Choosing a transistor

[The Master]

(No problem without heatsink).

Thats good news then. I dont even have room for TO220 packages. Theres no way i could get a heatsink to fit in there. Im not even using a heatsink for my voltage regulator since it never gets remotely warm with just a PIC chip and a MAX485 connected.

One thing i dont understand is how you know that 0.108W means you dont need a heatsink

[Luciano]

One thing i dont understand is how you know that 0.108W means you dont need a heatsink

Hi,

From the datasheet. The thermal resistance junction-to-ambient is for when
there is no heatsink.

IRLR/U2905 datasheet:
http://www.irf.com/product-info/data...a/irlr2905.pdf

The thermal resistance Junction-to-Ambient of an IRLR2905 is 110 °C/W.
Its operating junction temperature range is -55 to + 175 °C.

Example ambient air temperature = 45°C: 

175°C − 45°C
__________    =  1.18 Watts 
   110°C/W

With an ambient air temperature of 45°C, without 
heatsink an IRLR2905 can dissipate 1.18 Watts. (Absolute Maximum).

Best regards,

Luciano