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Relay not switching immediately if mains off to battery like as backup power. I'm looking for zero cross SPDT relay switching for my project. please help. design is simple two power sources (mains and battery) and load will be at relay common pin.

Zoals
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  • Where is the Ground terminal of the 7812 connected? – Peter Bennett Oct 01 '20 at 15:33
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    The trick is to detect mains failure before the 12V supply fails. So, D5 would be more useful connected to the 7812 INPUT which must be at least 14V for the 7812 to work, and I'm guessing your "expert" determined it was over 16V which would turn Q1 on. –  Oct 01 '20 at 15:33
  • Thank you very much, I'm using 15 volts 2 amp DC as mains source, I forgot to update in the question. still do you suggest I need to use 16 volts zener as D5 or 14v zener at 7812 input? – Zoals Oct 01 '20 at 16:22
  • Ground terminal of the 7812 connected to common ground (both mains and battery [ - ]) – Zoals Oct 01 '20 at 17:15
  • A switching delay is to be expected with a relay. Many UPS's run through the AC->DC->AC path all the time. At the voltages involved you can probably use power semiconductors rather than a relay. – Chris Stratton Oct 01 '20 at 19:00
  • I had to depend on relay switching as I need SPDT switching for mains and Battery, I don't know if this is possible with MOSFET. this circuit is just one part I have charging with auto cutoff and discharge protection will be attached to this once this part of function success – Zoals Oct 02 '20 at 03:45

1 Answers1

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Here's a simple solution, using Schottky diode switching.

enter image description here

The output voltage of the 7812 power supply is to be kept marginally higher than that of the battery in order to reverse bias the Schottky diode and isolate the battery as long as the power supply is available.

That would be possible by increasing the reference voltage with an additional diode in series for every 0.7 V increase.

enter image description here

vu2nan
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  • I had to depend on relay switching as I need SPDT switching for mains and Battery, I don't know if this is possible with MOSFET. this circuit is just one part I have charging with auto cutoff and discharge protection will be attached to this once this part of function success – Zoals Oct 02 '20 at 03:46
  • Hi Zoals, Thank you for the information. What's the battery type? – vu2nan Oct 02 '20 at 04:34
  • its 12 volts 7.4Ah or lesser like 1.3 Ah lead acid battery – Zoals Oct 02 '20 at 08:11
  • Thank you, Zoals. I am sure you will arrive at a likely solution. – vu2nan Oct 03 '20 at 14:47