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Re: Internal resistance



Ludwik Kowalski wrote:

....
> Vo=6.0 V1=4.2 I=1.33 A
....

OK, the data is absolutely clear now.

The only interpretation I can think of is that there is
something desperately wrong with this power supply. The
fact that the behavior is non-ohmic is the least of our
worries. Instead I am siezed by the fact that the apparent
impedance is at least two orders of magnitude larger than
it has any right to be.
-- Perhaps the sense leads are not hooked up properly;
check for loose or missing or mis-configured jumpers
on the back (or front?) of the instrument.
-- Perhaps the unregulated part of the circuit (upstream
of the regulator) has failed, possibly due to a
fried diode, so that the regulatory feedback loop
has almost nothing to work with, and you are seeing
regulated behavior at zero current but unregulated
behavior at the other reported current levels.

For a properly-working regulated supply, at the special
point where the sense lead meets the power lead, the DC
impedance should be unmeasurably small. The impedance
you observe should be microohms or milliohms depending
on how close you can attach to the special point. (If
you manage to attach slightly upstream of the special
point you will observe a negative impedance ... ain't
feedback wonderful!)



To late to think (it is 10:30 p.m. I stayed in the
lab to collect data). But here are row data that
I just collected. All at the same room temp.

Circuit consists of a regulated power supply
on whose terminals there was a volt-meter in
parallel with a branch containing a resistor
R, an amp-meter and a switch.

Notation:
Vo volts measured when I is zero (switch off)
V1 when switch is closed (I as indicated below)
I current measured current (switch on)

1) Results for R1 (nominal 2.8 ohms, 12.3A)
Vo=6.0 V1=4.2 I=1.33 A
Vo=10.0 V1=2.2 I=2.39 A
Vo=16.0 V1=11.7 I=4.00 A
Vo=20.0 V1=14.7 I=5.06 A

2) Results for R2 (nominal 21.5 ohms, 4.4A)
Vo=6.0 V1=5.5 I=0.24 A
Vo=10.0 V1=9.3 I=0.42 A
Vo=16.0 V1=15.0 I=0.67 A
Vo=24.0 V1=22.7 I=1.03 A
Vo=30.0 V1=28.3 I=1.28 A