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Connecting MOV's in parallell

S

Skenny

Jan 1, 1970
0
Can you connect MOV's in parallel to double the joules rating?
 
H

Harry

Jan 1, 1970
0
Skenny said:
Can you connect MOV's in parallel to double the joules rating?
--Skenny

Short reply - No, use a bigger (larger diameter) varistor.

Detailed reply - Varistors are voltage dependant devices that have a VI
characteristic that means the voltage across them remains constant(ish) for
a wide range of currents. Varistors also have manufacturing tolerance. If
you put a two varistors in parallel, and one is at the top of its tolerance
range and the other is at the bottom, virtually all the current will go in
to one of them, and hardly any onto the other - hence the short reply of no.
This is the same problem that occurs if you parallel diodes etc.
If you fully understand the tolerances and VI curve charicteristics, and are
using varisors from the same batch, and have statistical infomaton on
manufacturing tolerance, you can parallel varistors, BUT you will not be
able to simply double the current rating, you will have to apply a de-rating
factor to allow for the matching of currents between the varistors

Joules - Note that I have carefully avoided mentioning joules above. It is
much better to work in volts, amps and time when dealing with varistors. You
can then derive joules if you need to, but never loose sight of the raw
electrical information of volts, amps and time. Joules confuse the issue!
Joules are more usefull for dieting for the metrically inclined! Let me
explain: if you have two varistors, and want to double the joule rating,
just put them in series. Your joule rating will be just about doubled, no
problem! The joule rating hides the fact that this not a very clever thing
to do!
 
S

Skenny

Jan 1, 1970
0
Ok, thanks for the info, but im a little confused here.
I thought MOV's conducted at a certain voltage, example if the voltage gets
to 130 or above, and the MOV is rated at 130 volts, the MOV conducts, or
passes current.
If you connected two in series, would this not double the votage at which
they pass current?
 
H

Harry

Jan 1, 1970
0
Skenny said:
Ok, thanks for the info, but im a little confused here.
I thought MOV's conducted at a certain voltage, example if the voltage gets
to 130 or above, and the MOV is rated at 130 volts, the MOV conducts, or
passes current.
If you connected two in series, would this not double the votage at which
they pass current?

Absolutely! Connecting two varistors in series will double the voltage
across them (not actually what you want) and hence double the joule rating
(sort of what you asked for!).

I was trying to encourage you to not use joules, as they do not help you.
Just work with volts, amps and time and you won't get caught out.

What prompted your original question? What current rating are you trying to
pass and for what time?
 
S

Skenny

Jan 1, 1970
0
Well, I am making a "surge protector" out of a standard 15 amp 120 VAC
duplex receptacle with a MOV connected across the hot and neutral terminals.
Both mounted in a handy box.
I got the MOV at radio shack, all they had was a 700 joules one, so I was
thinking I could get more protection by adding another one in paralell.
 
E

ehsjr

Jan 1, 1970
0
I'm in agreement with your reply, but I'm looking at
a different aspect than you: two separately housed
MOV's plugged into the same circuit. The positioning
of my reply is for comparison, not disagreement.
Short reply - No, use a bigger (larger diameter) varistor.

Short reply - Yes, but a little different than discussed.
Individual point of use surge protectors, each with their
own MOV's, plugged into the same circuit, increase the
joule rating. If the first one to conduct blows open,
the second one conducts, so the total joule rating is
additive. If the first one to conduct blows shorted, the
second one is irrelevant, until and unless the short
blows open. (The typical failure mode is shorted. However,
if the 120 Vac supply or the surge energy is still present,
they open.) Since they are in separate housings, the heat
dissipated in one does not affect the other. But within a
single housing, use a bigger varistor, just as you say.
Detailed reply - Varistors are voltage dependant devices that have a VI
characteristic that means the voltage across them remains constant(ish) for
a wide range of currents. Varistors also have manufacturing tolerance. If
you put a two varistors in parallel, and one is at the top of its tolerance
range and the other is at the bottom, virtually all the current will go in
to one of them, and hardly any onto the other - hence the short reply of no.
This is the same problem that occurs if you parallel diodes etc.
If you fully understand the tolerances and VI curve charicteristics, and are
using varisors from the same batch, and have statistical infomaton on
manufacturing tolerance, you can parallel varistors, BUT you will not be
able to simply double the current rating, you will have to apply a de-rating
factor to allow for the matching of currents between the varistors

Joules - Note that I have carefully avoided mentioning joules above. It is
much better to work in volts, amps and time when dealing with varistors. You
can then derive joules if you need to, but never loose sight of the raw
electrical information of volts, amps and time. Joules confuse the issue!
Joules are more usefull for dieting for the metrically inclined! Let me
explain: if you have two varistors, and want to double the joule rating,
just put them in series. Your joule rating will be just about doubled, no
problem! The joule rating hides the fact that this not a very clever thing
to do!

