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Internal resistance



The following results were obtained in a
lecture-demonstration today. They show
that the internal r of a power supply is
not a constant. It was found to decrease
monotonically from 1.4 ohms, at I=0.85 A,
to 1.0 ohms at 3.8A. Any comments?

938 AC/DC power supply from LabVolt

We used the DC output (designed for
0-36 V and up to 5A). First we measured
V1, the DOP between the terminal when
there is no current. Then our resistor of
2.8 ohms (Imax=12 A) was connected to
the DC terminals. Both the current I and
the DOP across the 2.8 ohm resistor, V2,
were measured at room temperature. The
internal r was calculated as (V1-V2)/I.
Ludwik Kowalski