Load Wire Inductance - Prodigit 3270 Series Operation Manual

20 PRODIGIT
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Load wire inductance

The load wiring has an inductance (L). When the current (I) varies in short time period, It
generates a large voltage at both ends of the wiring cable. This voltage applies to all of
the load input terminals of the 3270 series when the impedance of the EUT is relatively
small. The voltage generated by the load wire inductance (L) and the current variation (I)
is expressed using the following equation.
In general, the wire inductance can be measured approximately 1 μH per 1 meter. If the 10
meters of Load wires is connected between the EUT and the electronic load (3270 Series)
with the current Variation of 2 A/μs, the voltage generated by the wire inductance Will be 20 V.
The negative polarity of the load input terminal is the reference potential of the external
Control signal, Therefore, the device connected to the external control terminal may get
malfunctioned.
When operating under the constant voltage (CV) mode or constant resistance (CR) mode or
constant power (CP), the load current is varied by the voltage at the load input terminal, so the
operation can be affected easily by the generated voltage.
The wiring to the EUT should be twisted and the shortest as possible.
If the load wire is long or has a large loop, the wire inductance is increased. Consequently, the
Current variation that results when switching occurs will cause a large voltage drop.
When the value of instantaneous voltage drops under the minimum operating voltage depends
on the generated voltage at the load input terminal, the response of recovery will be
extensively delayed.
In such event, the electronic load (3270) may generate unstable oscillation.
In such condition, the input voltage may exceed the maximum input voltage and Cause
damage to the 3270 Series.
E = L x (ΔI /ΔT)
E: Voltage generated by the wire inductance
L: Load wire inductance
ΔI: Amount of Current variation
ΔT: Variation period of current