I've now wired my PSU for 'flexible fan control'
I've got the original PSU fan-power wires running out of the case, the original Zalman (PE) fan installed, with leads running out of the case; and a second thermistor glued to the heatsink next to the built in one. So I can power the fan from either the PSU's own temp-controlled supply, or my Zalman fan-mate supply, and monitor the temps. What else would one do on a Thansgiving weekend?
I was surprised by what you said at first ...
The PSU fan controller feeds ~5.5V to the fan, so the drop is only 1.5V.
I measured 9V across the built-in PSU power lines (un-loaded - ie no fan connected). But you are right - as soon as you hook up the fan, the voltage drops to about 5.5V. Strange; the zalman fanmate supplies a given voltage when it's unloaded, and maintains pretty much that same voltage when a fan is connected. Makes comparisons harder ...
Anyway I did a bunch of tests with both the built-in power supply lines and the zalman fanmate. With the PSU open, I inserted a card between the fan and the heatsink (to reduce the cooling effect of the fan), and measured the voltage on the actual fan at different heatsink temps. For the record, I'll record them here!
Heatsink temp and voltage using built in fan power supply:
20 ---- 5.9
35 ---- 5.9
36 ---- 6.0
40 ---- 6.1
42 ---- 6.2
43 ---- 6.3
44 ---- 6.4
45 ---- 6.5
46 ---- 6.6
46.5 -- 6.7
46.7 -- 6.8
47 ---- 6.9
47.6 -- 7.0
After inserting a 22 ohm resistor in series with the fan supply (half-watt used - hope that's ok!)
39 ---- 4.63
40 ---- 4.69
41 ---- 4.7
42 ---- 4.77
43 ---- 4.81
44 ---- 4.9
44.9 -- 4.98
45.4 -- 5.04
46 ---- 5.12
46.5 -- 5.2
47 ---- 5.38
47.5 -- 5.5
48 ---- 5.6
I also did the above tests using a 3 watt, 25 ohm variable resistor, and measured the voltage drop across it - and it was pretty consistently 1.35 to 1.45 V (as Mike suggested).
I also tested with the fanmate, at min and max settings. At max setting, unloaded voltage is 11.03 V, loaded (fan connected) is 10.42. At min setting, voltage is 5.02 (loaded or unloaded - makes no difference).
So - bottom line is, with a fanmate at the minimum setting, the fan is getting 5.02 V, while with a 22 ohm resistor (and a cold PSU heatsink!), the fan is getting 4.63V - half a volt lower, and slightly quieter. And as Mike said, I have the knowledge that if the heat rises, the voltage supplied will rise also.
I pointed a can of compressed air at the heatsink to cool it down even more, briefly, and with the heatsink temp at 32C, got a voltage reading of 4.58.
It seems that the built-in circuit really starts to increase the voltage to the fan at about 40C - increasing about 0.1V per degree, and ramps this up more at about 46C.
Another observation - with the PSU case off, and no fan, the heatsink temp rises quickly; I gave up measuring when it hit 50C. Clearly this PSU can't be run fanless without some serious modifications. (well, I was too chicken to see if it stopped rising once it hit 50C!).
Also noticed that the PSU case greatly helps in cooling (with the PSU out in the open, not enclosed in the PC case). With the PSU case off, for a given fan speed (when I was using the zalman fanmate, which is not temp controlled), the PSU heatsink temp was 35C, and with case on, 27C. So the PSU case really helps 'funnel' cold air over the heatsink.
Whew ... what a waste of a day that was ... Now I need to get back to my real purpose in life and scan some pictures!