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 Post subject: How to identify dielectric material on modern film caps
PostPosted: Feb Wed 28, 2018 2:22 pm 
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I was wondering how one can identify the type of dielectric material used on modern axial film caps. Is it by color of the case? ive got cases here that are white, bright yellow, and a dull yellow. or is color just whatver the manufacturer used? also seen older film caps in a blue case.

And of course we have generic orange drops. ive also notices a bright orange and a dull orange case among the ones i have.

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Feb Wed 28, 2018 2:55 pm 
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There might be some esoteric measurements you could make, but the brute-force approach would be to take one apart.

You can get some information from relative physical size and comparing the dielectric constant and strength of different materials. ( high dielectric strength gives higher breakdown for a given size, and high dielectric constant gives higher capacitance)

Before going to all this trouble, ask yourself if / why you really care. The easy way to know what you are getting is when you buy it.

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Feb Wed 28, 2018 5:13 pm 
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Quote:
There might be some esoteric measurements you could make, but the brute-force approach would be to take one apart.


Quite the contrary: measurements are easy if you have the right equipment. On the other hand, if you did bust a cap open, how would you identify the type of plastic you found inside it without doing a chemical analysis or at least a flame analysis?

The measurement used is dissipation or dissipation factor (DF). It is a measure of the energy lost in the dielectric when an alternating electrostatic field is applied. This depends to a great extent on the nature of the plastic film. It is typically measured on a capacitance bridge at 1-kHz; most of the better LCR meters also perform this measurement. Polyester (Mylar) has a typical DF of about 0.5% while polypropylene is an order of magnitude better, eg. 0.02%. By comparison, tubular paper capacitors have DF's around 3%.

It is also worth noting that plastic tends to deteriorate with age, temperature, and exposure to ozone. So old, well used plastic film caps can be detected by an increase in DF compared to what it measured when the caps were new.

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Feb Wed 28, 2018 6:52 pm 
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trainman wrote:
I was wondering how one can identify the type of dielectric material used on modern axial film caps. Is it by color of the case? ive got cases here that are white, bright yellow, and a dull yellow. or is color just whatver the manufacturer used? also seen older film caps in a blue case.

And of course we have generic orange drops. ive also notices a bright orange and a dull orange case among the ones i have.


Sometimes the letter markings on the caps will give you a clue:

PP - Polypropylene
PS - Polystyrene
PET - polyethylene terephthalate
MF - Metallized Film (polypropylene, typically)

If there are manufacturer's markings, look to Google for a datasheet or manufacturers website for possible data.

You mentioned Orange Drops - made originally by Sprague (marked Sprague or with a circled 2), then by Sprague-Barre Electronics (marked SBE), and now by Cornell Dubilier, marked CDE. 225P (flat) and 418P (round profile)series are all polyester film, and 715P and 716P are Polypropylene film. There are other, esoteric series in the Orange Drop line, with the most common being the 2PS/4PS/6PS/16PS series, a distributor-based product not used by the big OEMs, but as an aftermarket replacement line. They use BOTH Polyester and Polypropylene :shock: , depending on value. Most of the lower value stuff in the PS line is polypropylene, good for it's pulse handling capability.

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Thu 01, 2018 2:09 am 
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Chris108 wrote:
Quote:
There might be some esoteric measurements you could make, but the brute-force approach would be to take one apart.


Quite the contrary: measurements are easy if you have the right equipment. On the other hand, if you did bust a cap open, how would you identify the type of plastic you found inside it without doing a chemical analysis or at least a flame analysis?

I stand corrected---sort of.

Looking at the guts, one presumably can get major clues. Polyester, e.g. has some distinctive physical properties, and the thickness of the dielectric gives a big clue.

Perhaps we would need both electrical and mechanical info for a definitive answer.

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Thu 01, 2018 4:15 pm 
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I don't know if my tester can measure DF well enough to tell which is what, but I would go with mfr data in the case it is available. That is not a problem with US brands usually, but Asian made caps could be anything. I would expect in most products they use the most compact and cheap type, as in Polyester. There were some weird US products during the transition from paper to plastic. I have some wax caps that are "polyester impregnated" which does not mean they are totally plastic, but they all test no leakage and very accurate values. European caps are also distinguishable as to whether they are one or the other. Just have to research it. I know Elna caps are different colors, that may be of some significance.


