I bought a UPS, it has output of 220v AC, the problem is the output is square-wave, which causes distortion on TV/LCD, is there circuit that can take this Square-Wave as input, and output Sin-Wave?

  • \$\begingroup\$ What power or current do you need for your TV/LCD? \$\endgroup\$ – Andy aka May 7 '14 at 9:20
  • \$\begingroup\$ approximately 120w, but there maybe other devices plugged into the UPS, the UPS's running load is 225w maximum \$\endgroup\$ – user41468 May 7 '14 at 9:29

You can use a 2nd order low pass filter made from an inductor (in series with the AC) and a capacitor to neutral. You will also need a resistor: -


simulate this circuit – Schematic created using CircuitLab

1 Henry and 10uF form a 2nd order low pass filter that will attenuate the higher order frequencies from the sq wave and make the output signal more sinusoidal in nature. It might work fine or it still might give your TV problems if it is particularly sensitive to harmonics.

The resistor is needed to keep the Q of the circuit low and prevent high ringing voltages.

Q = \$\dfrac{1}{R}\sqrt{\dfrac{L}{C}}\$ = 3.16 using the values above.

Q needs to be about unity so I'd advise to make C bigger and L proportionally smaller but be aware that the value of resistance is in series with the TV and whatever current the TV takes, it flows through the resistor. For instance, 110 watts needs 0.5 amps from 220 V and this means the I^2R losses in the resistor are 25 watts - choose big wirewound resistors.

  • \$\begingroup\$ what about the rms losses? \$\endgroup\$ – user41468 May 7 '14 at 10:16
  • \$\begingroup\$ That is the power loss based on rms values of current and voltage which, presumably you gave. \$\endgroup\$ – Andy aka May 7 '14 at 11:04
  • \$\begingroup\$ oh, ok, i will try to make this circuit, the capacitor and inductor are not special? just the resistance is 1 watt? \$\endgroup\$ – user41468 May 7 '14 at 13:11
  • 1
    \$\begingroup\$ every component is special. The cap stage needs to be Aw to withstand ac voltages of 220 volts and the resistor is probably better rated at 50 watts to avoid aot of heat. The inductor is very special too. \$\endgroup\$ – Andy aka May 7 '14 at 14:45
  • \$\begingroup\$ The passive filter is a valid approach and can be cheaper than a sinewave invertor if you already have a squarewave one and lamp ballasts in your junkbox I didn't use axternal resistance but the ballast had some dcr closer to 10 ohm than 100ohm I have recommended similar bodgy fixes and never had any nasty ringing problems \$\endgroup\$ – Autistic Aug 13 '15 at 3:45

There is no practical solution in your case. Some big changes must be done in the inverter circuit to make it give sinusoidal output. And it is not an easy and cheap work for an end user to do.

There must have been a warning message on the box of the product, giving information about flaws of the product, which is understandable by any non-technical customer. If there wasn't, you can sue them on for it.

  • \$\begingroup\$ there is no way to chop down the square-wave ac output into sin-wave form? \$\endgroup\$ – user41468 May 7 '14 at 9:32
  • \$\begingroup\$ If you connect several cascaded big inductors and capacitors with correct values, you can roughly clear higher order harmonics from the square wave and make it more like sine wave. But designing a filter like that requires a lot of mathematical background for the designer. The implementation process will cost a lot of money and work. And there is no guaranty that your TV will work alright with the new resulting waveform since it still won't be purely sinusoidal. \$\endgroup\$ – hkBattousai May 7 '14 at 9:37
  • \$\begingroup\$ and a power conditioner is useless in my case? \$\endgroup\$ – user41468 May 7 '14 at 9:43
  • \$\begingroup\$ Connecting even a single capacitor is useful. But we can't know if it will be useful enough or not. \$\endgroup\$ – hkBattousai May 7 '14 at 9:47

An Ott filter will do a good job of changing your square wave output to a sine wave without causing an inductive load on your inverter which other low pass filters do and can cause damage to your inverter.


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