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xinxing
Visitor II
June 29, 2024
Solved

LM335 (LM235) damage issue

  • June 29, 2024
  • 1 reply
  • 1270 views

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A high-frequency DC testing power supply product that uses LM235 to measure the temperature of the heat sink (aluminum), and the measurement circuit is shown in the above figure:
The temperature range of the radiator is also within the temperature range of LM235 (TO-92/-40 ℃,+125 ℃). The product is placed in a high-temperature box for 40 ℃ full power aging test. The maximum temperature of the aluminum heat sink is about 90 ℃, and the aging test time is 6 hours;
During the aging process, if the temperature value detected by LM235 (voltage between measurement positions ① and ②, refer to the circuit diagram) is about 20 ℃ higher than the actual temperature (aluminum radiator), it will cause the product to have a false alarm (HOT-ERR);
Question 1:
This temperature measurement circuit has not been adjusted in any way, and LM335, which has been used for 23 years, has basically not had this problem; Recently, there have been multiple instances of LM335 being damaged, and now replacing it with LM235 also results in damage (with a significantly higher probability of failure than LM335). What are the reasons that can cause such a large measurement error in LM235 (LM335)?
Question 2:
The capacitance of C111 in the circuit diagram is 470nF. What is the impact of adjusting it to 1uF on the temperature sampling value?

    This topic has been closed for replies.
    Best answer by Peter BENSCH

    Welcome @xinxing, to the community!

    The LM235 is technically identical to the LM335 and only has guaranteed parameters over a wider temperature range, so it must behave identically at 90°C. However, your problem is not entirely clear: by damaged, do you mean that the LMx35 shows a voltage that is 200mV too high than it should be for the respective temperature?

    As far as your second question is concerned, the purpose of capacitor C111 is not clear to me, especially as it is in parallel with another capacitor with 2.2µF. You already have a time delay due to:

    • the low development speed of the temperature and therfore the LMx35
    • the integrator that the TL084 forms with the 4.7nF in the feedback

    However, the LMx35 is not typically used with capacitors in parallel, which is why I would ask you to remove both capacitors (470nF and 2.2µF).

    Regards
    /Peter

    1 reply

    P
    Peter BENSCHAnswer
    Technical Moderator
    July 10, 2024

    Welcome @xinxing, to the community!

    The LM235 is technically identical to the LM335 and only has guaranteed parameters over a wider temperature range, so it must behave identically at 90°C. However, your problem is not entirely clear: by damaged, do you mean that the LMx35 shows a voltage that is 200mV too high than it should be for the respective temperature?

    As far as your second question is concerned, the purpose of capacitor C111 is not clear to me, especially as it is in parallel with another capacitor with 2.2µF. You already have a time delay due to:

    • the low development speed of the temperature and therfore the LMx35
    • the integrator that the TL084 forms with the 4.7nF in the feedback

    However, the LMx35 is not typically used with capacitors in parallel, which is why I would ask you to remove both capacitors (470nF and 2.2µF).

    Regards
    /Peter

    X
    xinxingAuthor
    Visitor II
    August 7, 2024

     Hi,Peter:

       Thanks for this answer!

       yes,the LMx35 shows a voltage that is 200mV too high than it should be for the respective temperature;

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