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I have an NMOS with me so I tried to design this 12 V DC UPS circuit after looking at another similar PMOS circuit, but when I open the dc switch, the output is 18kV!! What is the problem with this circuit?

Update:
I remove a pull-down resistor, and now there is no simulation error. I also tested it on the breadboard and it seems to be working with a 9V battery. I have yet to buy a 12V battery.

Background:
My apartment has an automatic generator backup (yay!), but the power goes down for around 10s before it powers up. A similar thing happens when the power comes back online. On a few days there is a large number of power cuts and comebacks. I use a direct fibre router which takes around 3-4 mins to start up and so I wanted a circuit like this. The router uses 12V power, so I plan to use a non-rechargeable or small lead-acid battery.

Similar PMOS circuit:
P channel mosfet battery charging/power selection circuit help

Circuit I made:

Multisim Live: 12V DC UPS

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  • \$\begingroup\$ Instead of a link, include (a screen dumb of) the circuit here as a picture. The 18 kV must be an issue with one or more of the models (opamp, NMOS) used in the simulator as there's no sane reason why this circuit should generate 18 kV. The choice of opamp is critical as this circuit expects it to work properly with both inputs working near the positive supply voltage, not all opamps can handle that. There's also the risk that current flows into the - input via the ESD diodes inside the opamp. A resistor in series with the - input can limit that current and prevent damage. \$\endgroup\$ – Bimpelrekkie Jun 19 '18 at 8:17
  • \$\begingroup\$ i was also thinking there would be some simulation error. i will add 2 resistors to the opamp inputs and one to opamp output and see how it works. \$\endgroup\$ – wolfram77 Jun 19 '18 at 13:25
  • \$\begingroup\$ also i wanted to ask what this circuit would normally be called, if i wanted to buy it in the market. thanks. \$\endgroup\$ – wolfram77 Jun 19 '18 at 13:26
  • \$\begingroup\$ @wolfram77 I imagine that if you searched for "router ups" you would find something. \$\endgroup\$ – Andrew Morton Jun 19 '18 at 14:26
  • \$\begingroup\$ yes, you are right. i had seen one available in banggood but i was thinking it shouldnt be that costly. thanks. \$\endgroup\$ – wolfram77 Jun 19 '18 at 16:13

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