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Power supplies and Power Factor

Discussion in 'Power Supplies' started by multicast, Oct 7, 2014.

  1. multicast

    multicast Senior Elf Generous Elf

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    Just been working on a system with a lot of Switch mode Power Supplys in it, and though that there was something worth sharing, might save someone else some issues.


    This is a particually bad SMPS, See the power factor, just .46. If you see the two measurements for RMS voltage and RMS current, and multiply ten, you'll get the apparent power of 70.77W.. But in reality this is only using .46 of this, or 32W.


    If a load was purely restive, Current and Voltage would be in sync with each other. But because a SMPS is mostly a 'reactive' load. ( i.e., inductive and capacitave ), the Current gets phase shifted with respect to the voltage.. Its a big topic for a few classes in Electronics 200 at university..


    Net result, if you have lots of devices with low Power Factors, you'll simply not be able to get the power out of a circuit that you expect, before you pop your uses.


    A 230V circuit at 10A would be 2300W right? that is true if PF = 100%.


    If PF= 0.46 like this nasty bit of gear was, that when your pulling 10A, you'll only be getting 2300W * 0.46 or 1058W.


    And theres your problem. If you are stacking lots of SMPS's up ( like some of you are ), you need to factor this in.


    Now this device is particually bad. A PF of less than .8 needs dealing to. It has no PFC ( power factor correction ) built in. Good Power supplies will have Power Factor Correction built in. The Meanwell SP-320's for example which many of you use do have this. Some of the cheap and nasty kooza brands don't, and you too might see things like this.











    [​IMG]
     
  2. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    Good advice and a great topic and the reason i use quality power supplies like the meanwell HRP model which has a very high power factor of greater than 0.95
     
  3. BundyRoy

    BundyRoy Senior Elf

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    So are the power factors stated as part of the specifications of a PSU. Can you work it out yourself by testing it somehow. Sounds like testing may be outside the abilities of the uneducated.
     
  4. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    Power factor is stated as part of the specification for Meanwells and should be with other name brands as well. Not sure about the cheap brands as many dont specify it

    Here is a link to a calculator for powerfactor, but im not sure how good it works and if you will get any reliable data because i wonder how reliable are the specs of a cheap power supply.

    http://www.rapidtables.com/calc/electric/power-factor-calculator.htm
     
  5. BundyRoy

    BundyRoy Senior Elf

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    Thanks Eddy. I was wondering about the reliability of any of the specifications on cheap chinese gear. I figured the specs wouldn't necessarily be too accurate.
     

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