Passive inductors are often used in test equipment for Design Verification (DV) and Functional Testing (FT) of automotive and other vehicle loads. Test equipment manufacturers should be aware of the characteristics that determine the physical sizes of these components.
Inductors are often used with series resistors to either simulate the shape of the initial current transient by the L/R ratio or to simulate the energy storage and resulting transient voltage generated by the L*di/dt at circuit turn-off.
The size of the inductor is determined by the current flowing thru the inductor, the duty cycle of that current flow, the inductance required and the DC resistance of the inductor.
To maintain a stable current throughout the duration of the testing process, it is necessary to keep the DC resistance of the inductor relatively low. The inductor is most likely wound with copper wire. If the inductor wire size is not properly picked, it will heat up during the testing process, resulting in lower test currents in the devices under test (DUT) after a period of testing. This then results in lower test currents, lower stored energy and lower transient voltages being applied to the DUT.
Actual loads on a vehicle are allowed to heat up and may have lower operating currents over time. But when providing short-term tests, it is desirable to keep the test uniform for all devices being tested.
Designing for relatively low DC resistance in the simulation inductor may make the inductor larger than the actual tested component. Further, a passive resistor is also used to maintain a stable total-circuit resistance. The resistance of the this resistor does not change much during long testing periods due to its low coefficient of resistivity. In selecting this resistor, the engineer must be aware of the heat produced and its effect on the test circuit layout.
Quite often design engineers, in trying to minimize the physical size of the overall test circuit instrumentation, neglect the importance of properly sizing the passive inductors and resistors.