The measurement of some characteristics of passive components, can also become instrumental using less specific than a Q-meter or a bridge RF.Un pulse generator and an oscilloscope, normal job in a laboratory, can be used in specific cases and parameters. Different measuring this practice provides a method of measuring the equivalent series resistance (ESR) of a capacitor capacitor. Un can be represented by the following equivalent circuit:
Where: Rs: sum of the losses in the terminals, joints and plates of the capacitor. Ls: inductance of terminals and plates. Rir: dielectric losses. Rdie: losses due to molecular polarization of the dielectric. Each of these matters or not, according to the type of application of the capacitor is the range of frequencies and it is subject.
A parameter of particular importance in capacitor under high ripple current, is called Equivalent Series Resistance (ESR), respnsable of the temperature rise of the component. The ERS is defined as:
- DFDIE: dielectric dissipation factor
- Capacitor manufacturers usually give the value of ERS at 100 Hz and 100 kHz. At high frequencies the prevailing term is Rs.
Experimental Circuit
The experimental circuit used for the determination of Rs is as follows:It must meet the rise time of the pulse delivered by the generator should be much smaller than the constant. Load capacitor. Under these conditions, during the pulse rise time, the capacitor fails to take significant load, which only become apparent resistance and inductance Ls Rs. In the oscilloscope, with the fastest time basis, shall be a signal.
The damped oscillation is observed, is due to resonance with the parasitic capacitance Ls presented to the input of measurement circuit. After the ringing, the voltage stabilizes at a value Voffset is given by the ratio of resistors R1 and Rs such that: By the Laplace transform can be deduced that: where a is the time it takes the input signal to reach VI.
In practice, the determination of the value of Ls is difficult due to the parasitic capacitance introduced by the probe, the oscilloscope and due to wiring, so we simply measure the value of Rs.