And even if they work in that configuration, it's not too
good - the clamping voltage will be doubled. And when
the first one opens, the second one is useless.

Ed
 
H

Harry

Jan 1, 1970
0
Let me support W_thom's post. Making a DIY surge protector is a scary thing
to do! You need to ask yourself "what happens when the varistor comes to the
end of its life?, what disconnects the varistor from the circuit ?". If you
don't know the answer, it could be that your house burns down as a result!
This is what UL 1449 2nd issue addresses. Using 130V varistors on a 120V
supply, though common practice, is also a cause of many failures as there is
not a lot of headroom for supply voltage variation. If your supply is over
8% high, then you could be heading for a nasty thermal runaway failure.

Transients can occur between any pair of conductors. So you need to protect
between line and neutral, neutral and ground, line and ground. If you are on
the type of supply where neutral and ground are bonded together in your
building, and are very close to the bonding location, then you can obviously
simplify things.

In your context, adding a second varistor in parallel will at worst do next
to nothing, and at best, if you are really really lucky, it will double the
current rating.
 
H

Harry

Jan 1, 1970
0
..> I'm in agreement with your reply, but I'm looking at
a different aspect than you: two separately housed
MOV's plugged into the same circuit. The positioning
of my reply is for comparison, not disagreement.

Short reply - Yes, but a little different than discussed.
Individual point of use surge protectors, each with their
own MOV's, plugged into the same circuit, increase the
joule rating. If the first one to conduct blows open,
the second one conducts, so the total joule rating is
additive. If the first one to conduct blows shorted, the
second one is irrelevant, until and unless the short
blows open. (The typical failure mode is shorted. However,
if the 120 Vac supply or the surge energy is still present,
they open.) Since they are in separate housings, the heat
dissipated in one does not affect the other. But within a
single housing, use a bigger varistor, just as you say.

You make an interesting point. Conventional practice/wisdom says that a
single shot current rating corresponds to a current that a device can endure
with no more than a defined amount of degradation, which is usually quite
low. Hence if I buy a device that is rated at 10kA, and I test it with10kA,
I don't expect to see bits fly off, acrid black smoke, or flames!

You could indeed have a new definition: Single shot current rating WITH
PERMITTED PARTIAL CATASTROPHIC FAILURE (the capitals are for emphasis, not
shouting!) and what you say then might work and make sense. Users would need
to be fully aware of the difference. Marketing men would need to avoid the
temptation to mislead the unwary.

You stated "Since they are in separate housings, the heat dissipated in one
does not affect the other." During the transient event, any heat dissipated
is actually absorbed (adiabatic behaviour), ie the heat raises the
temperature of the varistor itself, and does not have time to be conducted,
convected or radiated anywhere else. In this case it makes no difference if
the varistors are separate or not. On the other hand, a varistor suffering
from thermal runaway may well fail in a much slower manner if suitable
disconnection is not provided. Your separate boxes may indeed isolate the
mess caused by the failing device from a healthy device
 
E

ehsjr

Jan 1, 1970
0
Harry said:
.> I'm in agreement with your reply, but I'm looking at



You make an interesting point. Conventional practice/wisdom says that a
single shot current rating corresponds to a current that a device can endure
with no more than a defined amount of degradation, which is usually quite
low. Hence if I buy a device that is rated at 10kA, and I test it with10kA,
I don't expect to see bits fly off, acrid black smoke, or flames!

You could indeed have a new definition: Single shot current rating WITH
PERMITTED PARTIAL CATASTROPHIC FAILURE (the capitals are for emphasis, not
shouting!) and what you say then might work and make sense. Users would need
to be fully aware of the difference. Marketing men would need to avoid the
temptation to mislead the unwary.


Sorry, I'm missing the point in the above paragraphs.
I'm thinking of two physically separated in different
enclosures devices. Each would be tested independently,
if testing were done. It is my understanding that
MOVs that are tested are then discarded.

What happens in a real world surge is problematic
to predict:

A C E
S ----------+----------+-------+
O | | |
U | | |
R MOV1 MOV2 Equipment
C | | |
E -----------+----------+-------+
B D F

Assume a source surge V of 10,000.
Assume MOV1 conducts at 330 and clamps to 130
Assume MOV2 conducts at 332 and clamps to 130
A-B will go to 130, keeping MOV2 from operating
and holding C-D and E-F to 130, until the surge
stops or MOV1 opens. If MOV1 opens, C-D rises to
332 and MOV2 clamps to 130.

