jwp@larry.sal.wisc.edu (Jeffrey W Percival) (11/03/89)
I am getting a B/W TV back into working order, and want to replace a few capacitors. Being a novice at this (astronomy, not EE), I have some questions after reading through a parts catalog. My TV parts list gives a capacitance and voltage for each capacitor. The parts catalog (Digi-Key Corp.) lists many capacitors, but not always of the exact value I need. So, how do I choose? Always go to the next higher voltage rating? Thence to the next higher capacitance? Does it depend on what role the cap is playing in the circuit? Example: I need a 15V, 5uF cap. The catalog has this: voltage capacitance 6.3 47 6.3 100 6.3 220 10 22 10 33 10 47 16 10 16 22 16 33 16 47 25 3.3 25 4.7 25 10 25 22 Do I go all the way up to 25 volts to get the 4.7uF? Or do I simply find a better catalog? Do caps come so finely graded? Another question: does "bi-polar" mean the same as "non-polarized"? -- -Jeffrey W Percival (jwp@larry.sal.wisc.edu) (608)262-8686
brian@ucsd.Edu (Brian Kantor) (11/03/89)
It's hard to know what sort of fudge-factor you can apply to a part value without knowing what the part is being used for. For example, a capacitor used as a power supply filter or power line decoupling is usually not critical, and the next highest available capacitance will almost always be perfectly acceptable. However, if the capacitor is being used in a circuit as part of a RC time constant, such as a high-pass, low-pass, or timer circuit, the value is much more critical. Voltage ratings on electrolytic capacitors are much less of a problem; usually you can go to double the voltage rating without a problem. There are some guidelines; it is wise to keep in mind that if you use a cap with a voltage rating more than (say) 10 times the working voltage of the circuit, the electrolytic film can deteriorate and the capacitor's value will change. Thus it's probably ok to put a 25 or 50 volt capacitor in place of one originally spec'd at 16 volts, but it wouldn't be smart to use a 100 volt unit there. If you don't know what voltage the original was (perhaps you can't read what's left of the label after the old one exploded), you can feel pretty safe in most cases with the next voltage rating above the supply voltage; for example a 16v or 25v capacitor is a good bet for something like a car radio that runs off 13.8 volts. Most of the time you want to stick with the original type of capacitor; substituting a tantalum cap for an aluminum one can often be done but usually NOT the other way since the aluminum ones are looser tolerance and have different impedance, loss, and stability characteristics. Since tantalum's cost more, there probably was a good reason why they used the more expensive part in the first place. - Brian
vaso@mips.COM (Vaso Bovan) (11/03/89)
In article <109@cupcake.sal.wisc.edu> jwp@larry.sal.wisc.edu (Jeffrey W Percival) writes: >I am getting a B/W TV back into working order, and want to replace a >few capacitors. > >My TV parts list gives a capacitance and voltage for each capacitor. >The parts catalog (Digi-Key Corp.) lists many capacitors, but not >always of the exact value I need. So, how do I choose? Always go to >the next higher voltage rating? Thence to the next higher >capacitance? Does it depend on what role the cap is playing in the >circuit? > >Example: I need a 15V, 5uF cap. The catalog has this: > > voltage capacitance > 6.3 47 > 6.3 100 > 6.3 220 > 10 22 > 10 33 > 10 47 > 16 10 > 16 22 > 16 33 > 16 47 > 25 3.3 > 25 4.7 > 25 10 > 25 22 > >Do I go all the way up to 25 volts to get the 4.7uF? Or do I >simply find a better catalog? Do caps come so finely graded? > >Another question: does "bi-polar" mean the same as "non-polarized"? >-- Such reseller catalogs generally do not give enough information for a buyer to pick out the exact repalcement capacitor he needs. The buyer needs to match not only electrical characteristics, but also mechanical dimensions. I'd get a better catalog, from Sprague, or Nichicon, for instance. The degree of required electrical "match" depends greatly on the circuit application. An electrolytic cap using for filtering might not need to match closely. A (generally high grade) electrolytic cap used for timing applications had better be right on. Non-polarized is the preferred designation for a cap constructed so that it can be installed "backwards". The word "bi-polar" is best left to describe semiconductors. Most electrolytic caps are polarized ("non-polarization" exacts a volume penalty). If you install a polarized cap backwards, it will very likely explode. Therefore, you're not going to get any recommendations from me, except to be careful. :-)