember, simpler is almost always better since there are fewer things to fail.) On the other hand, if you have a 480V heater, you're probably going to need an external relay, mechanical or SSR. In this case, the internal relay can handle 250V/3A or 120V/6A.

Wires soldered to the pt sensor shouldn't influence much to the measurement, right?

You can extend the leads on these sensors. There will be some effect on the measurement due to the added resistance of the extensions. How much this affects your measurement is going to depend on how much resistance you add with your extension wires. I would expect this to be minimal unless you're running a really long extension. It's likely that any such error can be compensated for when calibrating the system when it's setup in its final configuration.

There's a lot of information on these controllers here: http://ink-bird.com/asset/file/ITC-100_%20Manual_%20V1.0_6.30.pdf

If you look at Section 7 of the manual I linked above, it shows sample system diagrams of the various modes in which the controller can be set up. It also indicates which model (There are 3) of the controller they are using for that setup.

DLS3141
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  • Thanks for advises. I have a low-voltage heater. As I realised, that temperature controller actuallz provided in different modifications : for use external SSR (which I dont need) and with internal, as well as with high voltage output (120V-250V) and low (12V-24V). Howewer, for me its still unclear if I can use for instance 12V heater with low voltage controller or it would supply 24V and my heater will burn. On other hand, power of the heater is >3W, and that load can be too much for low-voltage controller. And for the external power, I need contactor o something like this. – Antonio Jun 03 '16 at 14:37
  • So the main stuff which still remains unclear for me is the supply of the heater. I thought that I can include it in the circuit, and the internal relay simply connect or disconnect it, but as I see now, by default the controller not only work as switch but also supply output voltage. And I didnt understand how can I control it. – Antonio Jun 03 '16 at 14:39
  • Well, in spite of that I looked through the manual I stil don't completely understand the supply of heater. I mean, if understand them right, they just use the same voltage supply which gets controller (just use the same pin to supply the heater by voltage). And then there would be 2 possibilities: 1) to use the same voltage, and add voltage divider to get the voltage which I need for my heater 2) to use external voltage source and connect it to the normally close pin 6, heater to public port 7. Is it like that? I am really not sure about the wiring and these pins. – Antonio Jun 04 '16 at 12:27
  • I meant, between normally opened pin 8 and public port 7. Since I assume that controller should turn relay on when temperature is too low, and then normally opened-central pin circuit should be closed. – Antonio Jun 04 '16 at 21:50
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  1. Wires soldered to the pt sensor shouldn't influence much to the measurement, right?

A "pt100" sensor has a resistance of about 100 ohms (hence its name!) and changes resistance by about 0.385 ohms per degree C. Your question doesn't say what you mean by "high precision" but those two numbers should give you an idea of what lead resistance is acceptable for your application.

If you want to use long leads and still get "high precision", it would be better to use a 3-wire RTD probe, as shown in the ITC-100 documentaton. That configuration uses a Wheatstone bridge type of circuit, and only the difference in resistance between the three leads contributes to the error. Three-wire probes can use leads up to about half a kilometer long.

alephzero
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