Let's say there is 230 VAC power line. How can I convert it to let's say 223 VAC? Expected current is up to tens of Amps.

I could do one the fllowing:

  1. Use 10 bidirectional diodes to convert these 7 extra volts to a heat. This is the most simple but not efficient and can be a tricky thing for big currents.
  2. Use a autotransformer. As I understand most of the windings can be done with a thin wire. The winding which will take all current will have relatively small amount of turns. This is much more efficient than p.1 but the transformer will be a big piece of steel and copper = big and expensive.
  3. Use SMPS. But I can not figure out the best topology for this type of implementation. In simple this should look like this:

enter image description here

SMPS solution looks most promising for its efficiency but it is a bit tricky for me.

Could you please put some comments on the SPMS part? Or probably I missed something else worth considering?


I just tried one idea and at least simulation looks pretty good:

enter image description here

This schematic will need a feed-back as the voltage difference depends on a load resistance. Moreover I will need to switch the switches simultaneously and to deal with switching bad processes but anyway this looks very promising. Is there any drawbacks I did not aware of?

  • \$\begingroup\$ There is a chance you don't actually need to take it down to 223 volt. Maybe you just need to reduce the energy in this AC power, and for that you might get away with a simple TRIAC-based lamp dimmer (making sure that it will handle 10 A). And if you think that your autotransformer will be expensive but your SMPS for a 10 A/223 VAC solution will be cheaper, than you're delusional. :) \$\endgroup\$
    – pipe
    Commented Dec 10, 2015 at 9:22
  • \$\begingroup\$ @pipe Thank you for the input! Let me comment on the ideo of using TRIAC dimmer: the question I asked is not about dimming the power of the lamp. I exactly need to reduce the amplitude (application is: one directional data transfer over power line). And this is the main part of my technical enquiry. \$\endgroup\$ Commented Dec 10, 2015 at 9:31
  • \$\begingroup\$ veriac or in-series inductors \$\endgroup\$
    – user16222
    Commented Dec 10, 2015 at 9:56
  • \$\begingroup\$ You know that nominally '230 V AC' mains won't always be exactly 230 V, don't you? Do you want to reduce the voltage by a fixed amount, by a fixed proportion, or to a fixed RMS value? I can't help feeling there must be an easier way to achieve what you're after. \$\endgroup\$
    – nekomatic
    Commented Dec 11, 2015 at 9:08

2 Answers 2


Point number one. You seem to want to work with mains connections. Know what you're doing, take all necessary precautions. Don't kill yourself, and then try to sue me for giving you this information.

For a very small voltage change, the simplest is to use an auto transformer.

Let's assume that you want to be able to change up to 10 volts at 20Amps. Although your total throughput is 20Amps x 240v = 4800 watts, the power required for the transformer is just 10 x 20 = 200 watts.

If the variation you need is fixed, you can simply buy a mains transformer with the required power and secondary voltages. Say you bought a 200VA mains transformer with two 5v 20A secondary windings. You would then be able to vary your mains voltage by putting one or both secondaries in series with the incoming mains, using the appropriate phasing to either increase or decrease the voltage by 5v or 10v.

If you want continuous variation, then there are three options.

a) Buy a 200VA transformer kit, which has the mains winding already done, and wind your own custom secondary, suitable for 20A, with as many taps as you need, one per turn if you like!

b) Buy a 20A Variac. This would be very big and heavy, but would give you continuous variation from 0v to a little above mains voltage.

c) Buy a 1A variac, and the 10v output 200VA transformer. The 1A variac is much smaller and cheaper than the 20A version. Power the input of the transformer from the output of the variac. The transformer output will now give you continuous variation of 0v to 10v and a current capacity of 20A. Connect with the appropriate phasing to increase or decrease the mains voltage.

Note, strictly speaking, the 10v mains transformer I have described is not an auto-transformer, but an 'ordinary' transformer. However, with the connection I have described, it does function as an autotransformer. At this high ratio, there is practically no copper to be saved in making parts of the windings common.


From your comment to @pipe it appears that you are trying to do 'power line communication'. The normal way of doing this is to superimpose a high frequency carrier signal onto the mains somehow and modulate it to transmit the data. See Power-line Communication on Wikipedia for a start.

Your question reads as though you're trying to figure out how to pull the mains voltage down by 7 V to transmit the signal. To do this you would need a bank of high powered heaters or lamps flashing on and off at the data rate and it would hurt you badly through your electricity bill.

  • \$\begingroup\$ Thank you fr your answer! I aware of the high frequency communication, however I need to communicate under protocol which was given. Regarding the loads: the load will be LED drivers with current stabilisation. So 1) There will be no any problem 2) If it is - it is not my problem :) \$\endgroup\$ Commented Dec 11, 2015 at 9:21

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