Zen-light article -- error in Fig 3 or have I lost it?

[rif]

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 non-zero!

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.

[Bill Fitzpatrick]

So where's the thing to look at?

[rif]

Details, details

http://www.passdiy.com/projects/zenlite1.htm

[Nelson Pass]

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.

[rif]

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.

[wuffwaff]

R is always V/I wich is almost the same as dV/dI if you keep the d small enough.

William

[rif]

Originally posted by wuffwaff
R is always V/I wich is almost the same as dV/dI if you keep the d small enough.

William

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?

[Nelson Pass]

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+.

[rif]

Originally posted by Nelson Pass
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+.

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)

[Nelson Pass]

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.