Metal Oxide Resistors (thin film)

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

Graham

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

FORGET IT !!!

What you just asked is insane.

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

....... Phil

The voltage spikes last only 10 micro seconds.

Piss off - half wit.

...... Phil

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

"redbelly"

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

Divide by zero and the universe collapses..

Right back to a singularity......

...... Phil

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,

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

are.

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.

Graham

are.

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

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

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.)

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):

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