Square wave in for a sine wave device : Harmful ??

Discussion in 'General Electronics Chat' started by ranatungawk, Apr 5, 2015.

  1. ranatungawk

    Thread Starter Senior Member

    Oct 30, 2008
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    Is it harmful, If I supply 230V, 50Hz "square wave" power-input to a electronic device with SMPS (Such as TV,PC.. originally designed for 230V, 50Hz ) ? Or due to the SMPS wont it be a big problem ?

     
  2. Papabravo

    Expert

    Feb 24, 2006
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    It depends on the device and each one is different. Some might work, but no doubt others will be compromised.
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    Hi Ranatungawk,

    It really depends on how the transformer, assuming it has one, will respond to the higher order harmonics.
     
  4. tcmtech

    Well-Known Member

    Nov 4, 2013
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    In general very few SMPS based device are all that fussy about what sort of input power they get and will work anywhere from DC to several hundred Hz AC as long as the supply voltage is below their rated maximum voltage even if they are listed to use 50 - 60 Hz power.
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    It's more about how the transformer will deal with higher frequencies. Power transformers are low frequency. Higher frequencies could cause humming, best case, or transformer failure in the worst case.
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Where does the OP mention anything about a transformer? You do know what a SMPS is right? o_O
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
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    Designed my first switching regulator about 40 years ago. That was long before you could buy a switching regulator IC that gave you examples of buck/boost applications, so you needed to understand the theory...

    Most equipment has a transformer that steps line voltage down to some reasonable level so you don't have to use caps and rectifiers rated at hundreds of volts. If said equipment didn't have a step down transformer, discussing it on this forum would be disallowed. Read between the lines...
     
    Last edited: Apr 5, 2015
  8. tcmtech

    Well-Known Member

    Nov 4, 2013
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    I only read what is posted and the OP only mentioned SMPS's. It saves me from a lot of unnecessary speculation and pointless debate over unknowns that likely do not exist or are not a presently relevant concern to the OP. :rolleyes:

    The OP never mentioned iron core 50 - 60 HZ transformer based power supplies. Only SMPS which by design convert the incoming power to a HV DC making its initial incoming form and frequency largely irrelevant.

    If I were to speculate on what he did say I would take that when he mentions ' If I supply 230V, 50Hz "square wave" power , is likely the power being supplied by a cheap power inverter to which effect most typical SMPS's are unaffected by.
     
  9. dl324

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    Hi tcmtech,

    He mentioned TV and PC which usually have step down transformers. Sometimes people don't articulate their questions clearly, either because English isn't their first language or they're not a trained engineer. Instead of jumping to conclusions or giving answers that would be wrong if one took the time to ask clarifying questions, it's better to make sure you know what question you're answering.

    And we know we're not allowed to discuss equipment that connects to line voltage without using a transformer. The OP was talking about supplying line voltage to something that contained a SMPS. If the SMPS didn't use a step down transformer, we should know that so we don't violate Forum policy and help him do something dangerous.
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
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    Trained engineer trying to help people for free. Retired and nothing better to do... How about you?
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    @tcmtech : That last remark really was uncalled for.

    Most TV/PC and similar SMPS have a transformer in them. The transformer is designed to work with a much higher frequency which is provided by the switching mechanism at the front end on the primary side which uses feedback from the secondary side. Discussing the potential effect of harmonics on this circuitry seems quite reasonable.
     
  12. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But do not forget that every off-line smps has a bridge rectifier and 400V bulk capacitor at the input. So who cares if the input voltage is a sine wave or square wave?
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    I went ahead and deleted said post.
     
  14. Reloadron

    Active Member

    Jan 15, 2015
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    Papabravo said:
    That would be my thinking. Devices like my home computer and my TV as well as cordless telephones have no problems at all with power from UPS units which output a "Modified Sine Wave" which pretty much looks like a square wave. :) Some devices with linear power supplies using some transformers may not care for the square wave or modified sine wave input. So in a simple nutshell it all comes down to the device. Some will and some won't. Never gave it much thought but some of my power tools that are mains operated (120 VAC 60Hz) may not operate well on a square wave and I am not about to try the microwave oven. :)

    Ron
     
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  15. Roderick Young

    Member

    Feb 22, 2015
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    I don't know about TV's. I know for sure that PC's today use line switching power supplies, and the first thing that they do is rectify the line voltage into high voltage DC. They will work fine off a square wave, modified sine, or even high voltage DC *. Every brick-type computer supply I've seen is the same way, as are the tiny, light phone chargers. Only the older "wall wort" adapters actually have a line transformer in them, and you can usually tell those simply by their heavier weight for their size.

    I have heard, but have not personally verified, that AC motors will not run well off a square wave - that they can overheat, draw more power than expected, or even burn out.

    * With the exception that if the supply uses discrete rectifiers for a full wave bridge, only two of the parts will carry the current, and could over heat if the supply is pushed to the extreme limit.
     
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  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Many off-line switching power supplies have power factor correcting front ends, and these care very much about the shape of the input voltage waveform. In this type of supply the bulk capacitor comes after the boost converter stage. The boost converter is relatively complex, as it shapes the input current to track the input voltage while maintaining a constant output voltage. The control loop bandwidth is not wide enough to handle the fast edge of a square wave.

    Loke almost everyone has said, it depends on the device - for more reasons that you might think.

    ak
     
  17. ranatungawk

    Thread Starter Senior Member

    Oct 30, 2008
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    Thanks!! Mr. Young!!! It's a very clear answer for my question.

    I don't know if i'm correct, but as I think, with in a SMPS , AC power (Sine or square wave) goes to a bridge rectifier and then it is converted to DC as the first step. Even though the input is Sine or square it wont be a matter for other stages after the bridge rectifier of the SMPS..



    **** as i have seen some of AC motors wont run with a square wave at all!!! when AC with square wave is supplied, it gives a hum (Drrrrrrr....)and fan blades begins to vibrate but rotate.
     
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