Power supply AC input Cut off voltage?

Discussion in 'General Electronics Chat' started by SamEricson, May 1, 2015.

  1. SamEricson

    Thread Starter Member

    Apr 25, 2015
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    For linear and switching power supplies, when lowering the AC input voltage from 115vac down under 90Vac will turn the power supply off. Some power supplies can go under 90VAC and the power supply will will output a stable DC voltage. What determines a power supplies cut off voltage or threshold? I can't find anything that tell on the datasheets on power supplies about this. Linear power supplies react differently compared to a switching power supply when lowering the AC input voltage under 90vac. What kind of power supply can the output of a power supply can stay stable with an AC input under 80VAC?
     
  2. tranzz4md

    Member

    Apr 10, 2015
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    Don't expect to find any. That's over 30% droop in input voltage.

    The term "power supply" covers a lot of ground. Knowing a bit more about the type of use such a unit is intended for, both by the OEM and you, would help.

    There are various devices, that someone might call a power supply, that will detect their nominal supply voltage and "switch" to operate properly, but they switch between common nominal utility power voltages (480 / 380 / 277 / 240 / 220 / 208 / 120).
     
  3. SamEricson

    Thread Starter Member

    Apr 25, 2015
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    But what is it called in the datasheets to look for how low the AC input can do with a stable output DC voltage?
     
  4. #12

    Expert

    Nov 30, 2010
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    1) The way it's designed.
    2) You will never find the drop-out voltage of power supplies which is below the designed drop-out voltage because they are not designed or guaranteed to operated below the designed drop-out voltage.
    3) (The kind that are designed to operate below an 80 volt input.
     
  5. #12

    Expert

    Nov 30, 2010
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    The input voltage range.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Switching power supplies are inherently very different from linear supplies in the way they handle an input undervoltage condition. Most switcher control chips have an input voltage monitor circuit built-in. Because of component tolerances, it usually is designed to cut off the conversion stage at a trip point a few volts lower than what is on the data sheet. for example, if thesupply is rated for operation down to 85 VAC, the low-voltage cutoff might be designed around 80 VAC.

    Linear supplies usually have all control on the secondary side of the transformer, so there is no explicit input voltage monitoring. What happens with low AC is that power line ripple starts to show up on the output DC voltage. The is very dependent on load (and, or course, how the regulator is designed).

    ak
     
  7. SamEricson

    Thread Starter Member

    Apr 25, 2015
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    So when the AC input to a linear power is very low the DC output ac ripple voltage gets larger? why is that

    Which is better linear or switching to operate at 80VAC and get a stable DC output with no ripple?
     
  8. #12

    Expert

    Nov 30, 2010
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    1) Because the voltage on the power supply capacitors will not stay high enough to supply the proper voltage.
    2) Neither of them are better at working below their design voltage while outputting no ripple. That's like asking, "Which car is better at floating in water?" They aren't designed to do that. That's why it doesn't work.
     
  9. SamEricson

    Thread Starter Member

    Apr 25, 2015
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    The ripple gets larger because the input voltage is lower, that doesn't make sense explain please
     
  10. #12

    Expert

    Nov 30, 2010
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    The ripple gets larger because it is rippling BELOW the designed output voltage instead of ABOVE the designed output voltage, as you appear to think.
     
  11. SamEricson

    Thread Starter Member

    Apr 25, 2015
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    So the filter cap values are only going to filter out the ripple at 120VAC not at 80VAC?
     
  12. #12

    Expert

    Nov 30, 2010
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    The filter capacitors filter out ripple within the designed input voltage range of the power supply.
     
  13. SamEricson

    Thread Starter Member

    Apr 25, 2015
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    your formale for calculating a power supply ripple is:

    Vripple P-P = I/1.414 x C x F

    Where does it say it's depended on the AC input voltage?

    How do you change the formula
     
  14. #12

    Expert

    Nov 30, 2010
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    1) It doesn't say that.
    2) with algebra
     
  15. SamEricson

    Thread Starter Member

    Apr 25, 2015
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    so it's just magic?

    it happens for a reason with a formula
     
  16. #12

    Expert

    Nov 30, 2010
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    It's a perfectly good formula that has no relationship to your question. You keep asking what happens when you give a power supply inadequate voltage. The formula that I copied from a National Semiconductor audio handbook works for power supplies that have proper voltage supplied.
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The capacitors on a linear supply are designed for a certain % ripple at the maximum rated current, lower the voltage and % ripple figure will rise, but consequently if the voltage is lowered and the load remains the same the consequential current will be lower.
    You need to specify all parameters.
    Max.
     
  18. #12

    Expert

    Nov 30, 2010
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    The question is:

    The answer is: It doesn't.

    This is not the formula for how linear supplies pass power line voltage ripple through their power transistors. It is the formula for how much ripple will appear on the power supply capacitors under PROPER OPERATING CONDITIONS.
     
  19. SamEricson

    Thread Starter Member

    Apr 25, 2015
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    The ripple rises because of the filter cap values are only ment for filtering at 120vac and not 80vac

    Does the ripple rise like this in switching power supplies or just linear power supplies?
     
  20. SamEricson

    Thread Starter Member

    Apr 25, 2015
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    Correct, the formula needs to be changed to be able to calculate for Improper operating conditions like if the AC line voltage goes under to 80VAC , what is the ripple and how do I calculate it?
     
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