# A question about reading and calculating thermal resistance of a voltage regulator from its datasheet

I have this voltage regulator from this supplier and the exact model name is given as: "LM317T/NOPB".

I plan to use this circuit to power a windwane's potentiometer which is around 10 kΩ:

As you see I want to set the excitation voltage for the vane pot to around 9 or 10 V where the op-amp will be supplied directly from a 12 V switch-mode power supply. For this purpose I will use an LM317 to obtain 9 or 10 V for the vane excitation, so I will set the LM317 for 9 or 10 VDC.

I will mount the vane in a box around 50 m high which will stay there for at least 3 months.

Since I will use a linear voltage regulator I decided to find out whether I need a heatsink.

I have encountered two challenges which I put here as questions:

1. In the datasheet, when I check the thermal resistances, I can't figure out which model I should use:

Mine is T0-220, but in the datasheet there are two types namely KCT and KCS, but my model is "LM317T/NOPB", and on it it is written "LM317T +P". So should I use the thermal resistance for KCT or KCS?

2. Here is how I calculate the ΔT:
ΔT = θJA × P
In my case:
P = V2 / R = 102 / 10000 = 0.01 W

But for junction to ambient temperature I used to use the following:

Junction to Ambient = Junction to Case + Case to Ambient

But in the datasheet there is no Case to Ambient but there is Junction to Ambient. Does "Junction to Ambient" already include "Case to Ambient"? Should I in my case only use Junction to Ambient? If so, in my case ΔT becomes like 4°C, and I conclude I don't need heatsink. Did I calculate correctly and does the duration (three months) matter when it comes to using a heatsink?

• With a 10k pot, the LM317 only needs to source 1 mA. So its power consumption is only 2 mW. You're way over-thinking this. Commented Sep 9, 2017 at 21:18
• If the part you bought actually has the National Semi logo on it, that means its been sitting around in RS's stockroom for at least 5 years. If you were using enough power to actually care about the answer to this question, it would be better to buy new parts so you can get a datasheet that actually corresponds to your part. Commented Sep 9, 2017 at 21:26
• Is my using thermal resistance using only "Junction to ambient" (θJA) is enough ? Commented Sep 9, 2017 at 21:29
• And where in the datasheet I can find out the minimum input voltage needed for this regulator to output 10V? Thanks Commented Sep 9, 2017 at 21:31
• If you have multiple questions, ask them with multiple posts. But search first to see if the question has already been asked. (Hint: the term to search for is "drop-out voltage") Commented Sep 9, 2017 at 21:32