MYLAR Non polarised Capacitors

[Musicmanager]

Hi Guys

Does anybody know, is there anything special about a Mylar non polarised capacitor ?
I've got a schematic that specifies 'mylar' so I looked on Google which suggests mylar is polyester film; Rapid shows them like small disc caps; RS shows them like ceramic blocks. I'm confused !!
Anybody know ?

Thanks
S

 

[JimB]

Have a look at the picture:

These are Polyester capacitors, a plastic film like mylar.

JimB

 

[Musicmanager]

Hi Jim

OIC, so they are quite special then. This is for a function generator to get some 'sample' waves for my 'scope. I found a simple schematic that I understand and can build which includes a rotary switch with 4 positions sine - triangle - square - sawtooth . there a 4 of these caps one at each position 1uf; o.1uf; 0.01uf and 0.001uf specified as mylar non-polarised capacitors. See schematic below

Good to hear from you
S

 

[kubeek]

I think those chips are quite obsolete, so I suggest you find a source of the chip and then decide if you want pursue this any further.

 

[Musicmanager]

Still available on Ebay !!

 

[Musicmanager]

Kubeek, my apologies. I did not mean to be rude. I thought your comment was part of a dialogue already started with JimB.

I found the schematic on the web today, from something or somewhere called Wolfden Press. I recognised it was quite old so I checked to see if the main components were still available. You're right the chip is obsolete, non of the usual - RS, Rapid or Farnell have any - but there are loads still on Ebay.

Thanks for your help

S

 

[ronsimpson]

You are building a signal generator.
Do not use caps like this: +20%-80%. This is ok for power supplies but not for timing.
Many of the low cost caps for power supplies have bad temperature stability. The value changes with temperature.
Ceramic caps can make a microphone. Vibration will effect the cap.
Mylar is called for because they are trying to keep you form using ceramic. Years ago we used silver mica.

I have one of these signal generators. Many years ago.
Can you get the XR-2206CP?

 

[JimB]

there a 4 of these caps one at each position 1uf; o.1uf; 0.01uf and 0.001uf specified as mylar non-polarised capacitors.

If you can find "Mylar" capacitors that is OK, if not try looking for "polyester".

JimB

 

[Musicmanager]

Hi Ron
Thanks for the information. I'm a complete novice . I bought a cheap 'scope last week to play with and get used to when & how to use. I've spent the weekend stripping it down and removing several Kgs of dust etc, reassembled and all is OK. I decided to get a signal generator to use to play with the 'scope to get used to it. I don't want to buy one I want to build it, just for the hell of it really, and to extend my experience.
If I can use ceramic caps, thats fine, I've used before so they are familiar. The XR2206CP is obsolete at most suppliers now, but there are many available on Ebay.
Good to hear from you
S

 

[audioguru]

The round cylindrical "mylar" capacitors were made about 50 years ago. The rectangular block polyester ones are the new kind. Get them with a tolerance of 5%.

 

[Musicmanager]

Sorry, just realised I've misunderstood what you said Ron - don't use ceramics !!

 

[KeepItSimpleStupid]

The tolerances you can get in Polyester or metalized polyester. These caps are not polarity sensitive.

Polystyene is used for stability but the cost is much much higher. Working voltage should allway be >= 2x higher than expected in the circuit.

 

[BobW]

The primary reason for not using ceramic caps in this type of function generator circuit is because their capacitance decreases as the voltage across them increases. This results in distortion of the waveform. Sawtooth and triangle waves will have curved rather than straight ramps, and the sinewaves will be similarly distorted. Polyester/mylar caps have a constant capacitance regardless of the voltage across them, resulting in low distortion waveforms.

 

[k7elp60]

Here is a link to the function generator kit at Jameco:
https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_20685_-1,