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I've got a couple of these 9001-05-01 reed relays. I've tried activating them by applying 5 VDC voltage through pins 2 / 3. This connects pin 1 to pin 4, as I expected. But, after disconnecting pins 2 and 3, the pin 1 remains connected to pin 4 indefinitely (for days, at least). I thought this particular relay is not a latching relay. Have I misunderstood something, or do I have defective units? Or, do I need a diode?

To the best of my knowledge, I do not have strong magnetic fields around.

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    \$\begingroup\$ I would expect the relay to release as soon as the applied voltage drops below the "release voltage". \$\endgroup\$
    – Simon B
    Jan 14, 2017 at 21:17
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    \$\begingroup\$ What did you have going through your relay contacts when you switched it and it stayed on? As Spehro says below, that might explain it. \$\endgroup\$
    – TonyM
    Jan 14, 2017 at 22:16

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If you attempted to switch a large-ish current (more than 500mA in this case, or a capacitive load that causes a surge that exceeds 500mA) then you may have damaged (welded) the contacts.

It's possible that you can tap the relay and the contacts will open, but you will have permanently damaged the precious metal plating on the contacts.

Reed relays are really most useful only for switching low currents.

Note that there is a variation of the relay 900x-xx-02 which has a bias magnet to hold it closed, but that is not the part number you state. The version you have incorporates a series diode+zener so you should observe the correct polarity as given in the datasheet.

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