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I'm powering a relay from a 12v power source, however this source does not immediately turn off, rather falls gradually for a 'fade' rather than straight off.

Unfortunately this makes my relay chatter multiple times in rapid succession open/closed just before it opens for the last time until next power on, as the voltage is just fluctuating around the critical level.

How can I avoid this? I tried some capacitors and while it helped I still wound up with it chattering at the end (Although slightly less).

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  • \$\begingroup\$ you need to use a control circuit to activate the relay \$\endgroup\$
    – jsotola
    Commented Jan 27, 2020 at 0:33
  • \$\begingroup\$ What is your 12V source? What is causing slow turnoff (capacitors?) Can you build a crowbar or discharge circuit to ensure a clean shutdown? \$\endgroup\$
    – EasyOhm
    Commented Jan 27, 2020 at 0:58
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    \$\begingroup\$ Can you draw the schematics of what you are asking? It will really easy for others to understand the setup clearly \$\endgroup\$
    – User323693
    Commented Jan 27, 2020 at 2:20
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    \$\begingroup\$ Normally relays have hysteresis. That is: the activate voltage and release voltage are far apart with release << activate. It should not chatter unless it switches off some equipment which then causes the operating voltage to rise again. I think we need a diagram of how it all is connected together. \$\endgroup\$
    – Oldfart
    Commented Jan 27, 2020 at 6:02
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    \$\begingroup\$ Probably your drive circuit is at fault because relays alone don't do that, though their drop-out at low voltage may be a bit sickly and slow causing more contact wear. Maybe you need to implement UVLO (undervoltage lockout). Show us a schematic... \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jan 27, 2020 at 15:26

1 Answer 1

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You can use an op amp to operate the relay:

schematic

simulate this circuit – Schematic created using CircuitLab

When power is turned off, voltage starts to drop. once it reached 11 V, the op amp will deactivate the transistor base and turn off the relay.

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    \$\begingroup\$ For 12V coils you might want to pick a lower zener voltage though. 12V coils can typically operate reliably down to 9V or so. \$\endgroup\$
    – Lundin
    Commented Jan 27, 2020 at 7:50
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    \$\begingroup\$ Suggest changing your opamp for a comparator, which is designed for exactly this sort of purpose. If you only have an opamp you can do it, but it's not the optimal solution. \$\endgroup\$
    – awjlogan
    Commented Jan 27, 2020 at 9:39
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    \$\begingroup\$ @ElectronSurf - "faster" is not the reason to choose a comparator for this (although for a simple application an opamp will work, especially if you have a spare one on board already). Opamps are not designed to run open loop and have large differential voltages on their inputs, for example. Additionally, I would suggest putting in some hysteresis. \$\endgroup\$
    – awjlogan
    Commented Jan 27, 2020 at 11:14

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