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We are experiencing a problem where the mechanical switch of a SPST NO relay (COTO 9007-05-01) is not opening back up when the coil is de-energized. Current keeps flowing through the switch until we give the relay a (somewhat gentle) flick.

We measured voltage and current on an o-scope (a Tektronix current probe for the current), and everything appears to be in order: On/Off voltage connected to coil appears good, On/Off current through the coil appears good, on-current through the switch well below rated current, open voltage across the switch well below the rated voltage, no unreasonable spikes, the coil has a fly-back diode installed...

We even replicated our setup with different everything (including relays) and got the same results.

Any suggestions on what to do/check?

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    \$\begingroup\$ A link to the relay datasheet would also be helpful. \$\endgroup\$ – JYelton Oct 2 '14 at 21:50
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    \$\begingroup\$ data sheet: media.digikey.com/pdf/Data%20Sheets/Coto%20Technology%20PDFS/… we are using the COTO 9007-05-01 \$\endgroup\$ – user2913869 Oct 2 '14 at 21:52
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    \$\begingroup\$ I meant to say the "relay coil" in my comment above. Looks like you have to drop below 0.4V for the relay to turn off. Any chance your circuit is holding the coil voltage above that? Can you post a schematic? \$\endgroup\$ – Dan Laks Oct 2 '14 at 22:04
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    \$\begingroup\$ What I was trying to ask (before I ran into technical difficulties) was if the load on the relay switch is inductive (not the relay coil), and if so, does that have a flyback diode? I am thinking of the possibility that due to contact bounce, there could be some arcing when the contacts are closing causing the sticking. \$\endgroup\$ – Tut Oct 2 '14 at 22:23
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    \$\begingroup\$ Or if the load has a high capacitance then the surge current at closure could weld the contacts together. Relays are inherently unreliable - I avoid them whenever possible... \$\endgroup\$ – Bruce Abbott Oct 2 '14 at 22:33
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I am in agreement with @BruceAbbott's comment, above. I suspect that although your static load current is well within the relay's capability, the surge current when it closes, probably due to charging caps and/or firing up an SMPS, is welding the contacts. I suspect this because I have seen precisely the behaviour that you're describing, down to the sharp tap on the relay body causing the contacts to release.

My solution was to cause the device that I was switching (that was powered by an SMPS) to soft-start using an LCR network between the output of the relay and the input to the device.

If the cause of your problem is SMPS inrush current, and you have control over the SMPS, you may be able to put it into a "soft start" mode, either because it has the capability and just needs to be configured to use it. Alternatively, try my solution. Apologies, it was 10 years or so ago, I can't recall exactly what I used; a small series resistor (<10R) and large inductor (100uH?) and various smallish caps (1-10nF) to ground sounds vaguely familiar.

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  • \$\begingroup\$ Thanks for the detailed comments everyone. I will have to do some digging to find out the exact characteristics of our load (it's somewhat of a black box right now). We are speculating an inductive load right now...but we are going to try to make sure. We will try cutting a problematic one open and look for welds. \$\endgroup\$ – user2913869 Oct 3 '14 at 13:20
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It sounds like the relay is close to something that has become magnetized enough over time - because of the making and breaking of the relay's magnetic field - that its biasing the reeds OFF when they're supposed to release.

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