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We are facing issue with relay PCN-112D3MHZ. We are switching a FAN of 80 Watt at 220V AC with this relay. we are operating relay with transistor MPSA42. Micro controller is driving this transistor through 1K resistance at base. Coil voltage is 12V DC. Diode is also connected parallel to relay coil in reverse direction.

Some times relay get stuck at on position. when we send off signal from micro controller, transistor becomes off, but relay contacts are not released to make FAN off. when we switch off the complete supply(both AC & coil DC) for some time and switch it on again, relay still is in on position.

As per specification relay is of 3A. Any hints?

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  • \$\begingroup\$ You may find an R-C snubber across the motor or across the relay contacts will attenuate the switching spikes enough to prevent welding. Somewhere around 0.1uF (X2 rating) and 47 to 100 ohms as a suggested starting point. I'm just suggesting a line of inquiry, hence comment rather than answer. \$\endgroup\$ – Brian Drummond Oct 5 '15 at 12:07
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It is because the fan is an inductive load. The relay is rated for 3A resistive load. Sometimes you will be switching the relay off when the current through the fan motor is at a peak, and hence you will get a high voltage surge - causing the contacts to arc, this tends to weld the contacts together.

I suggest you use a solid state relay with zero crossing switching for this instead.

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  • \$\begingroup\$ Hmm, Agilent recommends "When you are switching a motor load, typical industry practice is to derate to 20 percent of the resistive rating." So in this case, 0.36A become 1.81A... still below the 3A rating of the relay. \$\endgroup\$ – Fizz Oct 5 '15 at 9:28
  • \$\begingroup\$ That's a guideline not a guarantee. Clearly the contacts are welded shut in the OP's case as he can turn the circuit off and back on, and 'relay still is in on position'. An alternative solution might be a capacitor or other transient voltage suppressor across the contacts. \$\endgroup\$ – Icy Oct 5 '15 at 9:44
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    \$\begingroup\$ Yeah, another appnote gives however 5 to 10 times the current for motors. With 10 times that would be 3.6A, so exceed the relay spec. That note also contains cheaper ideas for fixing this, like RC network or varistor across the load; TVS might be good too. \$\endgroup\$ – Fizz Oct 5 '15 at 9:45
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    \$\begingroup\$ And electronics.stackexchange.com/questions/42131/… has some example calculations for the RC snubber. \$\endgroup\$ – Fizz Oct 5 '15 at 9:59

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