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A friend wants to party with dance music this weekend at his cabin in the woods and wants to borrow an amp. However, he has been measuring line in voltages (on a kilawatt device) down to 96 volts. what is the safe minimum voltage to require before risking one of my amps (I am thinking of my early 90's Denon)in this adventure?
"When the demon is at your door, in the mornin' it won't be there no more"
Steely Dan
Follow Ups:
96VAC on a (unloaded) residential branch circuit is below spec. Your friend has either some crap wiring, an off-spec service drop, or both.
Either the back of the amp by the power source or certainly the owners manual will have listed the voltage 'spread' the device can operate at.
All this is governed by ANSI C84.1.
See linky below for my utility's explanation.
required to provide AC voltage +/- 10% of the nominal 120 VAC at the meter of the home. If not, they may be liable for any damages. When my voltage (commercial) was measured at 105VAC, the local utility came within 45 minutes to measure and although the voltage had risen to about 109VAC, they came back with a crew later on that night to up the voltage (switch transformer taps). They did NOT want to be liable for any damages.
Stu
Edits: 08/29/12 08/29/12
Hi stu,Just guessing the meter for the Op's friends cabin is more than likely mounted on a power pole several hundred feet from the cabin in the woods.
Again just guessing the power line from the main disconnect, below the meter setting, was not sized properly for VD, voltage drop.
NEC only recommends VD be considered when calculating the size of a conductor.
Again just guessing about the power line feeding the cabin.
Best thing the Op could do is measure the voltage at the cabin with a volt meter with everything powered up in the cabin.
Unloaded then loaded....
Jim
Edits: 08/29/12 08/29/12
The ANSI spec they follow is (now) +/- 5% of 120VAC baseline.
There is a 2011 Rev to that spec, but since nothing is free anymore, ANSI wants $66 for it. 'Prolly has an expanded computer power section, or maybe the baseline has been bumped to 125V. I don't know.
Could it be the accuracy of the measuring device?
My Panamax will shut the outlets OFF if line voltage drops below ..... 90vac......
don't forget that as voltage drops, current increases....for any given amount of power.
Too much is never enough
don't forget that as voltage drops, current increases
AC squirrel cage motors yes..... Not a power transformer found in audio equipment..
VD = lower power, watts, Va.
Ohms Law
Amps = voltage / resistance
Yes, in my post I specified 'for any given power'.
Now, I guess I have a question. 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?
I'm curious now....
You mention VD and Va......do you mean change in voltage=VD and some reference to power factor? VA? As near as I can figure, PF is always <1. Does that change when line voltage changes? It would seem that the reactance of the PS would pretty much remain the same? No?
I have a Kill-a-watt meter. I'll record line voltage vs the current draw of my computer for a few days. It is almost a resistive load, PF=.98
Too much is never enough
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 bulbP = E x I
100 watts / 120 volts = .83 amp.R = E / I
120V / .83 = 144.58 ohmsI = 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
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
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
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
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 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
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
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
Another ohms law
Power = voltage x current
This is where he is getting if voltage goes down current goes up.
But this is only true for constant power
Alan
However, he has been measuring line in voltages (on a kilawatt device) down to 96 volts.
From what connected loads?
VD, voltage drop, is directly proportional to the load, amps.....
Depending how big, watts, your amp is and how loud the amplifier will be driven the VD could go below 90 volts. With dynamic VD sags in the middle 80 volt range.
Hope you are not supplying the speakers......
......but the dance music party in the woods part would give me pause about sending my gear on that vacation, LOL!
Well it really depends on the device but 96 is pretty low for just about anything. I would suggest either a switched tap or better a ferro-resonant line conditioner.
The ones I have come across makes a BIG noise!
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