New scenario, except this time MOV2 conducts at 330
and MOV1 conducts at 332. Now we need to know the
voltage drop A-C. MOV2 will draw a bucket of
current through that wire, and there will be some
inductance as well, so it is quite possible that
MOV1 will see in excess of 332 while MOV2 is clamping
C-D at 130. So - MOV2 absorbs part of the surge, and
causes MOV1 to conduct. MOV1 now clamps to 130, turning
MOV2 off.

In either case, a surge in excess of rating that causes
MOV destruction will cause both MOV's to participate.
In the second case, a non-MOV-destructive surge *may*
cause both MOV's to participate.

But, bottom line, in either case, the total MOV joule
rating is increased by parallel MOV's. What is not
increased is the amperage, except for the brief time in
the second case where both MOV's conduct at the same time.
The parallel MOV's extend the duration of protection,
but would seem to have little effect on the total current
they can conduct.

That goes back to something you mentioned, advising to look
at the amperage rating.

You stated "Since they are in separate housings, the heat dissipated in one
does not affect the other." During the transient event, any heat dissipated
is actually absorbed (adiabatic behaviour), ie the heat raises the
temperature of the varistor itself, and does not have time to be conducted,
convected or radiated anywhere else. In this case it makes no difference if
the varistors are separate or not. On the other hand, a varistor suffering
from thermal runaway may well fail in a much slower manner if suitable
disconnection is not provided. Your separate boxes may indeed isolate the
mess caused by the failing device from a healthy device

Right - during the non-destructive to the MOV transient
period, the heat in one doesn't affect the other. I'm
referring to an event that raises the temperature in the
MOV to the point of destruction. The flames (sometimes)
in MOV 1 will affect MOV 2 ! I don't know if the kinetic
energy of an exploding MOV (speculated) would adversely
affect a second MOV in the same housing. I have no data
on "exploding MOV's" - don't know if it happens, nor even
what the definition of "exploding" would be in that case.

Ed
 
H

Harry

Jan 1, 1970
0
Sorry, I'm missing the point in the above paragraphs.
I'm thinking of two physically separated in different
enclosures devices. Each would be tested independently,
if testing were done. It is my understanding that
MOVs that are tested are then discarded.

What happens in a real world surge is problematic
to predict:

A C E
S ----------+----------+-------+
O | | |
U | | |
R MOV1 MOV2 Equipment
C | | |
E -----------+----------+-------+
B D F

Assume a source surge V of 10,000.
Assume MOV1 conducts at 330 and clamps to 130
Assume MOV2 conducts at 332 and clamps to 130
A-B will go to 130, keeping MOV2 from operating
and holding C-D and E-F to 130, until the surge
stops or MOV1 opens. If MOV1 opens, C-D rises to
332 and MOV2 clamps to 130.

New scenario, except this time MOV2 conducts at 330
and MOV1 conducts at 332. Now we need to know the
voltage drop A-C. MOV2 will draw a bucket of
current through that wire, and there will be some
inductance as well, so it is quite possible that
MOV1 will see in excess of 332 while MOV2 is clamping
C-D at 130. So - MOV2 absorbs part of the surge, and
causes MOV1 to conduct. MOV1 now clamps to 130, turning
MOV2 off.

In either case, a surge in excess of rating that causes
MOV destruction will cause both MOV's to participate.
In the second case, a non-MOV-destructive surge *may*
cause both MOV's to participate.

But, bottom line, in either case, the total MOV joule
rating is increased by parallel MOV's. What is not
increased is the amperage, except for the brief time in
the second case where both MOV's conduct at the same time.
The parallel MOV's extend the duration of protection,
but would seem to have little effect on the total current
they can conduct.

......

Aside from the point you are trying to make, I think you may have
misunderstood
how varistors work and muddled them up another suppression technology:

Varistors are voltage limiting devices with a continuous VI characteristic
where V progressively increases with I, but in a non-linear manner. A
130V varistor NEVER clamps at 130V.

This is in contrast to voltage switching devices such as gas discharge
tubes, spark gaps and thyristor based devices that switch to a lower voltage
once a threshold voltage is reached, which is the behaviour you describe.

The VI characteristic (mid-tolerance) of a 130VAC rated varistor is very
approximately:
205V @1mA
220V @ 10mA
240V @ 100mA
250V @ 1A
270V @10A
300V @100A
350V @1kA
450V @10kA

And about 10 micro amps at 130V!

Perhaps my point make more sense now.

