varistor calculations needed

[ehsjr]

michael nikolaou wrote:

Guys

I will be more specific about my points . The relay i will use is
5V 120mw . Contact rating is 5Amps at 220V . Its clever as i understand
to use NPN,PNP to drive the relay but i imagine a clever diode
arrangement can drive this relay from an MCU pin.As i understand what
Ross is trying to say is the differential modes of noise (turn off relay
transient ) is dealt with diode but common mode (armature leakage ) must
be dealt differently by isolating low voltage circuit from hi side. Now
the thing is, after checking tyco notes, do all these things apply to a
120 mw relay about turn off time expansion etc., or i'm after ghosts
?.My feeling is that a simple diode 1n4148 should be sufficient. So that
will be checked.

The relay i'm driving is drawing 30ma at 220V .But creates fast rising
spikes without any protection. Actually it is a common din-rail relay
found almost everywhere.I imagine a 600 W avalanche diode should cope
with that and the thing about these components is small size and smd
case compared to varistors.

The motor i'm driving is rated at 25W @ 220Volts .
I will measure the inductance as it is described in TYCO notes .
From that point how do you calculate if 600 W is enough or 1500 W is
the minimum transorber or not ??
The motor operates every minute one time and the relay 50 times per day.
Any help would be appreciated .






Ross Herbert wrote:

On Tue, 10 Mar 2009 02:46:18 -0700 (PDT), westom <westom1@gmail.com
wrote:

:On Mar 9, 10:45 am, "michael nikolaou"
:<michaelnikolaou_remove_...@yahoo.com> wrote:
:> The motor i'm driving is operated every minute for 5 seconds so i get
:> 24*60=1440 transients per day.
:> I will actually speed thing up for testing and see what will happen
after 5
:> years of operation.
:> I imagine it will be enough .So your suggestions will have iummediate
:> actions.
:> Now regarding the 12VDC and isolation pls check my post to Ross .
I know
:> it's ancient story but the fact is
:> that the new relays are extremely low power so somebody could check
the
:> minimum solution .
:> Can somebody use a TVS unidirectional zener in shunt with the 12V
relay
:> inductor to protect spikes originating from the wiper as you
mentioned
:> since the zener-diode should create a similar behaviour ?
:> The 1.5PKE seems fine though and you have my thanks for that.
:
: Testing without first doing calculations is a bad design practice. 5
:year testing is only to confirm those calculations; not to see if a
:circuit works.
:
: Varistors are not intended for repeated spikes. Manufacturer
:datasheets demonstrate the concept. With use, they degrade.
:Therefore varistor type devices once used for protection are now
:routinely replaced with semiconductor type devices. Avalanche diodes
ie Transorb, Transil, etc) do not degrade. Either it works the first
:day or it does not work at all.

What do you mean by "repeated"? Is it once every second or once per
minute or
whatever? Provided that the varistor is selected to have the right
voltage and
current surge rating for the task it will not fail as you suggest.

:
: Varistors must never fail as implied by "lawsuits", etc. A failed
degraded) varistor must have no visual indications of that failure.
atasheets make that obvious. Varistors are intended for limited
:events. With each use, varistors degrade - as datasheets define with
:numbers. Any varistor that fails by creating a visual indication
:leaves the circuit designer liable for a lawsuit. A completely
:degraded varistor has no indication of its failed - a normal event
:defined by the size and number of 'spikes'. And varistors are
:typically used where rare, not normal, and excessive currents might
:exist.

What you failed to state is that varistors degrade when they
repeatedly have to
absorb surges "beyond" their specified ratings of voltage and current.
Provided
that the transients absorb do not produce excessive surge currents
they will
continue to limit transients for many years. It all depends on the
correct
device selection. Transorbs which are not appropriately selected for
the task
will fail just as easily as varistors. It is just that when they do
fail it is
usually on the first occurrence and the destructive effect is more
violent. I
have seen a transorb blown to pieces just as I have seen a varistor when
subjected to ecessive voltage and current. I have also seen varistors
continue
operating in 240Vac operated environments for 30 years without failing.


:
: Current pulses induced by the wiper (armature or whatever) typically
:would be completely ignored by a transorb across the coil. Appreciate
:the two different types of current. A transorb across the coil may
:see no current (or voltage) as that current passes out the on one or
:both wires.


This statement makes absolutely no sense at all. What are the "two
different
types of current" you refer to? The fact that you use the term "wiper" in
relation to electromagnetic relays indicates you have little knowledge or
experience with them. A transorb or varistor across a relay coil will
"always"
see the voltage produced by the back emf as the relay is de-energised
- that is
the whole purpose of using such devices. The degree of current
produced by the
back emf is dependent upon the magnitude of the voltage transient
produced by
the back emf and the coil resistance through which this current must
pass.

:
: This is another reason for buffering the coil with a transistor.
:Transient current conducts through transistor to ground AND the large
:CB or drain to gate voltage in series with a resistor buffers the
:microprocessor.

In the case of the OP's 12V dc relay we know that this is controlled
by a solid
state device. As such, it must include some appropriately rated relay
driver
transistor whether this is discrete or in an IC package. Your talk of
producing
ground currents as a result of surges produced by relay coils is
meaningless.
Provided that a separate ground scheme is maintained purely for the relay
circuits there will be no deleterious effect on the sensitive digital
control
circuits. It is all about appropriate printed circuit design and layout
technique. Transorbs are more appropriate to low voltage dc operation
where
solid state control is used.

