Metal Oxide Resistors (thin film)

[kfel]

Hi,

I need to know the pulse handling capability of 3K9/2W MOR(thin film)
for a voltage pulse having a maximum voltage of 1600V and which decays
exponentially. Also this pulse repeats every 10ms and is superimposed
onto the line voltage of 230VAC/50Hz. I am told that MORs can withstand
upto 2.5 times their rated wattage for upto 3minutes. Is this true?

 

[Eeyore]

Hi,

I need to know the pulse handling capability of 3K9/2W MOR(thin film)
for a voltage pulse having a maximum voltage of 1600V and which decays
exponentially. Also this pulse repeats every 10ms and is superimposed
onto the line voltage of 230VAC/50Hz. I am told that MORs can withstand
upto 2.5 times their rated wattage for upto 3minutes. Is this true?

It'll depend on construction. You ought to consult the manufacturer for pulse
data too. You mean an average dissipation of 2.5x the rating for 3 mins. You
*might* get away with it in that size - it'll depend on the quiescent
dissipation of course. You'll have to get rid of the heat some way eventually
though.

Graham

 

[Phil Allison]

** Groper alert !

I need to know the pulse handling capability of 3K9/2W MOR(thin film)
for a voltage pulse having a maximum voltage of 1600V and which decays
exponentially.

** Expect very early failure.

Such high voltage spikes are way to much for the fine clearances between the
spiral cut into the metal oxide.

Also this pulse repeats every 10ms and is superimposed
onto the line voltage of 230VAC/50Hz.

** 230 x 230 / 3900 = 13.5 watts !!

FORGET IT !!!

I am told that MORs can withstand
upto 2.5 times their rated wattage for upto 3minutes. Is this true?

** Who cares.

What you just asked is insane.

You will need a series string of such 2W resistors to handle that abuse.





........ Phil

 

[kfel]

The voltage spikes last only 10 micro seconds.

 

[Phil Allison]

** Groper Fuckwit Alert

The voltage spikes last only 10 micro seconds.

** Lots little pricks can be very annoying.

100 per second is way more than any Koala can Bear.

Piss off - half wit.





....... Phil

 

[Eeyore]

The voltage spikes last only 10 micro seconds.

It's the energy in them that counts. Doesn't matter how long or short they are.

Are you familiar with integration ?

Graham

 

[redbelly]

It's the energy in them that counts. Doesn't matter how long or short they are.

Are you familiar with integration ?

Graham

I think Phil meant you could get arcing inside the resistor. In that
case, what counts is the voltage (between adjacent spirals) divided by
the **small** gap between them. Divide by a small number, and you get
a big number.

Mark

 

[Phil Allison]

"redbelly"

I think Phil meant you could get arcing inside the resistor. In that
case, what counts is the voltage (between adjacent spirals) divided by
the **small** gap between them.

** Precisely correct.

That is why ordinary, spiral cut, tubular resistors have low continuous
voltage ratings - 250 to 500 volts max.

Divide by a small number, and you get a big number.

** How VERY true.

Divide by zero and the universe collapses..

Right back to a singularity......





....... Phil

 

[Chris]

Hi,

I need to know the pulse handling capability of 3K9/2W MOR(thin film)
for a voltage pulse having a maximum voltage of 1600V and which decays
exponentially. Also this pulse repeats every 10ms and is superimposed
onto the line voltage of 230VAC/50Hz. I am told that MORs can withstand
upto 2.5 times their rated wattage for upto 3minutes. Is this true?

Hi, Kfel. From a hobbyist or even a one-off engineering prototype
standpoint, it's usually better to just nuke problems like this. It's
not worth the hassle to try to find the cheapest solution that probably
won't smoke. Unless you're talking about making these in quantity,
it's better to just overbuy and be done with it.

For your steady state power requirement ( P = V^2 / R = 13.6W), as well
as joule rating, I'd recommend a significantly higher wattage
non-inductive wirewound resistor. Surplus Sales of Nebraska has a 4K,
25 watt non-inductive Kool-ohm for $12 USD. It's their P/N (RNI)
04000-25A

http://www.surplussales.com/index.html

Wirewound resistors have great joule and overload ratings in comparison
to other types. This will do the job. Get it and move on.

And whoever gave you the story about 2.5X rated overload for 3 minutes
for a film resistor was spinning a fabulous tale. It'll be toast for
sure.