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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Thu 01, 2018 5:15 pm 
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I ah, thought there might be some easy way. like maybe color of the outer case had something to do with it. i have amassed a large amount of newer film caps, so i know they all have different dielectric material.

like for example, the no name yellow film caps JustRadios sells, once you mix up all the different values you have,you lose track. so, i guess unlee you know the manufacture part number, guess there is no easy way to know if you got a polyester cap, a plypropelene cap. or a metalized film. i was just curiou

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Thu 01, 2018 11:02 pm 
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Just plan on using the mystery dielectric parts for applications where it doesn't matter. Wherever an ordinary mylar (PETE, polyester) cap would be suitable, any of the other film dielectrics will work. ( And probably ceramic caps, too.)

For specific, more stringent applications, you may have to buy new. In order to be sure you are using the right thing. I can't imagine this would amount to much money, if these are for repairing old radios.

Ted


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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Fri 09, 2018 8:20 pm 
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A simple handheld LCR meter like the DER EE DE-5000 can easily measure dissipation factor and tell you the difference between polyester and polypropylene film caps. At last check they were under $100 online. Doesn't require any imagination at all, nor do you have to take your caps apart and try to figure out what they are made of.

One "oh by the way" they don't tell you is that many manufacturers--even name brands like Panasonic--will sometimes substitute polypropylene for polyester in caps when they run short ends of material. In most cases that won't make much difference to an end user but sometimes it does. Polypropylene caps have less loss, higher Q's, and higher self resonant frequencies. They will resonate more, and more sharply than polyester or paper--and at different frequencies. In audio applications nobody would notice the difference, but such things can and do matter sometimes at IF and RF frequencies.

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Sun 11, 2018 6:52 pm 
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Somewhat off topic, but...

Years ago I worked for a company making analog computers. Analog computers require resistors and capacitors whose value is correct to 10 parts per million. The computing caps were made slightly low in value and then trim caps were added to make them exactly right. But a problem was observed. Even with the caps wound as tight as possible, there is a little give in the cap. When voltage was applied the cap would squeeze together slightly. This changed the capacitance slightly but enough to be a problem. I don't remember if the caps were polypropylene or polystyrene, but the solution was to have the manufacturer heat the caps to just below the melting point. This fused everything together and cured the problem.

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Sun 11, 2018 8:35 pm 
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I loved the class on analog computers in college......a lost art....
When you say "fused", do you mean that air gaps were eliminated? If the dielectric melted or even softened, i would think the breakdown voltage would be compromised.

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Mon 12, 2018 12:35 am 
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I can readily believe that they'd have trouble with film capacitors changing value with temperature in analog computers. Some plastic films are not that stable with temperature, and any kind of wound capacitor can have voids or loose wraps if it is not specially treated.

A while back somebody showed me some very expensive tubular film/foil audiophool caps intended to go in speaker crossover networks. Some of the wraps were loose and they would resonate or buzz at certain frequencies. You want to stump an electronics "know-it-all," show them some waveforms while doing an audio sweep through a capacitor like that!

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Mon 12, 2018 6:01 pm 
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The dielectric was softened enough to adhere to the metal foil. I don't know if the voltage rating changed due to the heating or not. But the caps were custom made, so that could have been allowed for.

The caps were temperature sensitive. They were mounted in a metal enclosure with a heater. The temperature was regulated using a thermister to sense the temperature. The enclosure was insulated with what looked like quilted baby blankets. There were small holes in the enclosure to allow for adjusting the trim caps. An earlier computer used a refrigerator instead.

The larger caps were made up with .33 uf sections. 3 for the 1 uf cap and 30 for the 10 uf cap. They were supplied in sets of selected caps so that the value of the set was close to the desired value.

Despite having very low leakage, the leakage had to be compensated for. Each integrator amp was followed by a precision inverting amp. The output of the inverting amp was fed back through an adjustable network to supply a current equal to the leakage current but of the opposite polarity.

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 Post subject: Re: How to identify dielectric material on modern film caps
PostPosted: Mar Mon 12, 2018 7:48 pm 
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Amazing!!--thanks for sharing this.

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