a-bike said:
I have a problem of noise caused by the conmutation of a contactor that is
hunging the dsp in the control board of my system. Some one told me the
posibility of connecting a snubber or varistor in parallel with the
contactor coil in order to reduce that noise. Does anybody knows about the
method used to dimensioning the snubber?
A common AC (you didn't say whether the contactor coil is AC
or DC operated, so I guessed) coil snubber consists of a
resistor in series with a capacitor, connected across the
coil. When the driving contacts opens, the coil current
detours to the capacitor, which limits the rate of change of
voltage as it absorbs the energy that was stored in the
coil. The lower the value of the resistor, the better this
works. Except that when the driving contact closes at peak
voltage, there is a large inrush current as the cap is
popped up to that peak voltage, while the contact is
bouncing. Bad for both RFI and contact life. So a
compromise must be struck on the resistor value. Since the
coil draws essentially no current during the bounce time,
because of its inductance, I usually size the resistor to
limit the peak current to no more than the contact current
rating. The capacitor value and voltage rating must be
large enough to contain the peak stored coil energy without
exceeding its voltage rating. But way too much capacitance
causes the series resistor to get hot while the coil is
energized.
In some cases, I have used a smaller capacitor than could
safely store the peak coil energy, and limited its peak
voltage by paralleling it with an MOV.
So to design an effective snubber that doesn't get too hot,
or damage the contacts, you need to start with information
about the contact rating (that is driving the coil) the peak
driving voltage, the coil's peak current and its inductance
(to come up with the (I^2*L)/2 energy that gets transfered
to (V^2/C)/2 in the capacitor.
Or you but an off the shelf RC snubber rated for the coil
voltage and hope it works and survives.