What chris writes really makes sense. I am adding my extra's here too.
On Tue, 26 Feb 2008 04:43:44 -0800 (PST), Chris
wrote:
On Feb 26, 3:00*am, "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
Hi, Michael. First off, you should spec a 12V relay that's made to
handle inductive loads (you can see a HP rating). This type of relay
has contacts which open more quickly, and are farther apart when
open. A small standard relay might not even open up far enough to
stop an inductive arc at line voltage.
Very true!
Next, when using a relay to drive a relay, you have to be aware of the
delay-on-make, which can be several milliseconds, especially for
larger relays. That might help explain some of the curious results
you're getting. Turn-on delay can be affected by wear and aging. It
also varies from unit to unit, even in relays with the same part and
lot number. Trying to get this kind of timing accuracy might be the
wrong way to go.
There also is a relay-off delay. And the RC snubber makes the turn-off
delay even worse as the current will run a little longer. Expect
something here between 5 to 20 milliseconds.
It might be better to take a look at suppressing the arc, which you've
already started to do. Here's an intuitive way to start. First off,
remember your basic goal: you want the voltage across the contacts to
rise just slowly enough so the contacts can pull away without
sustaining an arc. That's all.
What you need to know is the current running through the contacts, and
the voltage spike you want to allow. What a snubber does is store the
energy of the coil in the capacitor. The resistors "eats" this up as
the capacitor discharges over the load. The voltagespike at turnoff
(aasuming an empty capacitor at that moment) is the load current *
resistor. So if you have R= 100, C= 47n, I=2A, you get a voltage spike
of 100*2 = 200 volt even before your capacitor charges. The load
inductance with the current give the stored energy: Q=L*I=C*U. So the
capacitor size should match the load inductance, otherwise you get a
high voltage there too. The capacitor will for example be loaded to
100 volts with a Q = 27n*100V = 2,7 uCoulomb. An inductance that
contains this would be L=Q/I= 2,7uC/2A = 1,35 uH. But the resistor
already "eats" up a lot, so the voltage will be lower. When you turn
off larger motors, transformers, inductors (the coil of a large
relay!), the capacitor must match the load to keep voltages limited.
The prevent oscillations, resistance values must not be too low,
ususally between 30 to 100 Ohm is ok. For large inductive loads I
would not take 100 Ohm but 47 Ohm as normal value (see below what
Chris wrote), you contact must ve able to handle that. The capacitance
could be anything you want, for larger devices 220n, 470n etc.
Remove the cover from the driving relay so you can see the contacts.
Next, find the current rating of those contacts, and use Ohm's law to
find a resistor which will result in about half that current.
For example, if you have a 220VAC load, and your relay can handle
10A,:
R = 220V / 5A = 44 ohms
Choose a 47 ohm, 1 watt resistor here (carbon comp is best). Now get
a selection of self-healing AC line-rated capacitors, and switch the
inductive load with the 47 ohm and C snubber directly across the load,
repeating and increasing the cap value until the contact arcing
disappears, or at least is minimized. Without knowing anything about
your 220VAC relay, it sounds like your 0.027uF cap might be on the
small side.
Also very true: the RC snubber also gives a current peak when you turn
ON the relay. Nice thing about inductive loads is that the current
does not rise fast, so the load and RC current do not occure exactly
at the same time (also depending on RC, where a smaller R gives a
higher rush-in current but also a better timing).
The peak where the snubber works is at turning the relay OFF again.
What I often use is 47 Ohm to 100 Ohm with 100 nF.
Note that ITW Paktron makes a series of Quencharc snubbers that you
can just plug in, which makes selection a snap. They're one-piece,
epoxy-encapsulated units, and are very easy to use.
http://www.paktron.com/pdf/Quencharc_QRL.pdf
If all else fails, remember that physical distance is also helpful.
Do what you can to place the arcing contact as far away as possible
from sensitive circuitry.
Good luck
Chris
You need TWO RC networks: one across the driving relay, one across the
large relay, and do not forget the diode (or also RC) across the coil
of the driving relay. So you have 3 things that need attention.
Do not forget that long cables (also with resistive loads like lamps)
act as inductance. 10 meters or more may also need a RC!
Regards,
Pieter