

December 17, 1986


To: N. Kelly


From: R. Briody


Subject: Diskless Node Power Supply - airflow and thermal test


Diskless Node Configuration: (1)68020 card, (2)2-Meg memory cards, (1)Ethernet,
	(1)Monochrome graphics card, (1)Display memory card, Cherokee QT6-R2,
	Monochrome monitor, Keyboard, Mouse

Misc. Parameters:
    Power supply compartment cross-sectional area = .1528 sq. ft. (5.5" X 4.0")
    Cherokee QT6-R2 cross-sectional area = .0633 sq. ft. (4.8" X 1.9")

The airflow through the power supply compartment was measured, using a Kurz
annemometer, at 105 FPM (feet/minute). The CFM over the supply is:

	105FPM * (.1528 sq.ft. - .0633 sq.ft.) = 9.4 CFM

The Cherokee power supply is rated 150 watts convection cooled at 50 degrees
Centigrade. The supply has a forced air cooling specification of 175 watts at
50 degrees Centigrade with 25 CFM of air. A close approximation of the supplies
true power capability for our application could be derived as follows:

	150 watts + [(9.4 CFM / 25 CFM)(175 watts - 150 watts)] = 159.4 watts

The system current and power consumption is as follows:

	+5 VDC:		26.50 A				132.5 W
	+12 VDC:	 0.35 A				  4.2 W
	-12 VDC:	 1.20 A				 14.4 W
	-5.2 VDC:	 0.00 A				  0.0 W
					Total power =	151.1 W

The power supply outputs are rated as follows:

	+5 VDC @ 25 A cont. 30 A peak
	+12 VDC @ 1 A cont. 1.5 A peak
	-12 VDC @ 1 A cont. 1.5 A peak
	-5.2 VDC @ 3 A cont. 4 A peak

Using the above calculations and the failure history of the Cherokee power 
supply, we see that QT6-R2 power supplies are NOT meeting their published
electrical specifications. The failures we are seeing aren't too suprising
after speaking to Larry Sharp, Test Supervisor at Cherokee. He specified that
power supplies are "burned-in at 25 to 30 degrees Centigrade"!!! I also spoke
with Bob Philips, an engineer at Cherokee, in regards to our +5 and -12 volt
supply overload. The 5 volt supply current is bound by the maximum forward
current of a schottky rectifier in the output circuit and the -12 volt supply
overload is of no consequence as only the load regulation of the -12 supply is
affected. 


cc:	N. Massa
	K. Brunskill
	F. Vernon
	C. Nguyen
