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The following shows a simple regulated transformer supply

230 to 5v DC power transformer

This unit needs 230 Volt Input to produce 5 Volt.

But I'd like to build a power supply that can handle both 120V and 230 Volt. Like this one here 5V Buck convert with variable input voltage. It accepts AC 85 ~ 265 v. What do I need to add to make it accept different voltages?

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    \$\begingroup\$ Do you want to [have to] flip a switch [between 120/230] or do you expect it to happen automagically? In the latter case, you can mostly forget about a linear supply, you'll want a switching supply instead. (A buck converter is one of those.) \$\endgroup\$ – Fizz Nov 7 '15 at 14:46
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    \$\begingroup\$ It works on a different principle to the circuit you have so there is no modify this to make that at all. \$\endgroup\$ – Andy aka Nov 7 '15 at 14:47
  • \$\begingroup\$ yes, I'd like it automagically. So i'll go for switching supply. thanks! \$\endgroup\$ – Eduscho Nov 7 '15 at 14:56
  • \$\begingroup\$ Easiest - get a 115/230V primary transformer and forget the electronics. \$\endgroup\$ – Chu Nov 7 '15 at 15:59
  • \$\begingroup\$ if you are asking the question, you might not want to be experimenting with mains voltages. Due to safety. \$\endgroup\$ – Brian Carlton Nov 7 '15 at 22:48
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This is about as simple a circuit that looks like it might work the way you want: -

enter image description here

About the only commonality between this and yours is the bridge rectifer if you used 1N4007 devices (rated at 1000 volts). Bare in mind that the circuit above has a lot of detail missing like the turns ratios of the high frequency flyback transformer (nothing at all like the transformer you have of course). There is also an opto coupler to sample the output side so that the TOPSwitch device can regulate the output voltage.

Here's one that uses a Viper 12 chip and this looks more reasonable in terms of all the components needed: -

enter image description here

You can get more details from this blog

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