Re: varistor calculations needed

Ross
Regarding the capacitors can i ask why a X2 capacitor is a bad choise and
one should use polypropylene type ?. The thing is as i see i must use a
varistor
so no overshoot should be observed to destroy the capacitor. I want to use
a 5mm
varistor though so i don't know if it is sufficient.
The thing that worried me though was a suggestion that i must not use a
varistor
since they seem to self destruct .
Do you have an opinion on that ?.
Now regarding the zener arrangement . As i know the spike is according
to -L(di/dt)
so the voltage produced is negative. I was thinking for the reverse
connection to the
zener as it is suggested so it is forward biased during the negative spike
and clip during turn on .
I read the article you suggested .It makes sense but i don't know if
applied to a 180 mw relay
there is actually such a critical behaviour that should make us consider a
speed up during turn -off.
Anyway as is see i must test it .
Ross your suggestions are invaluable . Thank's once again





"Ross Herbert" <rherber1@xxxxxxxxxxxxxx> wrote in message
news:i129r41sfkl4kopf4514priemvitko1pbv@xxxxxxxxxx
On Sun, 08 Mar 2009 19:34:16 +0200, michael nikolaou
<michaelnikolaou_removeme@xxxxxxxxx> wrote:

:Thanks Ross for the reply .
:The thing is in the end the cost and size of the components is more that
:the relay it's self . I've checked the FKP 1 capacitors and they seem
:monstrous for the design i had in mind . I know it's playing on the safe
:side but putting a product on the market without thorough examination
:leads only to more research .
:Can i ask why not only a Zener alone why should somebody use also a
:diode .If you forward bias a zener it will conduct as an ordinary diode
:.So why do we need a series diode also ? My supply is regulated at 12
:volts so a 12 volts zener should be ok as i think
:
:


I suppose the FKP1 could be too expensive and difficult to source so you
could
use a cheaper polypropylene self healing cap since the MOV across the
relay will
limit the voltage trying to maintain any arcing at the contacts.

With regard to your 12Vdc relay;
As you know a common method of relay coil suppression is a common silicon
diode
such as 1N4004. Used by itself, the release delay period of a relay is
often too
prolonged, which can lead to problems of extended arcing on any inductive
circuits controlled by that relay's contacts. Ideally, a relay should
release as
fast as possible within its mechanical constraints, but due to the
destructive
transients produced by the coil, some suppression is required. A simple
resistor, a combined capacitor + series resistor or a combined zener +
silicon
rectifier are options, each having its own advantages and disadvantages.
If cost
is a problem a simple resistor shunting the coil is often useful. A better
alternative is the C-R combination. LM Ericsson x-bar exchanges of the
60's era
used a 0.25uF + 600 ohm C-R unit across the coil of almost every general
purpose
relay. This produced the best compromise between relay release delay and
contact
protection for any contacts in the operate paths of the relays. As
semiconductor
relay control became common the simple resistor or C-R unit was not
appropriate
and subsequently the use of either MOV's or transient suppression diodes
(Transorb or other proprietary name, eg. 1.5KE series) became the norm.
Due to
the relatively high cost of Transorbs it was often much cheaper to use a
standard zener + diode to produce similar results.

This Tyco note gives a little information on the zener + diode approach.
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3264.pdf

A zener across the relay coil by itself must be connected with the cathode
towards the negative side of the coil and the anode to the positive side
if it
is to limit the back emf surge voltage produced by the coil during
release.
However, it is then obvious that, if used alone, the zener is now biased
in the
forward conducting state and will be permanently shunting the coil so that
the
relay will never operate. The silicon diode must be included and connected
so
that it is normally biased off and thus will not allow the zener to
conduct.

In your case a 12V zener is just too close to the supply voltage. Since
the
purpose of it is to reduce the peak transient induced by the relay to
below that
of the operating transistor voltage maximum, is all that is required.
Since most
relay operating transistors would be in the region of at least Vce/Vbe =
40V,
you can use a zener of say 20 - 25V without causing any problems for your
electronics.

:
:
:
:
:
:
:Ross Herbert wrote:
:> On Sun, 8 Mar 2009 10:15:31 +0200, "michael nikolaou"
:> <michaelnikolaou_remove_me_@xxxxxxxxx> wrote:
:>
:> :Lets make it clear what i want
:> :
:> :
:> :First i have one 12volt dc relay controlled by a microcontroller
:> :with contacts at 220Volt arming a large relay 220V 6W
:> :I also have another dc relay from the microcntroller arming an ac
motor at
:> :25W.
:> :I used R-C snubber at the contacts of the small relay with a 275V AC
X2
:> :capacitors, 100 R resistor
:> :Problem was the capacitor was failing by open circuit after some
thousands
:> :of operations .Measured the spike and it was over 1KV .The motor was
:> :producing nasty spikes during the turn off. I decided to use a
varistor in
:> :shunt with RC but i need minimum space so i need
:> :the calculations for minimum requirements .
:> :Test i made improved both symptoms so to me it is an accepted solution
.
:> :Varistor clamps
:> :and capacitor smothens dv/dt
:> :Symptoms from the innapropriate snubber were microntroller reset due
to
:> :watchdog.
:> :and limited relay life due to arcing.
:> :As i understand i must not use a varistor since it's not for
repetitive
:> :usage and use avalance diodes?????
:> :
:> :My second question is related to the 12 dc relay .Instead of a 1n4001
why
:> :not use a zener in the 12V dc relay coil ?.I have ,as i understand,
switch
:> :off protection since the zener is forward biased and also if there is
a
:> :spike (not related to the relay switch ) in my supply clamp over the
12VDC
:> :.Is my assumption wrong ???
:> :In that case a 200 mw relay inductor can be protected by a small 500mW
zener
:> :in shunt???
:> :
:>
:> Start by reading as much literature as you can.
:>
:>
http://www.littelfuse.com/design/application-literature/application-notes.html
:> and
:> http://relays.tycoelectronics.com/appnotes/
:>
:> I see you have also included a CR snubber across the series contact but
the
X2
:> rated cap is failing due to high peak voltage across it. The source of
the
high
:> voltage, I assume, is the 220Vac relay so you need to suppress this
directly
at
:> the relay coil. I would suggest a Littlefuse V250LA40C varistor will do
the
:> trick.
:>
:> Provided you select a suitable varistor for the relay there is little
chance
of
:> failure over an extended period of operation.
:>
:> http://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_MOV_CIII.pdf
:>
:> Using an X2 cap for the CR snubber is not a good idea. I would suggest
a self
:> healing polypropylene type. The WIMA FKP1 looks to be an ideal type.
:> http://www.wima.com/EN/fkp1.htm
:>
:> For your 12Vdc relay coil you can use a 24V zener in series with a 1A
silicon
:> diode - NOT a 1N4001 btw. I would use a 1N4002 or preferably a 4004 -
there
is
:> no point in trying to specify as low a voltage as you can get away
with.


.

Quantcast