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Types of suppressors: Passive devices. The resistor capacitor circuit It eliminates the power dissipation problem, as well as fast voltage rises. With a proper match between coil and resistor, approximate capacitance value can be calculated from: C = 0.02xT/R, where T = operating time in milliseconds R = coil resistance in kiloOhms C = capacitance in microFarads The series resistor must be between 0.5 and 1 times the coil resistance. Special consideration must be taken for the capacitor inrush current in the case of a low resistance coil. The record shown opposite is performed on the same relay as above. The operation time becomes: - time to start the movement 2.3ms - transfer time 1.2ms The major difficulty comes from the capacitor volume. In our example of a relay with a 290 W coil and time delay of 8 ms, a capacitance value of C=0.5 uF is found. This non polarized capacitor, with a voltage of 63V minimum, has a volume of about 1cm3. For 150V, this volume becomes 1.5 cm3. 
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