The bifilar coil

The principle is to wind on the magnetic circuit of the main coil a second coil shorted on itself. By a proper adaptation of the

internal resistance of this second coil it is possible to find an acceptable equilibrium between surge voltage and reduction of

the opening speed. To be efficient at fast voltage changes, the coupling of two coils must be perfect. This implies embedded

windings. The volume occupied by the second coil reduces the efficiency of the main coil and results in higher coil power

consumption. This method cannot be applied efficiently to products not specifically designed for this purpose.

The resistor (parallel with the coil)

For efficient action, the resistor must be of the same order of magnitude as the coil resistance. A resistor 1.5 times the coil

resistance will limit the surge to 1.5 times the supply voltage. Release time and opening speed are moderately affected. The

major problem is the extra power dissipated.

Semi-conductor devices

The diode

It is the most simple method to totally suppress the surge voltage. It has the major disadvantage of the higher reduction of

contact opening speed. This is due to the total recycling, through the diode, of the energy contained in the coil itself. The

following measurement is performed once again on the same relay. Operation times are given by the upper curve:

- time to start the movement 14ms

- transfer time 5ms

These times are multiplied by a coefficient from 4 to 8.

The lower curve shows the coil current. The increase prior to NO contact opening indicates that the contact spring dissipates

its energy. At the opening time the current becomes constant as a result of practically zero opening speed.

Due to this kind of behavior, this type of suppression must be avoided for power relays. For small relays which have to switch

low currents of less than 0.2 A, degradation of life is not that significant and the method may be acceptable.

Date of issue: 6/00

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