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LM317: Best way to measure device temperature

Robert Campbell
Prodigy 30 points
Part Number: LM317
Other Parts Discussed in Thread: LM337

I am using the LM317 and LM337 on a dual rail +-15V PSU design.  They are in the TO-220 package and have heatsinks attached.

I'm going to be pulling around 1A per rail.  Although I believe my heatsinks should be up to the job, I want to measure the temperature on the devices to ensure I am staying well below the maximum recommended operating temperature.

I have two questions related to this:

1. Where is the correct place on the LM317/337 devices to measure the temperature?  Is it on the plastic body or the top metal tab?

2. What kind of temperature measuring device is most practical to use?  I am thinking of the very small surface area available on the device, especially if I have to measure the top metal tab temperature.  There are plenty of low cost thermocouple based Probes e.g. 

https://www.amazon.co.uk/KT550D-Thermometer-Temperature-Adjustable-Emissivity-thermocouple/dp/B07TKJYGWC/ref=pd_ybh_a_14?_encoding=UTF8&psc=1&refRID=57BZHQ9272FTKXMWZXQH

but I'm unsure whether I'll get an accurate temperature reading based on touching the probe tip to the device.

This is the first time I've attempted this type of temperature measurement, so your advice would be appreciated.

Regards

Robert

  • 0
    TI__Mastermind 37965 points

    Hi Robert,

    See this app note to guide in making your thermal measurement and junction temperature calculation. Since you are using a heat sink, you should use equations 5, 6, and possibly 7 to estimate the thermal performance of the part + heat sink given the thermal properties of the heat sink.

    As discussed in the app note, the correct place to measure the temperature (if you are measuring the case temperature) is the top of the plastic case where the temperature is highest. This is usually in the center of the top of the case. 

    You can use a thermocouple to make your measurement, but you will have to be careful to minimize the heat sinking nature of the thermocouple itself. Alternatively, you can use an IR camera to get the reading.

    Regards,
    Nick

  • 0 in reply to Nick Butts
    Prodigy 30 points

    Hi Nick

    This is really helpful advice which I will follow.

    Thanks

    Regards

    Robert