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6th May 2004, 08:54 PM  #1 
diyAudio Member

Zenlight article  error in Fig 3 or have I lost it?
Since I (and others) are using Mr. Pass' articles as an excellent learning tool, I need to know if I have this correct.
Fig 3 shows the I vs. V curve for 2 lightbulbs. They are straight lines as expected, since they are linear Ohmic devices. I'm confident I've got this part correct. 1) But shouldn't they also cross the origin if extended? From what I see, it looks like at 0 Volts, current is nonzero! 2) The article says resistance is calc'd by dividing V by I at any point. This won't work here  I believe it should be the inverse of the derivative of the curve since the line is shifted. Change in V divided by change in I to get resistance. So the top curve is 50 ohm, the bottom 100 ohm. 
6th May 2004, 09:09 PM  #2 
diyAudio Member
Join Date: Jun 2001
Location: Eugene, OR

So where's the thing to look at?

6th May 2004, 09:19 PM  #3 
diyAudio Member


7th May 2004, 12:34 AM  #4 
The one and only

These are the measured curves, and they are only extended
down to 40 volts. The minimum voltage shown on the scale is 35 volts. Filament heating increases the resistance of the light bulbs, but the nonlinearity that results is subsonic, and the residual audio distortion is taken out by feedback. 
7th May 2004, 12:52 AM  #5 
diyAudio Member

Is the resistance in this linear region equal to V/I or dV/dI? I'm guessing dV/dI  change in V divided by change in I.

7th May 2004, 10:22 AM  #6 
diyAudio Member
Join Date: Aug 2001
Location: Ingolstadt Germany

R is always V/I wich is almost the same as dV/dI if you keep the d small enough.
William
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7th May 2004, 05:38 PM  #7  
diyAudio Member

Quote:
The d's are for derivatives, but you probably knew that already . The values will be very different for most situations. (if y=ax+b, y/x =dy/dx only if b is zero). Ohm's Law is usually confused with V=IR if I remember correctly  I'll look for my old univ. notes over the weekend. I guess I just need to know if R is defined as V/I or dV/dI. Or look at it the other way: is transconductance defined as I/V or dI/dV? 

7th May 2004, 07:12 PM  #8 
The one and only

At audio frequencies with a healthy DC voltage you can pretty
much treat it as a resistor, and you see that the 150 watt bulb is about 80 ohms and the 300 watt bulb is about 40+. 
7th May 2004, 07:52 PM  #9  
diyAudio Member

Quote:
Thank you again! But which is the proper way to get R? V/I or dV/dI? They give very different answers (50&100 vs. approx 40&80depending on the voltage) 

8th May 2004, 02:22 AM  #10 
The one and only

Since you can't change individual light bulbs, you either put
up with what you've got or you parallel different types to get what you want. 
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