S
Skenny
- Jan 1, 1970
- 0
Can you connect MOV's in parallel to double the joules rating?
Skenny said:Can you connect MOV's in parallel to double the joules rating?
--Skenny
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?
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!
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.
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.
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
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.
......
Do it to learn - but in a location
where human life would be threatened.
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.
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.
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:
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?
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.