In Crawlspace ventilation, I talked about wiring up some little 5V fans, a voltage regulator, and a solar panel to ventilate the crawl space under my house. Today I wired up another regulator board (with a green LED this time). The wiring is a bit neater this time (practice helps!):
I tested the regulator with 1, 2, 3, and 4 of the fans: http://www.digikey.com/product-detail/en/FAD1-06025BBLW12/Q620-ND/2600074 I got a constant 5.02V output independent of the load when powering the regulator from a 9.25V power supply (nominally 9V). The 9V supply is pretty beefy (6A capability), so the tiny loads of the fans did not cause much change in the voltage at the input to the regulator (maybe 0.02v IR drop).
I was curious what a larger IR drop in the wiring to the regulator would do, so I tried putting a resistor between the 9.25V supply and the regulator:
With the 8Ω series resistance, the 4-fan load had a hard time getting started, and whined a bit before starting. Once one of the fans got up to speed and reduced its current draw, the others quickly came up to speed also. The regulated voltage was only 4.9V, and fluctuated a bit, rather than the constant 5.02V with smaller loads or less series resistance between the 9V power supply and the regulator.
With the 10Ω series resistance, the 4-fan load could not start at all, but just whined. The output of the regulator was only 2.2V while the fans were stalled. If a 4th fan was added while 3 fans were already running, the fans ran, but with a 4.11V regulator output. (The three fans had already reduced the regulator voltage to 4.99V).
It seems like the voltage regulator works fine as long as the input voltage is at least 7V (as claimed in the specs), but if the IR drop in the wiring to regulator is enough to drop the voltage below 6.8V, the regulator may not be able to supply enough current to start the fan motors. Each fan takes about 80mA@5V (400mW) once up to speed, so the regulator seems to have an efficiency around 74%—considerably less than what I expected from the spec sheet. I’ll have to investigate that more closely.
I may have a little trouble with the fans each morning as the power comes up gradually—they may have trouble starting if the panel is not yet putting out enough power. If that turns out to be a problem, I may need to add some circuitry to detect low voltage on the regulator and turn off one or more fans.