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I have been needing a 3.3 volt power supply that can use at least 12-24 volts ac dc in. I have looked online and managed to make this schematic

schematic

but I have no idea if it works.

I was hoping if you guys know if this might work

(please don't test I just want to know if theoretical work)

Link to project.

EDIT

  • this circuit should power as esp12-f and a relay and it would be nice to have some head room

  • the second regulator can handle 20v (I think) (from the nodemcu board)

  • I'm not certain that the first (7805) regulator is the best choice due to the high heat production as stated by Spehro Pefhany

  • if you know an alternative solution I'd be happy to know

sorry for any inconveniences I have caused I mainly do the software side of things

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  • \$\begingroup\$ For under 500mA, I would first use a MC34063 to drop to 8V and then a 7805 \$\endgroup\$ – Indraneel Mar 24 at 1:04
  • \$\begingroup\$ I'd use one of those $1 adjustable buck modules, but I'm lazy. \$\endgroup\$ – Jasen Mar 24 at 2:39
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Nobody uses linear regulators for this type of application anymore. The issue with using linear regulators with large Vin to Vout differentials, as pointed out above, is power dissipation.

A far better approach is to use a buck switcher like the LT1076 (www.analog.com). Although the device costs a bit more than a linear regulator, you can easily get 1 ampere or more from them. There is a good example schematic on the first page of the LT1076 (for example) data sheet.

A good way to think of this problem is to prefer switchers unless: 1) Currents are low (<100mA) AND 2) Vin-Vout is low (<5V)

Sure, a switching design is a little more complex and you'll need to select an appropriate inductor but the benefits of cool and efficient operation generally make it a far better choice when building non-trivial power supplies.

Linear Tech/Analog Devices offer an excellent Spice simulator you can use to evaluate any of their devices. Similar switchers are available from TI and other semiconductor manufacturers.

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It will provide 3.3V out, however you have not specified the current, and the efficiency will be miserable.

For example, with 24VAC in the efficiency will be around 10% (and the input voltage will be uncomfortably close to the maximum the 7805 can withstand- a slightly higher than nominal mains voltage will stress the chip beyond the absolute maximum 35V input).

That means if you want 0.1A @ 3.3V (about 1/3 of a W) the losses will be 3W in the 7805 and 200mW in the NCP1117, so 3.2W loss for 0.33W out. That means a fairly large heatsink on the 7805 and maybe a fan.

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  • \$\begingroup\$ It's also worth noting that the 1117 is picky about its output capacitor, and you can't be certain it won't oscillate if we don't know what the ESR is. \$\endgroup\$ – Hearth Mar 23 at 23:40
  • \$\begingroup\$ @Hearth Good point, the NCP1117 specifically states tantalum or ceramic "should be" adequate. \$\endgroup\$ – Spehro Pefhany Mar 23 at 23:42

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