Tweakers' Asylum

RE: voltage drops question

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jea48

A

In Reply to: RE: voltage drops question posted by pictureguy on August 29, 2012 at 14:19:50

When powerline voltage drops what does happen? The resistence of the circuit is the same......so something's got to give. IF power stays the same by P=IE current must rise. So, what changes? Voltage drops and current drops too?
Power does not stay the same.
Example.
A 100 watt incandescent light bulb
P = E x I
100 watts / 120 volts = .83 amp.
R = E / I
120V / .83 = 144.58 ohms
I = E / R
96V / 144.58 ohms = 0.66 amp.
P = E x I
96V x 0.66 amp = 63.36 watts.
.
A squirrel cage motor on the other hand tries to maintain its power rating.
A drop in voltage results in an increase in current.
Jim

Edits: 08/29/12 08/29/12

Follow Ups:

Two things - Jon Risch 20:37:30 08/29/12 (6)
1st, the light bulb is a dynamic load, and with 96 volts present, it will not be at the same effective resistance, it will be at a higher resistance because it will not be running as hot at the lower voltage. Otherwise, your general idea is correct.

2nd, while he was referring to an older Denon amp, which undoubtedly has a conventional linear power supply with a honkin' transformer and big filter caps, many modern devices use switching power supplies, and when they attempt to maintain a given rail voltage, as the AC line voltage goes down, the current draw from the line goes up.
Jon Risch
- RE: Two things - rick_m 09:28:49 08/30/12 (1)
  Ummm, I thinks you got it backward, the filament has a positive TC and it's resistance increases with temperature.
  
  The power supply bit is right though...
  
  Rick
  - RE: Two things - Jon Risch 19:43:27 08/30/12 (0)
    Yes, I did get it backwards, the filament is a positive temperature coefficient device. The point being, it is not a linear resistor.
    Jon Risch
- I gave a simple example. Not technical...... - jea48 08:10:20 08/30/12 (3)
  1st, the light bulb is a dynamic load, and with 96 volts present, it will not be at the same effective resistance, it will be at a higher resistance because it will not be running as hot at the lower voltage. Otherwise, your general idea is correct.
  
  Splitting hairs....
  
  All one has to observe is the difference in brilliance, lumens, of the light bulb at 120V vs 96 volts. Less power is being consumed at 96V.
  Simple ohms law.
  
  If the 100 watt light bulb were connected to a variac, a volt meter connected across the load, and an AC ammeter connected in series with the load I doubt the test would revile a rise in current with a reduction of applied voltage. As the voltage was lowered the current would also be lower...... Power consumed lower.
  
  2nd, while he was referring to an older Denon amp, which undoubtedly has a conventional linear power supply with a honkin' transformer and big filter caps, many modern devices use switching power supplies, and when they attempt to maintain a given rail voltage, as the AC line voltage goes down, the current draw from the line goes up.
  
  That may be for a piece of equipment that uses a switchmode power supply vs a power transformer... You are the EE here.
  But does a drop of 24Vac cause a rise in current to maintain the rated power consumption? And Does equipment that use switchmode power supplies use voltage regulators? How do they come into play?
  
  As for the Denon amp, I believe, the amp will output less power at 96Vac than 120Vac into a 8 ohm nominal speaker load.
  
  AC power consumption of the Denon amp will be less at 96Vac than at 120Vac. I could be wrong.... I been wrong before.....
  Jim
  - RE: I gave a simple example. Not technical...... - Jon Risch 19:59:15 08/30/12 (2)
    RE the light bulb at 96 volts, as rick_m noted, I mis-stated the resistance swing, it should be lower not higher. Of course, the point was, that the light bulb as a load is non-linear.
    
    "But does a drop of 24Vac cause a rise in current to maintain the rated power consumption?"
    
    Not so much a matter of power consumption, but of supplying the devices load with enough power. With switching power supplies, if the incoming voltage goes down, the incoming current has to go up to supply the demand from the load.
    
    "And Does equipment that use switchmode power supplies use voltage regulators? How do they come into play?"
    
    The vast majority of them do, if for no other reason, that the switchmode supply has to self-regulate anyway, and the few added parts that it take's to make that self-regulation slaved to a reference voltage for the output rails to remain stable and regulated are so cheap, that it is common practice.
    
    "As for the Denon amp, I believe, the amp will output less power at 96Vac than 120Vac into a 8 ohm nominal speaker load."
    
    Yes, it probably will, for while many of the better brands of receivers used regulated supplies for the low voltage circuits, such as for the preamp or tuner, they would not have done so for the power amp rails, as that would have required heroic sized components due to the current involved, and the linear/analog nature of the power supply system.
    
    I am not picking nits with you, I just thought it would be informative to others regarding the nature of the loading and current draw. Many people do not realize that switching supplies are likely to draw MORE current, and not less, when the AC line goes low.
    
    Jon Risch
    - Jon I appreciate your reply. - jea48 21:24:29 08/30/12 (1)
      I am not picking nits with you, I just thought it would be informative to others regarding the nature of the loading and current draw. Many people do not realize that switching supplies are likely to draw MORE current, and not less, when the AC line goes low.
      
      I learned something new today.....
      
      I would think a low AC line voltage would harm a switching power supply. Higher current = more heat.
      
      Jim
      - RE: Jon I appreciate your reply. - Jon Risch 22:26:29 09/01/12 (0)
        
        In Reply to: RE: Jon I appreciate your reply. posted by jea48 on August 30, 2012 at 21:24:29
        
        Yes, at some point, they trip the breaker when the line goes low enough, or the circuit is set to shut down the switcher, as the AC line current can exceed what the input components were designed to handle. This would include the nearly mandatory AC line filter block, often one of the metal can all-in-one filter blocks with an IEC inlet, and possibly a breaker built-in.
        Jon Risch

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