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Interference of compressor and heating elements in LM34


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#1 alberto

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Posted 11 November 2013 - 04:00 PM

Hello,

 

I'm measuring the temperature of a tank that is heated with a 6000 W heating element and cooled with a compressor, i measured the temp with a LM34 sensor with a 10 kohm resistor between the signal and ground (EI1034). I use a KYOTO 40 W SSR (KD20C40AX) to turn on and off the heating element and compressor. I have a Labjack U3-HV.

 

The problem is that every time the compressor turns on my temperature signal goes up about 0.3 °C and this deviation continues until the compressor turns off. And the same with the heating element but instead it goes down about 0.3 °C.

 

How can I avoid this deviation and what is causing it?

 

Thank you in advance.



#2 LabJack Support

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Posted 11 November 2013 - 04:14 PM

Section 2.8.1.4 of the U3 User's Guides mentions that sinking excessive current into digital outputs can cause analog input offset errors.  Perhaps that is what you are seeing?  Try using DAC0 to control the SSR rather than a digital I/O.  Connect DAC0/GND to the SSR in the sourcing configuration, then set DAC0 to 5V to turn the SSR on and 0V to turn the SSR off.

 

http://labjack.com/s...s-guide/2.8.1.4



#3 alberto

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Posted 11 November 2013 - 06:52 PM

Thank you, that was the solution. The problem is that I am handling a total of 8 SSR, is there another option to handel all eight?



#4 LabJack Support

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Posted 11 November 2013 - 07:59 PM

You need some solution that does not result in a digital output having to sink substantial current.  Section 2.8.1.4 mentions using a 2N2222 or ULN2003.  Other options are a PS12DC or 4x LJTick-RelayDrivers:

 

http://labjack.com/s...river/datasheet

 

http://labjack.com/s...s12dc/datasheet



#5 alberto

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Posted 12 November 2013 - 02:05 PM

I have been evaluating the ULN2003AIN, Im planning to connect:

  1. U3-HV terminal EIO4 to pin 1 of the ULN2003.
  2. U3-HV GND to pin 8 (E) of the ULN2003, with a 100 ohm resistor.
  3. V+ of a 12 V power supply into pin 9 (COM) of ULN2003.
  4. V- of 12 V power supply to pin 8 (E) of ULN2003.
  5. - terminal of my SSR into pin 16 of ULN2003.
  6. + terminal of my SSR to V+ of my power supply.

Is that correct?

If im using multiple blocks of the U3 should I wire each ground of each block to pin 8 of the ULN2003?

 

Another question, where should I connect the shielded ground (the one without insulation) of my shield cable? Can I connect it directly to the GND of the U3-HV?

 

Thank you



#6 LabJack Support

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Posted 12 November 2013 - 04:13 PM

Your connections sound right assuming the power supply is isolated (likely is) and you want to use 24 volts to control your SSR.

 

The LJTick-RelayDriver uses a 10 or 22 ohm resistor, rather than 100.  The LJTRD also has 100k pull-downs on the I/O lines, which I think is to make sure a channel is off when the I/O line is set to input.

 

Perhaps try just using VS (5 volts) from the U3 to control the SSRs at first.  So in your step 3 and 6 connect VS from the U3 to pin 9 of the ULN2003AI and + terminal of the SSR.

 

 

 

If im using multiple blocks of the U3 should I wire each ground of each block to pin 8 of the ULN2003?

 

All GND terminals are the same, so you just need 1 connection to any GND.

 

 

 

where should I connect the shielded ground (the one without insulation) of my shield cable? Can I connect it directly to the GND of the U3-HV?

 

What cable is this?  Where does the cable come from and go to?



#7 alberto

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Posted 12 November 2013 - 05:05 PM

The cable is a shielded cable, like the one in the picture below. I use this to connect my sensor to the Labjack (for example LM34). I read somewhere that to reduce the noise I should ground the shield using the wire without insulation. The problem is that I dont know if I could ground it to the GND terminal of Labjack.

 

Any ideas?

 

 726px-Shielded_wire_4F.jpg



#8 LabJack Support

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Posted 12 November 2013 - 05:51 PM

Yes, you can connect it to GND.




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