[Lmandrake]

Boy, I have killed a lot of fans in my quest for knowledge on these sensors. I doubt that you killed yours by using a powered voltmeter.

Most I have seen have three leads, positive, negative, and output. Some have four leads, positive, negative, and two outputs. I don't know how you can tell which one are which without the circuit board it was used on or a data sheet.

The first thing I would suggest is that you look for numbers on the hall effect chip itself and do a google search with the numbers. That may lead you to a data sheet on the chip. If not, the sensor you have may be one where almost all of the fan control circuits are integrated on the chip. I am using one of those now since I ruined my last one by handling it too roughly and breaking it off the circuit board. In the four pin sensor I have, there are positive and negative leads in for power. There are two outputs.

The outputs are not what you may expect. If this is a chip where the whole fan control circuit is on the chip, the outputs are probably not "hot" or positive outputs. They are, instead, tied to ground. When used in a fan, which usually has two coils, the 12v "hot" or positive lead is tied to one side of each coil and the other side of each coil is tied to these outputs. The fan control circuit, which is essentially two transistors controlled by the hall sensor, activates the coils by providing a path to ground (the negative side). When the coil is not activated, the output for that coil stays "high" - i.e. near or at the "hot" or positive input voltage. With no voltage differential across it, the coil does nothing. To activate that coil, the chip output goes "low" - i.e. close to ground and the voltage differential across the coil becomes great enough for it to become active and do work.

I would suggest that you either find data on the chip or experiment on the hypothesis that the two "output" leads are paths to ground. If you pulled the chip off a fan circuit board that just had circuit traces on it and not any or many components, I would bet that the whole fan circuit is in a single chip and it operates as described above. If you have some LED's with integrated resistors or LED's wired in line with resistors so they won't blow, you should try them with their "negative" sides tied to the chip outputs.

I have played with far too many hall sensors (I have a pile of discarded AT power supplies that I pull fans out of) and it is possible to burn up the fan circuit by operating it without a load, but I have only burned up one that way.

LMK if this helps. BTW, if you, or anybody else is interested, I would be willing to share how to use this thing to automatically shut down your box if flow stops.