In addition, are you under the misapprehension that varistors are single
shot devices
like fuses? Well the aren't, unless abused. In general, you should select a
varistor so it will give
you a reasonable lifetime, and so failure or overstressing is unlikely.

Coming back to your way of looking at things, if I buy a car a twin turbo
car, and the manufacturer tells me you can drive it at 200 mph for no more
than 10 minutes, I would expect it to fully work afterwards, but wouldn't be
surprised if it had suffered some very minor effects as a results. Perhaps
there might me some minor increse in emmisions etc. You would be
happy to have one of the turbos fail in the same circumstances, providing
200 mph was achieved for 10 minutes. I hope this
illistrates the point I am making.

On a technical level, your point is entirely dependant on the failure mode
of the varistors, which is indeterminate. Never the less, it is an
interesting way to look at things.

Finally, it may amuse you to know that an exploding varistor (if safely
contained)
is actually a comparatively benign form of failure compared to a slow
thermal
runaway.
 
P

Peter Pan

Jan 1, 1970
0
I had the idea that MOV weren't too good voltage clamps, and the
clamping voltage would depend on the current through the device. This
voltage usually being greater than the voltage at which ti triggers
(think gas gaps are better in this respect).

If this is the case, the second MOV would then have enough voltage to
also trigger, and help dissipate part of the transient.
 
S

Skenny

Jan 1, 1970
0
Well, thanks for everyone's help and opinions.
I feel any advice done in hosnesty is helpful.
Let me address the issue of burning down my house first:
The MOV will be enclosed in a metal "handy box", with a duplex receptacle,
the whole thing will be covered with a metal duplex cover, factory made to
fit the duplex receptacle and box. The wire entry into the handy box will be
with a metal cable clamp (romex clamp is what Ive always called them). This
wire will then go to a 10 amp button type breaker rated at 600 volts.
(breaker is on the hot wire).
Im not worried about catching anything on fire. Fire is always my first and
most important concern.
As fas as attracting lightning, I dont see how this could be, since the
electronic equipment will be plugged into the duplex receptacle, how could
adding a MOV "attract" more lightning than the electronic equipment alone?
(I could be wrong, not trying to start an argument, just trying to get
understanding.)
I could see where attaching two more MOV's from hot and neutral to ground
could be benificial.
But you still have the ground wire going back to the panel (house fuse box),
looks like any current going throug the MOV's will take this path or the
neutral, since they both go back to ground.
As fas as the regulations, I took apart a power strip, all it has is a MOV
between hot and neutral buss bars, and a reset button breaker on the hot
wire. Plus the case of the power strip is plastic, may be fire resistant, I
dont know, but I do know the metal handy box is.
Now, to address the 700 joules rating: I messed up, it is 70 joules, not
700, those typos and decimal points will get you everytime! LOL
Someone mentioned testing the MOV, how is that done? Except by visually
inspecting them?
 
B

Bud--

Jan 1, 1970
0
Peter said:
I had the idea that MOV weren't too good voltage clamps, and the
clamping voltage would depend on the current through the device. This
voltage usually being greater than the voltage at which ti triggers
(think gas gaps are better in this respect).

If this is the case, the second MOV would then have enough voltage to
also trigger, and help dissipate part of the transient.

MOVs don't trigger like a gas device. As with Harry's figures, the
current is very low until the "maximum continuous operating voltage" (at
1 mA), then the current smoothly, but very rapidly, increases as the
voltage increases.

If the 2nd MOV has a 'actual' higher voltage rating, it will always be
conducting at a lower current than the 1st MOV. And because the V-I
characteristic is so non-linear it will be conducting at a much lower
current than the 1st MOV. The currents and energies will split very
unevenly unless the devices are the same type and have very nearly the
same 'turn-on' voltage.

bud--
 
S

Skenny

Jan 1, 1970
0
Thanks, I didnt realize how ignorant of the subject I really was. All the
information discussed is very useful.
Thanks to all that have participated in this thread.
 
S

Skenny

Jan 1, 1970
0
thanks.
By reading over the sevral posts to this original one, I have drawn the
conclusion that MOV's may not be such a good lightning protector after all.
Transient voltages from other devices (such as refrigerator compressor
turning on or off) might be caught by the MOV, but certainly not high
voltages from lightning.
Of course, if a direct lightning hit the utlity wire outside, I doubt if you
would have much left in the house.
 
B

Bud--

Jan 1, 1970
0
Skenny said:
thanks.
By reading over the sevral posts to this original one, I have drawn the
conclusion that MOV's may not be such a good lightning protector after all.
Transient voltages from other devices (such as refrigerator compressor
turning on or off) might be caught by the MOV, but certainly not high
voltages from lightning.
Of course, if a direct lightning hit the utlity wire outside, I doubt if you
would have much left in the house.

