Ohm's Law, Impedance & Phase oh My!

[admin]

Are you flapped up with all the tech jargon surrounding Electricity like Ohms Law, Impedance, Phase, etc? It's understandable as these are very tech oriented topics. WE are here to clear the air and help identify them. This article will help you.

Discuss "Understanding Ohm's Law, Impedance And Electrical Phase 101" here. Read the article.

 

[JerryLove]

Why do we rate cables on DC impedance when amps are AC power sources?

 

[gene]

Why do we rate cables on DC impedance when amps are AC power sources?

B/C the dominant metric in speaker cables is resistance. All of the cables we measure we do at DC and AC (Inductance and Capacitance vs frequency) including AC resistance.

 

[PENG]

Why do we rate cables on DC impedance when amps are AC power sources?

Obviously the inductive reactance (XL= 2Πf L (ohms) of a cable is more significant at higher frequencies but for smaller cables such as 10 AWG and under, the resistance (DC) is much more significant relative to the inductive reactance (AC), at least for the lower frequencies. Also, in audio, the signal component at the higher frequency components are typically of much lower magnitudes. These are probably the two main reasons why for practical purposes, we generally only look at the DC resistance and ignore the inductive and capacitive reactances.

 

[PENG]

B/C the dominant metric in speaker cables is resistance. All of the cables we measure we do at DC and AC (Inductance and Capacitance vs frequency) including AC resistance.

We must be posting at around the same time but yours slightly ahead, making mine redundant.

 

[GO-NAD!]

I was doing OK, until you started talking about phase angles...

 

[Steve81]

I was doing OK, until you started talking about phase angles...

I can't imagine why. It's such a simple and straightforward topic.

 

[3db]

A beginner explanation of phase angles. Phase angles aren't tough once you know the ground rules. Phase angles only occur with AC, not DC. Phase angle is a measure between voltage and current conduction time. To help you visualize this, imagine first a pure capacitor with infinite AC resistance. If a voltage is applied to a capacitor, the voltage begins to build across its terminal but there is no current flow for the first 90 degrees of a sine wave. Voltage leads the current by 90 degrees. Once past 90 degrees, the volatge lessens and the cap begins to discharge, hence the current. With pure inductors with no winding resistance , its just the reverse. For the first 90 degrees, the current rushes through the coil but there is no voltage build up across its terminals because of the zereo resistance but a magnetic field is created. The voltage lags the current by 90 degrees. Once past 90 degrees, the current begins to drop causing the field to collapse causing a voltage to appear. All capacitors and inductors have some rsistance and this resistance causes the phase angles to vary from 90 degrees. I hope this helps.

 

[GO-NAD!]

A beginner explanation of phase angles. Phase angles aren't tough once you know the ground rules. Phase angles only occur with AC, not DC. Phase angle is a measure between voltage and current conduction time. To help you visualize this, imagine first a pure capacitor with infinite AC resistance. If a voltage is applied to a capacitor, the voltage begins to build across its terminal but there is no current flow for the first 90 degrees of a sine wave. Voltage leads the current by 90 degrees. Once past 90 degrees, the volatge lessens and the cap begins to discharge, hence the current. With pure inductors with no winding resistance , its just the reverse. For the first 90 degrees, the current rushes through the coil but there is no voltage build up across its terminals because of the zereo resistance but a magnetic field is created. The voltage lags the current by 90 degrees. Once past 90 degrees, the current begins to drop causing the field to collapse causing a voltage to appear. All capacitors and inductors have some rsistance and this resistance causes the phase angles to vary from 90 degrees. I hope this helps.

Pictures would help...and sock puppets...and interpretive dance...

 

[Steve81]

Pictures would help...and sock puppets...and interpretive dance...

There was a chart that was captioned with a link to sound.westhost.com; that article has one picture that illustrates voltage and and current out of phase by 38 degrees and a frequency of 45Hz.

 

[3db]

I knew I would get the order wrong. In a capacitive circuit, current leads voltage and an in inductive circuits, current lags voltage. Its been 30+ years since I played with this. As current begins to flow across the capacitor, a charge is being created, hence the voltage build up across the capacitor ....still 90 degrees difference in a pure capacitive circuit. With a pure inductive circuit, the voltage applied across the inductor causes a field to build across the coil which in turns induces current flow. My apologies for having that mixed up earlier.

Here's a pic that illustrates pure resistance, inductance, and capacitance circuits.

 

[shadyJ]

Thanks for this article, I started a thread about this very subject about a month ago, and this article has give it more clarity for me. I still have a lot more to learn yet, so I hope these sort of articles continue. Again, a big thanks!

 

[shadyJ]

What I am taking from the article is that phase angles are OK as long as they don't hover around 45 degrees? Is it as simple as that? So 60 degrees is fine and 30 is fine, but 45 is not fine, at least if you don't have a amp with good cooling? What would the most perfect, most ideal phase angle profile a speaker could have?

 

[Steve81]

What I am taking from the article is that phase angles are OK as long as they don't hover around 45 degrees? Is it as simple as that? So 60 degrees is fine and 30 is fine, but 45 is not fine, at least if you don't have a amp with good cooling? What would the most perfect, most ideal phase angle profile a speaker could have?

If you look at the table included in that section, specifically under the Power (Amp) column, you'll note that as the phase angle goes up, power dissipated in the amplifier goes up until you reach 45 degrees. At that point it falls off until you reach 90 degrees, which no real world loudspeaker will present. Nonetheless, at 30 and 60 degrees, while the power dissipated in the amplifier isn't as high as 45 degrees, it's still significantly higher than at 0 degrees, which is a purely resistive load.

 

[killdozzer]

"wreck havoc"

Sorry, I had to!
Hey, you calculated the impedance wrong and now it completely wrecked my havoc!