Why are voltage dividers said to be "highly inefficient"?

Discussion in 'Homework Help' started by pigpen, Jan 27, 2016.

  1. pigpen

    Thread Starter New Member

    Jan 26, 2016
    23
    1
    Sorry, I've always wondered about this -
    In systems where 220 or 250V AC is standard, you will often find that small transformers are used in case 110V is needed. Why not just use a voltage divider instead of these transformers that are very heavy, and can get dangerously hot? I googled this and the consensus seems to be that voltage dividers are highly inefficient, no explanation given. Is it that they are fine for DC use but not for AC? Or is it because you must first decide on a standard output current before designing one? Please someone give me a hint.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,796
    Is this trolling? Are you serious?
    If you think a transformer might get dangerously hot, just try calculating the power dissipation of a voltage divider across the 220V mains that draw even a small amount of current. Never mind the current that the 110V load would draw. The numbers are eye popping, not to mention the safety issue.
     
  3. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    So run some numbers and see for yourself.

    In the U.S. a typical 110 V circuit would be fused at 15 A. Let's say that you don't want the voltage delivered to the load to drop by more than 10%. What resistor values would you use? How much power would be delivered to the load? How much power would be lost in the resistors? What would the efficiency be?
     
    pigpen likes this.
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,565
    2,379
    Who says they are fine for DC?
    Any slight variation in load, AC or DC can cause a an unacceptable variation in output voltage.
    Max.
     
    pigpen likes this.
  5. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    A voltage divider will dissipate the same power whether used for AC or DC at the same load (RMS) current.
    Why would you think otherwise?
     
    pigpen likes this.
  6. pigpen

    Thread Starter New Member

    Jan 26, 2016
    23
    1
    No this isn't trolling. It's only because I seem to remember voltage dividers cropping up a lot in DC circuits, but for AC, I can't think of an example, except maybe for the circuit inside voltmeters... and the power supplies of desktops... I just have no concept of them being highly inefficient because they seem to be an unavoidable part of any circuit. I think we just had maybe a few minutes in class where the professor discussed voltage divider circuits. He never said please don't use them ever cos they are incredibly wasteful. And I'm not that far into LIEC but the part that I've encountered about voltage dividers doesn't make any mention of their inefficiency either.
     
  7. pigpen

    Thread Starter New Member

    Jan 26, 2016
    23
    1
    Now I see I only had to do some power calculations -- see that didn't even occur to me for some reason. So sorry.
     
  8. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    So now you can see that they are useful only as long as efficiency doesn't matter, which means as long as no significant power is needed. Thus they are used for manipulating voltage signals that convey information and not power. The same is true for zener regulator circuits -- they are seldom used for power regulators, but rather for signal regulators, such as generating reference voltages.
     
    pigpen likes this.
  9. pigpen

    Thread Starter New Member

    Jan 26, 2016
    23
    1
    Yes. And that's a great summary, thank you so much!
     
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    Say you have some circuit running off 15 volts an you need 5 volts. A simple voltage divider can do that with 10 volts across one resistor to leave 5 volts across the other. If you have a light load you may just run 10 ma in the divider, so the top resistor just has 0.1 watts in it. That isn't much power and most any resistor you buy will handle it fine.

    Now do the same thing off 115 volts. Your top resistor now has 110 volts across it, and for the same 10 ma it burns off 1.1 watts. That is a significant amount of power.

    Transformers are used because they are wonderfully efficient at converting AC voltages.
     
  11. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,015
    1,531
    And safer. Surprised this thread wasn't locked.
     
  12. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    No reason to lock it -- the TS is asking a question that is a quite reasonable question for someone new to electronics to consider.
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,015
    1,531
    But doesn't doing this, the original question, result in a "transformerless power supply"?
     
  14. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    The TS is giving absolutely no indication that the question is in relation to any physical circuit that they want to build -- it is a theoretical question regarding the efficiency of using a voltage divider verses a transformer.

    If you feel it is an issue, the Report the original post and the Mod Team will consider it. As an individual mod I see no reason to close it or report it, but if it IS reported I will go with the consensus view.
     
  15. pigpen

    Thread Starter New Member

    Jan 26, 2016
    23
    1
    Sorry it's just that we've been given so many power dissipation exercises to do in class it gets one used to the incredible inefficiency of ordinary devices. Even the incandescent bulb was known to be astonishingly inefficient but was in use for decades. Pre-inverter type airconditioners and anything that goes against a machine's natural tendency to produce heat are gobsmackingly inefficient but are ordinary household appliances. It's almost that the message has been pretty much everything is crazily inefficient (apart from solid state devices---maybe) and in wide use, it all distorted my view of what level of inefficiency is acceptable.

    Actually the knowledge you guys have offered here are the sorts of things I would only glean from an aside / supplementary info by a nice professor. I don't think this stuff is pointed out in our references... especially since we got voltage dividers explained as part of a sort of intro to the uses of resistors. I've had a few classes on Electrical Safety believe it or not but they never mentioned this stuff.

    If you can refer me to an online resource that you think could give me further perspective on this topic (honestly I have never heard of "transformerless power supply" either) I would really appreciate it.
     
Loading...