michael nikolaou wrote:
Hi

I have a 12 v relay driving an large 220 volt AC relay . Across the contact
of the driver relay i placed one RC snubber circut (27NF with 100 R
resisitor in series) to help with some spikes that were influencing the low
voltage driver circuits.
The driver circuit is able to detect mains zero crossing and fire the
driver relay at an angle i choose .
From what i read the best point to switch off the power relay is at zero
crossing . I did that and i show a large spike up to 1 KV at the relay
contact followed by a decaying 500hz waveform to 0 volts . After some
experimentation the best point came exactly when switching off at the peak
of the mains voltage .At this point there is smooth decaying waveform to 0
volt after 5 periods of 500 HZ but no overshoot. The relay presents no
arcing. If i remove the snubber and make the experiment the best place to
switch is zero crossing but i also see large SHARP spikes up to 500 Volts
Peak.
My question is
The switching with snubber must be made at zero crossing or at the peak of
an ac voltage waveform ?
What is the behaviour of the circuit ?.
As i understand any large spikes can harm the X2 capacitor i'm using so
what is the best operating practise ?.

Any help will be appreciated

Michael


Hello,


Two comments:


If the load is not purely resistive, there will be a voltage current
shift, aka Eli ICE Man... therefore in this case zero voltage crossing
is not at all zero current crossing and you may be creating more problem
by switching at the worst (or just a bad) time) .

BY looking at the signal that the rc snubber is trying to "tame" , you
adjust RC unitl you get a "critically damped response. you can dampen
more, but at the cost of higher stand by leakage via the RC, as it
becomes a part of the load too!

Good luck, this is an often asked problem, and the ability to visualizes
on scope and make changes and observations, will help you forever. This
is a re-occcuring problem and it is the variations in the load that
cause engineers to have to re-visit the solutions

Best regards

Marco