Application notes
N°007
SUPPRESSOR DEVICES FOR RELAY COILS
The inductive nature of relay coils allows them to create magnetic forces which are converted to mechanical movements to
operate contact systems. When voltage is applied to a coil, the resulting current generates a magnetic flux, creating
mechanical work. Upon deenergizing the coil, the collapasing magnetic field induces a reverse voltage (also known as back
EMF) which tends to maintain current flow in the coil. The induced voltage level mainly depends on the duration of the
deenergization. The faster the switch-off, the higher the induced voltage.
All coil suppression networks are based on a reduction of speed of current decay. This reduction may also slow down the
opening of contacts, adversly effecting contact life and reliability. Therefore, it is very important to have a clear understanding
of these phenomena when designing a coil suppression circuitry.
Typical coil characteristics
On the graph below, the upper record shows the contacts state. (High level NO contacts closed, low level NC contacts closed,
intermediate state contact transfer). The lower record shows the voltage across the coil when the current is switched off by
another relay contact.
The surge voltage is limited to -300V by the arc generated across contact poles. Discharge duration is about 200
mircoseconds after which the current change does not generate sufficient voltage. The voltage decreases to the point where
the contacts start to move, at this time, the voltage increases due to the energy contained in the NO contact springs. The
voltage decreases again during transfer, and increases once more when the magnetic circuit is closed on permanent magnet.
Operating times are as follows:
Time to start the movement 1.5ms
Total motion time 2.3ms
Transfer time 1.4ms
Contact State
Date of issue: 6/00
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