I have used buck converters that can move from 30 V DC to 5 V DC, their maximum current is set at 400 mA, I need 3.5 amps. I am looking to move from 120 V AC to 5 V AC. I'm then going to change it to DC current using the rest of the circuit. In my searching on Google I have found many many voltage regulators that can move small amounts of current and i'd like to move high current.


I am looking for a solution I can build instead of using a prefab solution. This is the way learning my power supply. Also looking for specifically a buck converter, as there are other options.. but I'd like to know that it can be done similarly with buck converters and which buck converter could do it.

EDIT: i have decided to use a isolated transformer circuit to step down from 120V AC to less voltage and then use a buck converter. This was for safety reasons. This post has a valid question that i no longer am pursuing an answer for. If there isn't a need for electrical isolation a buck converter circuit is practical. If someone answered the question with reference to a buck converter i accept the answer.

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    \$\begingroup\$ If you are going from AC to AC: why not just use a transformer ? \$\endgroup\$
    – Hilmar
    May 23, 2020 at 2:07
  • \$\begingroup\$ i am going from AC to DC i figured it is going to be converted after. I'd like to use a buck converter. I am thinking to use a high frequency transformer (for a different question, as i'm not sure exactly how it's going to setup) . I'd like to know if it's possible and which buck converter can be used from 120v to 5v ac, if i go that way. Most of the time i see these hulking transformers and that'd throw my PCB off. @hilman \$\endgroup\$ May 23, 2020 at 2:19
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    \$\begingroup\$ Look for a Power Supply, rather than a buck converter. A switch mode power supply will take 120 VAC input and produce a regulated DC output. Power supplies are available with a wide range of output voltages and currents. \$\endgroup\$ May 23, 2020 at 3:04
  • \$\begingroup\$ I am building a power supply @PeterBennett That is this circuit. I am getting the impression that most people here have only used prefabbed power supplys and such for their 120v projects. Power supplies are inexpensive in that they cost like $8-$15 each. I want 15 power supplies and it's more practical to make them and learn them because i am using them in the future as well. \$\endgroup\$ May 23, 2020 at 3:32
  • \$\begingroup\$ Look for a flyback AC/DC power supply design. \$\endgroup\$
    – Andy aka
    May 23, 2020 at 9:49

1 Answer 1


The easiest option is probably to use board mount AC to DC power supply brick.
Such as those made by Recom, XP Power, Meanwell, or CUI.

One example would be...
IRM-20-5 Vin=85V AC to 264V AC, Vout = 5V DC, Iout = 4A max.
The whole thing is only 2.06" x 0.94" x 1.07" and costs only $11.00

If you insist on doing it yourself Keep in mind that the peak output voltage of the isolation transformer needs to be at least a few volts greater than 5V.
1) Because you are going to 100se 1~1.5V in your bridge rectifier.
2) Because the input is AC you are going to have ripple at the output of your rectifier. Including ripple, your input must remain greater than 5V for a normal buck converter to give you a steady 5V output. If you don't leave yourself a lot of voltage headroom then the holdup capacitor on the output of your rectifier will need to be huge.

A common 10:1 isolation transformer with 120V RMS in will give you 12V RMS out (17V peak). Run the transformer output through a full-bridge rectifier with a large holdup capacitor on the rectifier output and you will have something near 15.6V DC. You can then use just about any 3.5A buck converter at that point.

Some Usefull Stuff for Finding Converters.

If you pick a converter chip from Texas Instruments you can use the free TI Web Bench tool on the Texas Instruments website. It will generate a schematic and list of components given some power supply criteria. With the click of a button it does like 100 designs and presents them to you so you can evaluate the trade offs.

If you pick a converter chip from Linear Tech/ Analog Devices you can simulate it using their free LT Spice program.

Distributor websites like Digikey.com or Mouser.com allow you to search for chips based on parameters. It makes finding what you need a lot easier.


  • \$\begingroup\$ I appreciate this answer, as even i have used digitkey and mouser for getting a more specific search and i didn't think to do that here. A buck converter is not an isolation transformer, i am looking for a buck transformer that can take the place of this isolation transformer mentioned in your answer \$\endgroup\$ May 23, 2020 at 2:41
  • \$\begingroup\$ I guess I was thrown off by your mention of a buck converter. If you are going to perform the conversion through the transformer then the converter is typically called an "Offline Switching Regulator". Try digikey.com/products/en/integrated-circuits-ics/… \$\endgroup\$
    – user4574
    May 23, 2020 at 2:51
  • \$\begingroup\$ interesting idea to do both the conversion of volts/amps and AC to DC in one, i plan to use a full bridge to convert AC to DC after it has been switched. Leaving ~9 volts or 12 volts for the rectifying circuit and voltage regulator to 5v exact. still i am just looking for a buck converter \$\endgroup\$ May 23, 2020 at 3:31
  • \$\begingroup\$ @kennethmods, for your safety, I highly recommend you design an isolation transformer into your system somewhere. If you also use it to step down the 120 Vac to 48 or 24 Vac, it will make your system much safer and also make designing the next stage (buck from 48 V or 24 V to 5 V) much easier. \$\endgroup\$
    – The Photon
    May 23, 2020 at 6:20
  • \$\begingroup\$ noted @ThePhoton , appreciate the suggestion. I am going to do so, though i have found some trouble gettign a very small transformer to use with my PCB design. This question reflects a need for a buck converter from 120v ac to 5v ac, which i no longer need. \$\endgroup\$ May 23, 2020 at 15:49

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