Good luck
Chris

 

[Eeyore]

I think Phil meant you could get arcing inside the resistor.

Because of the high voltage. You'd have to check the resistor spec.

In that
case, what counts is the voltage (between adjacent spirals) divided by
the **small** gap between them. Divide by a small number, and you get
a big number.

The inter-spiral voltage will be ~ Vapplied / Nspirals

Still doesn't help about the basic energy involved though.

Graham

 

[redbelly]

Because of the high voltage. You'd have to check the resistor spec.



The inter-spiral voltage will be ~ Vapplied / Nspirals

Still doesn't help about the basic energy involved though.

Well, yeah, I didn't mean to imply that pulse energy was not a concern,
though it probably seemed that way in that I posted as a reply to your
post. Sorry about that.

Energy per pulse, time-averaged power, and peak voltage are all factors
of concern.

Regards,

Mark

 

[redbelly]

"redbelly"




** How VERY true.

Divide by zero and the universe collapses..

Right back to a singularity......

Well, I wouldn't get carried away now. When you say stuff like that,
there are 1 or 2 people around here who might get the wrong idea.
(Referring to recent thread about Ohm's law and division by zero.)

Of course, even when you DON'T say stuff like that, there are still 1
or 2 people who get the wrong idea!

Mark

 

[kfel]

Can thick film MORs be used instead of wire wounds??

 

[Chris]

Can thick film MORs be used instead of wire wounds??

Your 2 watt 3.9K resistor will curse you as it emits it's fragrant
aroma and lets the smoke out.

As Graham mentioned, this is an application-specific problem that the
manufacturer's apps engineers can answer. Of course, if you place
enough of your 3.9K 2W metal film resistors in series/parallel
combination (5 X 5 will work, 3 X 3 is too close, 4 X 4 is probable but
you should contact the manufacturer). A lot of this has to do with
arcover at peak voltage, and a lot also with the joule rating of the
resistor (which again brings up the decay rate question -- you can't
calculate joules unless you have a decay rate).

Look, life is short, and there are always more things to do. You're
potentially looking at an earnest exchange of emails with a
manufacturer apps engineer who may take days to respond. If this is a
real problem, and there's no quantity involved, the very appropriate
technical response is to shoot the engineer, git r done and ship the
product. ;-)

Get something you know will work reliably. If you want to save money,
buy surplus -- it's only $12, even if your metal film resistors are
free. Go home on time, have a beer, give the kids a hug, and pat the
War Department on the fanny with a wink and a smile because you did a
good job -- you got r done.

Cheers
Chris

(By the way, thin film is a metal alloy. You generally don't start
calling them metal oxide film, though, until the resistance gets a lot
higher than 3.9K.)

 

[Chris]

Your 2 watt 3.9K resistor will curse you as it emits it's fragrant
aroma and lets the smoke out.

As Graham mentioned, this is an application-specific problem that the
manufacturer's apps engineers can answer. Of course, if you place
enough of your 3.9K 2W metal film resistors in series/parallel
combination (5 X 5 will work, 3 X 3 is too close, 4 X 4 is probable but
you should contact the manufacturer). A lot of this has to do with
arcover at peak voltage, and a lot also with the joule rating of the
resistor (which again brings up the decay rate question -- you can't
calculate joules unless you have a decay rate).

Look, life is short, and there are always more things to do. You're
potentially looking at an earnest exchange of emails with a
manufacturer apps engineer who may take days to respond. If this is a
real problem, and there's no quantity involved, the very appropriate
technical response is to shoot the engineer, git r done and ship the
product. ;-)

Get something you know will work reliably. If you want to save money,
buy surplus -- it's only $12, even if your metal film resistors are
free. Go home on time, have a beer, give the kids a hug, and pat the
War Department on the fanny with a wink and a smile because you did a
good job -- you got r done.

Cheers
Chris

(By the way, thin film is a metal alloy. You generally don't start
calling them metal oxide film, though, until the resistance gets a lot
higher than 3.9K.)

Sorry -- edit:

There are other considerations besides steady-state power and joule
raing which make a non-inductive (Ayrton-Perry winding) wirewound
appropriate, and these are specific to your application, which you also
haven't described. This is a good basic summary (wait for the intro
ad):

http://tinyurl.com/q6myw

This is a trade magazine article, "Specifying Resistors for Motor
Control Applications". It has some additional information which may be
of use. Wait for the splash ad.

Chris