radial electrolytic caps replace polystyrene caps?

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Hi,

 

I am looking to replace the Vectrex game console motherboard's polystyrene caps, which are quite expensive.  I am wondering if normal radial electrolytic caps like the below link can be used instead.  If not, can anyone suggest a cheaper replacement?  Thanks!

 

http://www.taydaelectronics.com/...

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Polystyrene caps are not used for the same applications as electrolytics. They may be used for precision timing, filters, sample and hold, etc. They also have very low leakage and dielectric absorption. Why do you think you need to replace them? They do not deteriorate with time and temp the way electros do. I have never looked inside my Vectrex, but it still works fine. How do you know these suspect caps are polystyrene?

 

Rick

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Electrolytic caps are quite different beasties than polystyrene. 

 

Because of the cost noted above, poly caps are not used on a whim. They are only used when they are really needed. Here are some of the differences:

 

                                               Electrolytic                                                                            Poly

Tolerance             Typically 10% or worse, sometimes -20%+80%                              Few percent or tighter

Temp Coeff          Typically pretty significant                                                              Typically pretty tight

Polarity Sensitive  YES, and may explode with reverse voltage                                    polarity insensitive

Leakage               Usually fairly high                                                                        usually very low

Q                        Usually not even spec'd                                                                 usually quite good (below 100KHz)

Value range         Few uF to few hundred uF                                                              usually around 1nf to 1uf or so

Voltage               Low cost ones to 50V or so, more expensive higher                          often 100V or more

 

If you have to look for a non-poly replacement, look for a ceramic, especially NPO/COG type. They come pretty close to film (poly). NPO have good tolerance and Tempco. You can get them over 1nf, these days, for pretty modest prices.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk wrote:

Electrolytic caps are quite different beasties than polystyrene. 

 

Because of the cost noted above, poly caps are not used on a whim. They are only used when they are really needed. Here are some of the differences:

 

                                               Electrolytic                                                                            Poly

Tolerance             Typically 10% or worse, sometimes -20%+80%                              Few percent or tighter

Temp Coeff          Typically pretty significant                                                              Typically pretty tight

Polarity Sensitive  YES, and may explode with reverse voltage                                    polarity insensitive

Leakage               Usually fairly high                                                                        usually very low

Q                        Usually not even spec'd                                                                 usually quite good (below 100KHz)

Value range         Few uF to few hundred uF                                                              usually around 1nf to 1uf or so

Voltage               Low cost ones to 50V or so, more expensive higher                          often 100V or more

 

If you have to look for a non-poly replacement, look for a ceramic, especially NPO/COG type. They come pretty close to film (poly). NPO have good tolerance and Tempco. You can get them over 1nf, these days, for pretty modest prices.

 

Jim

Thanks for a helpful answer. I was reading Wikipedia and couldn't determine a good answer from the text. Just too much info.

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unebonnevie:

 

"Why do you think you need to replace them?"