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I have been trying to use IRFZ44 as low side switch to ground the ground terminal of an optional 7805 that I wish to use whenever I want. For this I have connected the drain terminal to to circuit ground and the source to 7805 ground. The 7805 Vin is already connected to 12V. The MOSFET was turning on and off when the voltage regulator 7805 wasnt in the circuit. But after inserting it what I observed that when the MOSFET is turning On, the source voltage remains 0V, while the drain voltage attains a stable 9V! At the same time the 7805 isnt getting the circuit ground.

Cannot understand why is this happening. Any help is appreciated. Thanks in advance!

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  • \$\begingroup\$ This might theoretically be possible, but it sounds like a rather bad idea. What do you hope it will accomplish? How do you think the 7805 will behave with no ground connection? Chances are you should start over from a clearly expressed functional requirement, and (depending on what your goal actually is) look at more appropriate solutions for that such as a high-side switch as a bypass, or a regulator with an enable input. \$\endgroup\$
    – Chris Stratton
    Commented Dec 12, 2015 at 15:56
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    \$\begingroup\$ At the very least, you've got things backwards. Connect the source to circuit ground, and the drain to the 7805. \$\endgroup\$
    – WhatRoughBeast
    Commented Dec 12, 2015 at 16:39
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    \$\begingroup\$ If you disconnect the ground terminal of a 7805, I'd expect the output voltage to rise to 10 volts or so (although I don't think I've ever tried it). \$\endgroup\$
    – Peter Bennett
    Commented Dec 12, 2015 at 16:59
  • \$\begingroup\$ Can you, please, draw a simple schematic? \$\endgroup\$
    – jose.angel.jimenez
    Commented Dec 12, 2015 at 22:33

1 Answer 1

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If you are trying to disable the 7805 by disconnecting its ground connection with the FET, it likely won't work.

The 7805 is not designed or guaranteed to be off when its GND is open.

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  • \$\begingroup\$ Ya I was trying but as you rightly pointed, the 7805 isnt responding as I had thought. Now the issue has resolved when I connected the drain to 12V supply and the source to the input Vin of 7805. Thanks a lot! \$\endgroup\$
    – Brijesh Mishra
    Commented Dec 13, 2015 at 8:54

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