:
: Most modems do this slightly different. Rather than grounding the
:coil with an NPN transistor, the coil is powered from the V+ side with
:a PNP transistor. Even better. Therefore both types of transients
discharging coil and transient from relay contacts) are kept from the
:uprocessor and always have a direct connection to ground.

??? what the heck are you on about with this statement? It has absolutely
nothing to do with the OP's question.

:
: 12 volt zener on the coil does nothing useful. That diode must be
:nonconductive to powering current from the microprocessor. And it
:must be 0.7 volts conductive to discharge the coil. Using a 12 volt
:zener does nothing useful and adds costs. Better is to enhance
:buffering on the base or gate connection to the uprocessor.

What utter crap. For starters, if the relay supply is 12V regulated,
it makes no
sense to use a 12V zener across the relay coil. You have also fallen
into the
trap of thinking that the zener must not be conductive to the normally
applied
votage polarity. This is totally incorrect. The purpose of the zener
is to limit
the back emf produced by the relay coil when it is de-energised. If
you know
anything at all it is that the back emf produced by the coil is ALWAYS of
OPPOSITE polarity to that of the applied operating voltage.
Consequently, in
order to limit the back emf the zener MUST be conducting to the NORMAL
operating
polarity and only when subjected to the reverse polarity back emf does
the zener
"break-over" into normal zener mode. If connected as you suggest the
result is
no different to using a standard diode across the coil.

If the relay manufacturers (eg.Tyco, Siemens and others) suggest that
a standard
silicon diode MUST be used in conjunction with a zener diode for this
application, then I would suggest that you are not as smart as you
make out.

:
: Meanwhile, some MOSFETs are perfect because the coil's shunting
:diode is already part of the MOSFET.

Yes, but this is not relevant to the OP's question. Trying to
demonstrate your
knowledge of MOSFETS which include an integral diode is immaterial.

:
: Unmentioned is the reason for spikes from the motor. Best is always
:to eliminate the reason for spikes rather than suppress them - as
:discussed previously. And not just for circuit stability. Doing so
:also increases life expectancies elsewhere, reduces EMI, and other
:reasons. Best is to not create that harmful energy in the first
lace.


Oh great. And just how do you suggest that you eliminate spikes
produced by a
motor? Transients produced by motor windings are just part of the deal
- you
can't just "design them out" of the equation. As long as there are
inductive
windings in a motor there will be transient spikes - end of story. So
you are
stuck with having to suppress or moderate these spikes in exactly the
same way
as suppressing the back emf from relays. BTW, varistors are commonly
used to
perform this function in electric motors as well. Even motors rated at
hundreds
of horsepower have their winding transients suppressed by the common
varistor.
Admittedly these are much bigger than the disc types we commonly
encounter. It
would be logical to assume that in this particular application the
varistors
would be subject to repeated and constant transients, but do we hear
of frequent
failure of electric motors due to the failure of varistors which have
to endure
these transients? I would suggest not. It is all about selecting the
right
varistor for the job and it will rarely, if ever, fail.

I think you're chasing ghosts with regard to the speed of the
relay(s) dropping out. But in any event, simplify the issues.
As I understand it:
You have an MCU driving a small relay coil through a bipolar.
(Based on your figures, 5V 120mw that's 60 mA, so use a
transistor between the mcu and the relay coil.) The small relay
switches a big relay.

Draw us a circuit - something like this:

Big relay
V+ supply
V+ --+------------+ | +----+
| | | | |
| [Rly1] ---o [Motor] |
| | ^--o | +--- AC
--- /c | ---o +--- Mains
|MCU|---[R]---| [Rly2] ^--o |
--- \e | | |
| | | +----+
Gnd -+------------+ Big relay
Supply Ground

You have mentioned 4 issues (I think - there may be more):
1) Protect the MCU from too much current draw
2) Protect the NPN from spikes
3) Protect the contacts of Rly1
4) Protect the contacts of Rly2

For 1, the NPN is the solution
For 2, a diode across the Rly1 coil is the solution
For 3, a diode across the Rly2 coil is the solution
assuming the supply to Rly2 is DC

The above assumes that drop out delay is not an issue.

For 4, you may need a different relay.

There is an inconsistency in what you have posted.
You said

The motor operates every minute one time
and the relay 50 times per day.

There are 1440 minutes in a day, so something isn't right,
unless you mean that the motor operates for a duration
of 1 minute, 50 times a day. Or ...?

So please give us a fighting chance. Create (or modify)
a diagram that shows your circuit, and give us links
to the datasheets for each relay.

Ed

 

[westom]

On Mar 12, 1:09 am, Ross Herbert <rherb...@bigpond.net.au> wrote:

Quite frankly, I would get away from using relays at all. If you are intending
to use 5V logic to control a small relay and then use the contact on this small
relay to control the 220Vac relay, it seems a bit archaic.

This Ross paragraph is quite accurate. Using relays simply creates
more problems (ie massive noise transients from the electric motor),
less reliable operation, and noise problems for the microprocessor
that require additional transient protection.

For example, a simple MOSFET transistor between the uprocessor and
relay already has a relay suppression diode inside the package. And
since the motor is so trivial, then a zero crossing triac means more
reliable and easier to implement design.

Ross also suggests using an optocoupler. Also a good suggestion that
increases galvanic isolation (but increases costs). Using relays in
this situation is often considered obsolete design. Relays typically
are not used until 120 / 240 volt switching currents are tens of amps.