External Temp Sensor vs Internal Temp Sensor

Discussion in 'The Projects Forum' started by McMurry, Jul 12, 2008.

  1. McMurry

    Thread Starter New Member

    Jul 5, 2008
    Hi. I am currently working on a simple temperature monitoring device.

    The main question is:

    What are the disadvantages of using the internal temp. sensor (as in some MCU has built-in temperature sensor)? One main point I could think of is internal temp sensor has not much flexibility.

    I've read from other website, where it is written briefly that external temp sensor could actually respond faster due to its higher rise time. How correct is that fact?

    Also, am I correct to say that external temp. sensor typically gives a more accurate reading than internal temp. sensor?

    Is there any other fact that I could add in?
  2. bertus


    Apr 5, 2008

    Internal temperature sensors also measure temperature rise of the device itself.
    When the device has a lot of load, the temperature of the device wiil rise and influence the temperature measurement.
    My oppinion is an external is more acurrate.

  3. McMurry

    Thread Starter New Member

    Jul 5, 2008
    Hi. Thanks for the info. =)
    Hoping for more reply. Hehe :)
  4. SgtWookie


    Jul 17, 2007
    I would think that the uC internal temp sensor would be most useful for controlling a fan to cool the uC, rather than sensing external temperatures. Power consumed in the uC would offset the ambient temperatures, so it would not be very useful in monitoring ambient temperatures unless the uC spent most of it's time in "sleep" mode, and was thermally connected to a heat sink with good air flow across it.

    A higher rise time would mean a slower response. A lower rise time would mean a faster response.

    How quickly the temp sensor will respond is related to it's mass vs how well it's thermally connected to the source of heat/cold. Usually, one would pass a current through a thermistor and measure the voltage across it to obtain a reading relative to temperature, or place a voltage across it and read the current if it's a semiconductor such as an LM34/LM35/LM134/LM135/LM334/LM335. Note that the current passing through the thermistor causes it to dissipate power, which generates heat. The heat due to the current will offset the reading from the true temperature. This offset can become much more pronounced at lower temperatures.
    An external temp sensor has a better opportunity to be more responsive to temperature fluctuations. Accuracy depends upon the method used.
  5. McMurry

    Thread Starter New Member

    Jul 5, 2008
    Thanks, Sgtwookie. Now I understand a bit about the relation of rise time and the response time.