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I originally phrased this question very differently; the original text is retained below in case background information is useful to clarify the context.

When I using this digital multimeter to measure resistance across a diode, it shows 100-some ohms for a second or so (one update of the digital display screen) and then shows 1, for "open circuit". Why is this?


Original question

Everything that follows was the original body of the question. It is retained only to provide background information and context.

I have a heat gun that has stopped working. I don't know why; it wasn't working when I acquired it. I took it apart to hopefully figure out why. It looks like the surface reason seems to be that there is a fusible link that has opened because of high temperature. I'm now trying to find out why that would have happened.

The heat gun consists of two circuits connected in parallel: a heating element and a fan. I'm testing the fan. The fan has four electrical contacts, two of which were originally hooked up to the otherwise unmodified (except for a switch) AC in. They are unmarked. The other two are indicated + and -. When I hook up a 9V battery to either pair of contacts, the fan goes. When I hook up a multimeter in resistance mode to the +/- pair, the multimeter shows about 9 ohms of resistance.

This all seems pretty unexciting. However: when I hook the multimeter up to the unmarked pair of contacts — the pair that the circuit was hooked up to — the multimeter shows 100-some ohms for a second or so, then shows 1 (open circuit). Again, hooking up a 9V battery to this pair of contacts will result in a rapidly spinning fan.

Why am I getting this reading from the multimeter? Could this indicate a problem with the fan that could have caused the heat gun to overheat?

update: Setting the multimeter to diode-check mode — located in the resistance area (on this multimeter) and indicated by an icon of an arrow against a vertical line — gives a consistent reading of 1363±1 for the unmarked pair of contacts. I don't really understand what this is meant to indicate. Using diode mode for other circuits, including the +/- circuit on the fan, gives readings similar to those of the 200-ohm-max resistance mode.

update 2: Realized that the mysterious cylinders connecting pairs of unpaired contacts are diodes. Reassessing.

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    \$\begingroup\$ If the fan is functional, and it may well be, then the thermal link failed due to over temperature which CAN* be due to just obstructing air flow into or out of the gun. Block the air inlet or place the nozzle against something which obstructs flow and air temperature will rise. Do it long enough and badly enough and the fusible link fuses. If you can replace the link do so and see what happens. If it fails again use the wire in the heating element to make low temperatture coefficient resistors (as it's usually Nichrome) and get a new gun. If it blows not again, use it, but with more care. \$\endgroup\$
    – Russell McMahon
    Commented Aug 30, 2013 at 8:45
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    \$\begingroup\$ I disagree with the close votes on this one. However, the question could be improved by distilling it down to something like, "What are the terminals on this fan and how do I measure them?" Such a question would need some specifics on the fan, of course. \$\endgroup\$
    – JYelton
    Commented Aug 30, 2013 at 17:08
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    \$\begingroup\$ To be clear: the question doesn't say anything about trying to fix the fan, it just mentions trying to figure out why it doesn't work. :) \$\endgroup\$
    – intuited
    Commented Aug 30, 2013 at 18:53
  • \$\begingroup\$ Learned about diodes; posted new question. electronics.stackexchange.com/questions/80764/… \$\endgroup\$
    – intuited
    Commented Aug 30, 2013 at 19:22

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