Troubleshooting and component replacement

Discussion in 'Homebrew and Kit Projects' started by WA9SVD, Sep 15, 2021 at 2:22 AM.

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  1. WA9SVD

    WA9SVD Ham Member QRZ Page

    Hi,


    I was recently reading through some old ARRL Handbooks, (1988, to be exact,) and in the troubleshooting section, under component replacement of capacitors, they say "... if the same type of capacitor is not available, use one with better dielectric characteristics...also do not substitute polarized for non-polarized parts. Capacitors with a higher working voltage maybe used, UNLESS the replacement is an electrolytic." (Emphasis is mine.:rolleyes:) No explanation is given. Is there ANY rationale to such a statement:confused: (barring physical size limitations?)
     
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    None whatsoever. The 1968 Handbook is W---w---w---w---w-rong! Horrors and gasp!
     
  3. VK4HAT

    VK4HAT Ham Member QRZ Page

    Depends on the circuit, if its a filter or tuned circuit, the ESR of a electrolytic might be to high for the circuit to function right or be efficient. 1uf ceramic or 1uf electrolytic might be the same value but you would never use the later in a filter circuit, even if both are rated for 450V.
     
    US7IGN likes this.
  4. AI5DH

    AI5DH Ham Member QRZ Page

    Lower voltage electrolytes have lower ESR. Thus you want to operate as low of voltage as possible to operate in the capacitor's maximum effectiveness range. For example, the DC filter caps for 12-volt DC power supplies are rated 18 to 20 volts. Just enough for ripple and spikes.

    An equal capacitance but higher-rated operating voltage will have a higher ESR and impede the performance with the additional resistance.

    Electrolytic caps s are sloppy and unstable used where actual capacitance is not critical. They need to be operated in their peak voltage performance range of about 60 to 70% of the specified voltage rating.
     
    Last edited: Sep 15, 2021 at 5:54 AM
  5. SM0AOM

    SM0AOM Ham Member QRZ Page

    Exactly.

    When asking the electrolytic capacitor manufacturers for advice about what voltage rating
    to use in order to get optimum performance and life, they say "have a 25 or 30% headroom between the operating voltage and the voltage rating".

    I had to investigate this working with industrial electronics 40 years ago, and the rationale was that working at severe under-voltage may not form the dielectric oxide layer properly which increases ESR and reduces capacitance.

    Besides, electrolytics have such bad characteristics with regard to linearity, stability and dielectric losses that they are not meaningful to use in tuned circuits or other frequency-determining circuits.

    73/
    Karl-Arne
    SM0AOM
     
  6. KV4PD

    KV4PD Ham Member QRZ Page

    I worked in engineering at a photoflash manufacturer in Nashville for many years. Industry standard tolerance was +10/-20% for all the large flash capacitors. We would get samples from the various cap makers and run them through the gauntlet to see if they would be viable. Cornell/Dublier never made it as they couldn't take the temperature ratings (+50C). We used Nichecon for years and they were fairly consistent. Tried several others that couldn't stand the strain until we found a local businessman who had a capacitor manufacturing plant in China. Richie capacitors. They never failed and we hot-rodded them up to 525 volts (as opposed to 500) to see the results. They never failed. We never used anything else at that point. They are still going strong today.

    Paul Buff passed in March of 2015, but I will always be grateful to have worked hand-in-hand with him on some truly genius concepts.
     
  7. WA9SVD

    WA9SVD Ham Member QRZ Page

    I (sorta) understand, and I'd never even THINK of using electrolytic caps in TUNED circuits, particularly because of their WIDE "tolerance." But many manufacturers seem to use 16 Volt caps in radios, because they are supposed to operate at or below 13.8 Volts, +/-. 10%. But that leaves little "safety" factor. Would it really be so bad to use 25V, or even 35 Volt rated caps?
     

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