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 Post subject: "Non-Inductive" Electrolytic Capacitors
PostPosted: Dec Sun 03, 2017 4:23 pm 
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The schematic for my Zenith B513 (a 1959 AA5) contains the following notation:

"Use only Zenith type non-inductive electrolytic condensers for replacement."

I'm kinda new to this hobby and have never heard of this term. Is inductance a concern with modern electrolytic caps?
The schematic says to add a .047 cap (at a designated location) if other types of electrolytics are used.


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 Post subject: Re: "Non-Inductive" Electrolytic Capacitors
PostPosted: Dec Sun 03, 2017 4:43 pm 
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Real components are never a pure resistance, capacitance, etc. There are a minimum of three ways that a capacitor can be non-ideal: inductance, shunt resistance (leakage), and series resistance. I have been in design environments where every electrolytic cap was bypassed by a small ceramic or mica cap. This is especially important for RF circuits.

Inductance: even a short length of wire has inductance. Imagine two points in a circuit 1/4" apart. Install a large electrolytic there and you might wind up with a one-turn loop ~1" in diameter. At 10MHz or so, that would not be a good thing. Put a small ceramic cap directly between the two terminals and the problem is solved.

I don't know what makes a capacitor inductive except for it's size. But...eg...a high ESR would mean that net impedance at certain frequencies would be higher.

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 Post subject: Re: "Non-Inductive" Electrolytic Capacitors
PostPosted: Dec Mon 04, 2017 1:55 am 
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On some of the RCA models in the mid-30s there was a cap on the order of 0.1 in parallel with the larger filter caps. Along the way they sometimes got clipped out because "why would you need 0.1mf in parallel with 8mf?" Well, they served for circuit bypassing back to chassis ground because the old can electrolytics didn't do that very effectively. Without the 0.1 the radio might misbehave in various ways like oscillation, etc.

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 Post subject: Re: "Non-Inductive" Electrolytic Capacitors
PostPosted: Dec Mon 04, 2017 5:32 am 
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I always felt it was good practice to add a RF bypass cap across any filter cap that was used to bypass purposes as well as filtering. Caps can go high ESR, and unwanted RF feedback paths can develop.

Pete

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 Post subject: Re: "Non-Inductive" Electrolytic Capacitors
PostPosted: Dec Mon 04, 2017 2:25 pm 
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The technical term is ESL--equivalent series inductance. It is the inductance that is inherent in the leads, foils, and structure of a capacitor. Electrolytic capacitors typically consist of a piece of pure aluminum that is etched to give it a greater surface area, then anodized to create a dielectric layer. This is wound up in a spiral, placed inside a paper or phenolic separator, then placed inside an aluminum can. A lead is attached to the foil, which becomes the positive terminal. When the can is filled with electrolyte, the connection between the outside of the anodized layer and the metal can is completed, forming the negative terminal.

Now if they only used one lead attached to one spot on the positive foil, as was typical in cheaply made capacitors, the current has to pass through its whole surface of the foil from that one spot. This adds inductance in series with the cap. But there are various techniques such as attaching the lead over the whole length of the foil, in multiple points along the foil, etc., that will spread the current over the foil and lower the inductance.

What Zenith was basically saying is they used a carefully made electrolytic capacitor to eliminate a bypass cap. If you use somebody else's electrolytic, the radio may suffer from inadequate bypassing so then you have to add a bypass cap yourself. However, modern electrolytics are much smaller than the old ones and that alone means less ESL--not to mention the fact that manufacturers today use all sorts of techniques to reduce inductance so their HF specs are better. Practically nobody builds classic analog power supplies with 60-Hz or 120-Hz ripple any more; just about everything is switch mode at audio frequencies or higher. The radio will probably work fine with modern electrolytics even if you do not put the additional bypass cap in. In fact it would be an interesting experiment to try it with new electrolytics, then see if putting in the additional cap makes any difference at all. Symptoms of inadequate bypassing could be things like squealing, howling, "birdies" as you tune across the band, hum, or noise.

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 Post subject: Re: "Non-Inductive" Electrolytic Capacitors
PostPosted: Dec Mon 04, 2017 5:33 pm 
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Thanks guys for the replies. The information was very helpful. I think I'll recap and then tack in the bypass cap to see if it makes any difference.


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