HEATSINK.RTF		Rev0		John Macey		28 Jul 2012	

THERMAL DESIGN BASICS									

Circuit
	7805 regulator supplied by a 12V battery (allow for 14V MAX during charging)				Max voltage drop across the regulator = 14-5 = 7Volts							For a design load current of .5 Amp									Max regulator dissipation = 7V x .5A = 3.5 Watts															
Regulator Specs
	Case 221A								
	Thermal Resistance, Junction–to–Ambient RqJA 65 °C/W						Thermal Resistance, Junction–to–Case RqJC 5.0 °C/W							Max Operating Junction Temperature TJ 150 °C											
Thermal Design
	What is the max current with no heatsink?								With no heatsink, RqJA = 65C/W								
	Assuming a 50C ambient temperature, max allowable temp rise = (Tj - Ta) = (150-50) = 100C		Then max power = 100C/(65C/W) = 1.538W								Max current without a heatsink = 1.538W/7V = .22A = 220mA	

	For the nominated .5A, 3.5W load and an ambient temperature of 50C					We a heatsink and an overall RqJA of (150-50)C/3.5W =  28.6 C/W
	RqJC = 5C/W so we need a (case-ambient) RqCA = (28.6-5) = 23.6C/W
	Jacar $1 HH-8502 (19x19x9.5mm) 20C/W will marginally do
	{ alternatively, the Jaycar $1.45 HH-8504 12C/W is much better}								
	The junction temperature will be (20+5)C/W *3.5W = 87.5C above ambient [ Tj = 137.5C ]	
	{or with the 12C/W heatsink (12+5)C/W*3.5W = 59.5C above ambient [ Tj = 109.5C ] }								
	For the 12C/W heatsink, the case temperature will be 109.5C - 5C/W*3.5W = 109.5C-17.5C = 92C