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I am working with a device that has a relay on the 12v power supply to the micro controller that runs the device. Under certain unplanned circumstances this relay can chatter.

  1. What is the potential for memory corruption/unpredictable operation of the micro controller, and
  2. if the chattering relay causes problems for the micro controller, what is the most likely nature and effects of those problems.

The micro controller operates numerous other subsystems and has many digital inputs from sensors. The relay is located within inches of the micro-controller and its exposed I/O wiring.

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    \$\begingroup\$ Please provide a schematic of how the MCU and relay are connected to each other. Your description makes it sound like you've got 12V connected to the MCU through a relay. Part numbers for the MCU and relay would be helpful too. \$\endgroup\$ – Dan Laks Oct 19 '14 at 3:02
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    \$\begingroup\$ If you didn't plan for the chatter to happen, then instead of wasting time worrying about what might happen if you don't fix the chatter, you should make the chatter stop. \$\endgroup\$ – EM Fields Oct 19 '14 at 6:16
  • \$\begingroup\$ This is not my circuit and I can't do anything about its design. My question goes to the problems the microcontroller is likely have because of the relay. \$\endgroup\$ – LMS2424 Oct 19 '14 at 16:20
  • \$\begingroup\$ @LMS2424: If you're charged with being responsible for the device's behavior and you don't understand why the chatter's happening and you can't/aren't allowed to fix it, run as fast as you can and as far away from there as you can, because you'll certainly be blamed for whatever bad happens. Just saying... \$\endgroup\$ – EM Fields Oct 20 '14 at 0:24
  • \$\begingroup\$ @LMS2424: Meant to say that if you stick around and try to tough it out with your hands tied... \$\endgroup\$ – EM Fields Oct 20 '14 at 10:03
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The most common cause of relay chatter is the supply to the relay coil falling below coil hold on point...

this relaxes the load on the supply....

The supply then rises again above the coil pull-in point...

The supply sags again and the cycle repeats.

The solution in most cases is a higher current supply.

EDIT...... Sometimes it is not the supply current that is the problem but a high resistance between the coil and the supply. This could be because the driving transistor or FET not being driven hard enough or a bad connection.

Examples are dry solder joints or loose screw terminals

EDIT 2...

Also this rapid switching can introduce noise in to the supply that can affect sensor circuits. The supply rail going up and down will affect circuits that assume a stable supply rail. This could cause the wrong action to be taken by the micro (corrupt inputs) and in bad cases the micro to reset depending on the circuit.

The contacts opening and closed rapidly increases the production of EMI as well, each arc of the contacts sending pulses in to the environment and the rest of the circuit ...

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