[R-390] Replacement Capacitors

[Bob Camp]

Hi

One more wrench:

Back when R-390's were running down the production line, there was a significant difference between dielectrics in terms of cost. These days, not so much in some cases. You need to look at what they are selling you. 

If you go to Mouser and do a sort on 0.1 uf 1,000 V ceramic caps, leaded, in stock,  you get 11 matches. The lowest cost part is from AVX and it's an X7R. You get to number 3 on the list before you hit a Z5U.  All the rest of the stocked parts are X7R's. I'm not going to run out and pay $57 each for the ones down the list, but that's another issue. 

Off to Digikey (same sort), 5 parts in stock, all X7R. 

Switch to 0.01 uf at Digikey. Toss out the ones I need to buy a full reel of.  The first few parts are indeed unusual dielectrics, so take a look. Ok, that's -80% temperature characteristic at 85C. Next one is a -60%. First X7R is number 7 on the list. It's twice the cost of the -80% part. To get the same capacitance hot, I'd need five of the cheaper parts. If I bump up to the next one, I'd probably get away with two. At that point the "cheaper" part is actually more expensive.

Back to Mouser and 0.01 uf's. This one gets a bit tough to dig through, there are a lot of really poor capacitors on the list. The cheapest semi rational cap is about 1/2 the price of Digikey's X7R's. It's a Z5U, so you might have to use two. Mouser does not list any X7R's at 0.01 uf in single pieces. 

Off to Jameco. Yup, I can get some amazingly cheap caps. House brand parts only, who knows what the characteristics actually are. Not going in my radio…

Back in the day, X7R's were 10X the price of a Z5U. X7R was mil spec, and X5U went in TV's. They may still be if I'm buying billions of parts. From the places I'm likely to get them today, there's not much (if any) cost penalty to the X7R's. If I'm not spending more on them, I see no reason not to get the good(er) stuff. 

Bob

On Apr 26, 2013, at 3:55 PM, Charles P. Steinmetz <charles_steinmetz at lavabit.com> wrote:

> Bob wrote:
> 
>> There is an interesting tradeoff with the otherwise "really really good idea" ceramic caps. Even if you go to the X7R dielectric (which you should), their leakage is higher than most plastic parts. It's not something I would worry about, there's also leakage from dust and that spider web there. I don't believe the insulation resistance leakage is going to mess anything up, either with the ceramics, or between varieties of plastic.
> 
> As you say, nothing to worry about.  The starting leakage of ceramic caps is much too high to have any practical effect in bypass applications.  As long as the leakage does not increase over time, as it does with paper caps, there is no problem.  All "Class 2" disk ceramics (even those with lesser specifications than X7R, e.g. Z5U) have a well-proven track record in this regard.  If a radio has been underwater for a long time (months to years), the ceramic caps may get leaky enough to pose a problem.  Otherwise, they are very reliable.
> 
> I agree that X7R is the dielectric of choice for supply bypass capacitors, given a choice.  The reason to prefer X7R over other Class 2 ceramic caps is that they hold their capacitance value better at high temperature, over time ("aging effect"), with higher applied voltage, and at higher frequencies.  For example, an X7R cap is specified to be within 15% of its 25C value from -55C to 125C, while a Z5U cap is specified to be within +22% and -56% of its 25C value from 10C to 85C.  An X7R cap may decrease in value by ~10% over time, while a Z5U cap may decrease in value by ~20% over time.  However, for real-world use as a supply bypass in boatanchors, one can offset these effects by using two Z5U caps in parallel in place of an X7R.  (One Z5U of double the value should also work, but will bring the self-resonant frequency ("SRF") down by about an octave compared to the two smaller caps in parallel.)  The leakage of X7R and Z5U caps of a given value and voltage rating is approximately the same, and is entirely negligible in supply bypass applications.
> 
> Best regards,
> 
> Charles
> 
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