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The correct place to insert a temperature sensor?

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During the process of building my own dual-variable power supply, I decided to mount a cooling fan, controlled by a lm35 temperature sensor, to cool off the heat produced by the numerous of components.

Inside my box (old ammo case from WW2) there is very little space:

- 2 backlit displays (2x16 char)
- 1 Toroidal transformer 50 VA
- 1 Toroidal transformer 225 VA
- A total of 4 "TO220" voltage regulators (2x 7805, 1x 7905, 1x 7812) each one with it's fellow heatsink.
- A big heatsink where 2 LM338TK  (TO3) are mounted, this heatsink is mounted partly inside the case, partly outside.

I placed a 120 mm cooling fan driven by a PIC - LM35 based system.

My question is:

Where is the correct and most efficient place to put the LM35 sensor?

I thought about the big heatsink, but it will be tiepid most of the time, because of its size and the fact that it is, in part, outside the case.

I thought about the 220 VA Toroidal transformer, it should produce some heat under load.

I even noticed that the various LM7xx that populates my utility pcbs are often very warm (especially the lm7805 that powers the two displays, 3 pic and some leds).

I couldn't give myself a satisfying answer

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Connect the power supply to the full load current at the lowest voltage and measure the temperatures of various components. Put the sensor near the warmest device.

I wouldn't expect the transformer to get very hot, it'll be one of the voltage regulators. You shouldn't need any over temperature protection as the regulators have built-in thermal shut down but if you want to turn on a fan when it's too hot, it's probably a good idea.

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