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I am looking to make a very accurate steady 5V power supply that can be switched on and off using the digital IO pins on an Arduino. I planned to use a 9V power supply connected to a 2N2222 transistor switched on/off using the Arduino and the emitter pin of the 2N2222 connected to an LM7085 to get a regulated 5V (see image). Simulations say I won't get 5V out but I am not sure why.

Any help would be much appreciated.

enter image description here

Edited

enter image description here

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    \$\begingroup\$ Welcome! "very accurate" != 7805. How much accuracy do you need in numeric terms? Have you considered a linear regulator with an enable pin to simplify things? \$\endgroup\$
    – winny
    Commented Feb 20, 2023 at 17:20
  • \$\begingroup\$ @winny Probably on the order of 1mV. Is there a better option than an LM7085? No, I hadn't I didn't know they existed. Would have any suggestions? \$\endgroup\$
    – R Doyle
    Commented Feb 20, 2023 at 17:24
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    \$\begingroup\$ 1 mV accuracy is more precise than any integrated linear regulator I've ever seen, but who knows. If you go looking for 7805 alternatives anyway, you may as well select one with a shutdown pin. \$\endgroup\$
    – ocrdu
    Commented Feb 20, 2023 at 17:34
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    \$\begingroup\$ Close to 0 % chance on +-1 mV happening. Product recommendations are off-topic here but here are 1751 available 5 V regulators with enable pin to limit your search/point you in the right direction: digikey.com/en/products/filter/power-management-pmic/… \$\endgroup\$
    – winny
    Commented Feb 20, 2023 at 17:41
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    \$\begingroup\$ This sounds like an XY problem to me. What are you planning on doing with this 5 +/- 0.001 V power supply? How much current does it need to supply? Does it need to be stable over temperature changes? What transient response do you need? \$\endgroup\$
    – vir
    Commented Feb 20, 2023 at 17:46

1 Answer 1

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Simulations say I won't get 5V out but I am not sure why.

The NPN BJT is acting as an emitter follower and this means that the emitter voltage must be lower than the base voltage. Given that the base voltage is 5 volts, I estimate that the emitter voltage will be about 4.3 volts and no linear regulator in the universe is going to conjure up 5 volts on its output from 4.3 volts on its input.

Any help would be much appreciated.

I recommend you use a PNP BJT to do the switching and use an NPN to control the PNP. Better still, use a p-channel MOSFET for the switching device. Something like this: -

enter image description here

Modified image from here.

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  • \$\begingroup\$ Thanks Andy. Would something like the edited circuit above work? Where V1 is the arduino pin and ground. \$\endgroup\$
    – R Doyle
    Commented Feb 20, 2023 at 17:16
  • \$\begingroup\$ An Arduino could connect to the node called on/off but I don't know what you mean by V1. Grounds need to be common of course. @RDoyle \$\endgroup\$
    – Andy aka
    Commented Feb 20, 2023 at 17:24
  • \$\begingroup\$ I attached a modified version of my original circuit to my original post, which now uses a MOSFET. \$\endgroup\$
    – R Doyle
    Commented Feb 20, 2023 at 17:26
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    \$\begingroup\$ Yes, that will work but, the problem you have is in generating the floating 5 volt control signal. I strongly suggest you use the circuit in my answer because it will directly interface to an MCU like an Arduino. \$\endgroup\$
    – Andy aka
    Commented Feb 20, 2023 at 17:49
  • \$\begingroup\$ there's really no need for so low as 330R, another 10K would work there just as well, and save current.. \$\endgroup\$
    – Jasen Слава Україні
    Commented Feb 21, 2023 at 5:39

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