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I was doing on-off temperature control for around 40‎°C for an enclosed unit manually by switching on and off a 1100W heater(with a fan) with my hand. The accuracy is not very important roughly 40‎°C is fine. Now instead I want to automatize this on-off process. Basically 40‎°C corresponds to 8V output of the temperature transducer.

Here below is raw voltage output of the temperature transducer when it is heating and cooling around 40‎°C(sampled at 4kz):

enter image description here

Here is a close view of the raw data:

enter image description here

Now I tried the following comparator circuit where I set the threshold to 8V with a hysteresis of around 35mV, and fed the 4kHz above sampled input to the circuit as PWL.

Here the comparator triggers this zero crossing 25A SSR(D2425).

enter image description here [Left click to enlarge the view] (R5 is for input not to float when transducer is not connected)

Below shows the input as the sampled voltage from the transducer, the non-inverting input which shows the hysteresis level, and the heater current in order.

enter image description here

In the simulation the results look smooth, but if there is no fundamental problem with this circuit my questions are:

1.) Do I need R4 at the SSR input? In the data sheet input impedance for this D2425 is not given or clear(My ohmmeter doesn't show anything when measured at the input terminals):

enter image description here

2.) I don't want too much hysteresis not to effect the stability for the set temperature but too little is not good either if the noise exceeds hysteresis. Is the hysteresis level fine for this input or would an RC filter be needed?

Edit:

Regarding the comments some modification: enter image description here

enter image description here

Edit 2:

The sensor is set to 0-10V output and I wondering how would floating input turns off heater for this circuit. Is connecting 1Meg to Vcc load the transfucer or damage it? Transducer output can be anything between 0 to 10V.

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  • \$\begingroup\$ The SSR sheet shows max 12 mA possible for input current, which is fed from 12 V via R6 which is 1k. So the max drop across R6 is 12V. That may not work. Then to turn the coil off the 290x has to pull 12 mA to ground. The device is only rated for 10 mA over full range of conditions. Normally to resolve this one would add a npn transistor after the comparator to get the higher currents. The plots above show that it is working, but it may not always work depending on conditions and each 290x device. \$\endgroup\$
    – scorpdaddy
    Jun 3, 2019 at 12:07
  • \$\begingroup\$ If your sensor goes O/C or is disconnected the heater runs the whole time. Yes? To reverse this take R5 to Vcc rather than ground. \$\endgroup\$
    – Russell McMahon
    Jun 3, 2019 at 12:26
  • \$\begingroup\$ @scorpdaddy How about this? i.sstatic.net/jHPn5.png I added an NPN as follower and increased R6. \$\endgroup\$
    – user16307
    Jun 3, 2019 at 12:47
  • \$\begingroup\$ @RussellMcMahon I made that change as well now. Please see my edit i.sstatic.net/jHPn5.png But wouldnt this pass current into the traducer now? \$\endgroup\$
    – user16307
    Jun 3, 2019 at 12:48
  • \$\begingroup\$ @user16307 Yes - the 560k will pass about 10 uA into the transducer if it is low impedance and DC continuous. As it is usually at about 8V the grounded resistor will pass even more to ground. Depending on what the sensor is it MAY matter - but, does it? What is the sensor? If desired a high resistance divider from Vcc to ground with output of about 8.65 VDC could be used. Whatever you use, heater always-on in a fault condition is unlikely to be very desirable. \$\endgroup\$
    – Russell McMahon
    Jun 3, 2019 at 12:57

1 Answer 1

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Do I need R4 at the SSR input? In the data sheet input impedance for this D2425 is not given or clear(My ohmmeter doesn't show anything when measured at the input terminals):

No, you don't need R4, it serves no useful purpose. You do need to make sure that the current to the SSR input is between the range of 7 to 32VDC. The SSR input is current limited and will draw between 7-12mA.

enter image description here

I don't want too much hysteresis not to effect the stability for the set temperature but too little is not good either if the noise exceeds hysteresis. Is the hysteresis level fine for this input or would an RC filter be needed?

The hysteresis value will depend on the thermal rise time and the heat capacity of whatever you are heating. It will also depend on the location of the temperature sensor (further away from the heater and you will have a longer time for the temperature sensor to notice the heat, because heat takes time to travel through a material). There needs to be some distance between the heater and temperature sensor. Too much distance an the heater will "overshoot" and you'll have less temperature stablity.

Since thermal systems can be difficult to model if you've never done it before. I would experiment with the location of the temperature sensor. Odds are the thermal system is much slower than the comparator circuit and will have plenty of thermal RC. I'd include the option in the circuit just in case.

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  • \$\begingroup\$ I just need a control something better than or same as a human doing. Normally when I see the temperature exceeds 40C I turn off off the heater with my hands and when lower than 40C I turn on on heater. I act like a comparator and SSR. So I need to automatize it instead of staying there for one hour. And it is fine for my purpose. Then I turn off the heater after making a measurement . But could you also comment on how can I obtain heater off when the input is floating? See last comments under the question. Im not sure if the R5 connected to Vcc is a good idea(?) \$\endgroup\$
    – user16307
    Jun 3, 2019 at 16:59
  • \$\begingroup\$ If you like the answer, upvote, Edit your question with the new floating question. Also include what temperature sensor you have. \$\endgroup\$
    – Voltage Spike
    Jun 3, 2019 at 17:09
  • \$\begingroup\$ Thanks; Im on mobile phone at the moment if you can address my concern would be great meanwhile I can include those in the question. \$\endgroup\$
    – user16307
    Jun 3, 2019 at 17:19
  • \$\begingroup\$ I did an edit called Edit 2. \$\endgroup\$
    – user16307
    Jun 3, 2019 at 17:25

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