MOVs are the basic protection elements in both service surge protectors
and plug-in surge protectors. MOVs do provide effective protection from
most surges including those caused by lightning. They may or may not
protect from a direct hit to overhead wiring very near your house. Or
the surge suppressor may not survive but protected equipment will.

I would not recommend building your own surge protector.


I don't think it has been mentioned - the voltages are peak voltage. If
a MOV with a MCOV of 130 volts is connected to a 120 volt RMS line (168
volt peak) it will rapidly destruct.


------------
Martzloff, who was the NIST surge guru, has said the major cause of
surge protector failure is temporary overvoltage, not unusually large
surges.

bud--
 
E

ehsjr

Jan 1, 1970
0
Harry said:
Aside from the point you are trying to make, I think you may have
misunderstood
how varistors work and muddled them up another suppression technology:

It was a hypothetical, which should be obvious.
I was trying to force fit something as gently as I could
to point out that your statement could use a little work.
It is clear that my hypothetical only confused things.

I'll be blunt so the point won't be missed this time.
You said: "Varistors also have manufacturing tolerance.
If you put a two varistors in parallel, and one is at the
top of its tolerance range and the other is at the bottom,
virtually all the current will go in to one of them, and
hardly any onto the other - hence the short reply of no."

My stupid example was intended to illustrate that
even if one varistor operated as you say, with virtually
all of the current going through it, the overall rating
is in fact increased. The rating would be essentially
doubled, give or take innaccuracy in the rating.
Additionally, unless there is a large manufacturing
variation, both MOV's will conduct appreciable current
in a surge event that causes one of them to conduct heavily.


Varistors are voltage limiting devices with a continuous VI characteristic
where V progressively increases with I, but in a non-linear manner. A
130V varistor NEVER clamps at 130V.

Right. I did not specify a 130 v varistor. I
provided a bad hypothetical to try to illustrate
a point. You are right to challenge it - it stunk.
This is in contrast to voltage switching devices such as gas discharge
tubes, spark gaps and thyristor based devices that switch to a lower voltage
once a threshold voltage is reached, which is the behaviour you describe.

The VI characteristic (mid-tolerance) of a 130VAC rated varistor is very
approximately:
205V @1mA
220V @ 10mA
240V @ 100mA
250V @ 1A
270V @10A
300V @100A
350V @1kA
450V @10kA

And about 10 micro amps at 130V!

Perhaps my point make more sense now.

Not at all. Nothing you posted here addresses it.
In addition, are you under the misapprehension that varistors are single
shot devices
like fuses?

No. Are you? Some surges events are
destructive to MOV's, some are not.


Well the aren't, unless abused. In general, you should select a
varistor so it will give
you a reasonable lifetime, and so failure or overstressing is unlikely.

Coming back to your way of looking at things, if I buy a car a twin turbo
car, and the manufacturer tells me you can drive it at 200 mph for no more
than 10 minutes, I would expect it to fully work afterwards, but wouldn't be
surprised if it had suffered some very minor effects as a results. Perhaps
there might me some minor increse in emmisions etc. You would be
happy to have one of the turbos fail in the same circumstances, providing
200 mph was achieved for 10 minutes. I hope this
illistrates the point I am making.

Well, I don't know what your example has to do with
my way of looking at things. My way of looking
at MOV's in the real world is that some surge events
are destructive and some are not. Some surge events
may damage an MOV, without destroying it. Such events
degrade and therefore subtract from the capacity of
the MOV. Some surge events don't damage the MOV.

Testing (as opposed to real world) is assumed to
degrade the MOV, while a real world surge may not.



On a technical level, your point is entirely dependant on the failure mode
of the varistors, which is indeterminate. Never the less, it is an
interesting way to look at things.

That just dodges the issue. Putting MOV's in series is
useless as a means to double the joule rating, as you
pointed out. Putting them in parallel is effective
at doubling the rating. (Increase would be a better
term than double, because there may be some variation
from rating in the actual devices.) It is also increases
the current that can be drawn during a surge event,
because it will split between the two MOV's, but
in the case of current, the word increase does not
indicate essentially double the amount. The differences
between the MOV's would determine the proportion
of total current each MOV would draw.

I *think* we agree that joule rating may be misleading to
the public.
Finally, it may amuse you to know that an exploding varistor (if safely
contained)
is actually a comparatively benign form of failure compared to a slow
thermal
runaway.

Can you spell it out? Seems to me that if we apply
the same limitation - "safely contained" - to both
cases, they would be equally benign. So I'm not sure
what you have in mind.

Ed
 
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