I develop products that are sold all over the world. The line voltage may be one of (100, 120, 220, 240). I am currently considering options for handling this requirement. We typically have a 5V rail that needs a couple amps, and 15V rails that need under an amp. Perhaps another tap with a higher voltage.


  1. Transformer and Linear Regulators: This is what we presently do. We have a power entry module like this where the user can install the fuse differently to select the correct voltage.

    • Pros: Clean Supply (we measure extremely small signals with these devices), Simple to achieve Galvanic isolation with multiple secondaries, With undervoltage and overvoltage accounted for, the device is not damaged even if the fuse is installed incorrectly.
    • Cons: Bulky Transformer, We have to change the fuse drawer based on destination address which adds a manufacturing step. Also, we occasionally sell to OEMs who then resell their system to unknown addresses.
  2. Switched Mode Power Supply:

    • Pros: Can handle variable input voltage.
    • Cons: The switching introduces noise which will impact the performance of some of our products.

I am not asking for a decision from the above options, but rather wondering if there are any other options to put on the list. What do you guys do?

  • 1
    \$\begingroup\$ switchers can have multiple secondaries, and isolation. noise can be filtered or you can use a linear regulator after the switcher. \$\endgroup\$ Commented Nov 21, 2015 at 10:25
  • \$\begingroup\$ What frequencies would be most likely to muckup your sensitive equipment? \$\endgroup\$
    – Autistic
    Commented Nov 21, 2015 at 12:31
  • \$\begingroup\$ @Autistic DC to 10 MHz \$\endgroup\$ Commented Nov 21, 2015 at 12:40
  • \$\begingroup\$ Well dont give up you could go class E running above 10meg,its not that bad because the power is low. \$\endgroup\$
    – Autistic
    Commented Nov 21, 2015 at 12:53
  • 1
    \$\begingroup\$ Typically 2 is coupled with some LDOs for the most sensitive sub-circuits. More can't be said without specifics on the actual circuits involved. Also note that multiple output switchers (that Jasen suggested) are generally harder to regulate, so tend to have worse regulation of some of their rails than others. \$\endgroup\$ Commented Dec 23, 2015 at 5:52

2 Answers 2


You really have the only two answers there are: Linear or SMPS. There are some things you can consider though to make you designs more robust regardless of which you choose.

Linear: Designing a Transformer that can take each of those input voltages and output the correct secondary would not be too difficult. You can double wind the input transformer and have it select between 100-120Vrms and 220-240Vrms. Then with the linear regulators you simply allow them to regulate when the lowest rms voltage is present and burn more power then the highest rms voltage is present.


simulate this circuit – Schematic created using CircuitLab

SMPS: These are much harder to design. There are more regulations regarding their standards internationally. They are lighter weight. If you're worried about sensitive electronics, Use a constant switching frequency and then make a notch filter on the output specifically targeting that frequency.


Once I was involved with a very similar problem - a power supply for biasing a sensitive radio circuit, where a switching PSU was out of the question.

We used a linear power supply and then attempted to offer automatic 110/220 switching. The mains input would initially be connected to the 220 V primary, but if the DC voltage was low, it would then select the 110 V primary with a relay. This worked well.

One problem I had was that after disconnecting a 220 V mains supply, there was a short time while the smoothing caps discharged, during which the relay would pull in to the 110 V position. If the supply was reconnected during this time, it went straight to the 110 V primary and blew the fuse. This happened surprisingly often...

If you had better automatic voltage selection logic, perhaps using a microcontroller, or sampling the mains voltage directly not through the switched transformer, I see no reason why the automatic voltage selection wouldn't work. Once connected to mains it's not possible to suddenly change from 110 to 220 V.

The microcontroller making the decisions, and the relay(s) would need to be powered from a linear regulator with tons of headroom, so they run at half-voltage until the correct decision is made. Your loads might need to be inhibited until the supply was stable.

Im sure regulatory approval of the power supply would be more difficult too.

  • \$\begingroup\$ Thank you for sharing your experience with this approach \$\endgroup\$ Commented Dec 23, 2015 at 22:09

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