A Kit Change to a Tantalum Capacitor
After the TiCK keyer came out I realized that the 8 pin PIC TiCK chips used were perfect for keyers. I eventually came out with the PK-Basic keyer kit and have used PICs ever since. Eventually as new varieties of the PIC 8 pin chips were introduced I also updated the PK-Basic to the PK-2, the PK-3, the PK-4 and finally the MegaPK-II keyers. But I also made another change starting with the PK-2: the addition of a regulator chip, the LM2936 from National (now TI).
Initially I used a 47 uF electrolytic cap on the output of the regulator for stability. It was sized not so much on the cap value as for the ESR. This was mentioned in an earlier version of this app note:
http://web.mit.edu/sp.716/www/datasheets/AN-1148-1.pdf
Using the 47 uF cap continued with the PK-3 keyer. But one problem: the relatively large size of the 47 uF cap meant that after the keyer power was turned off it could take a fairly long time before the PIC chip power would decay to allow a "good" powerup reset. With the PK-2 this wasn't as much of a problem because users might want to leave the power on so they didn't lose their memories and settings. But the PK-3 had an on-board EEPROM chip (actually a separate chip) which allowed the PK-3 to retain the memories and settings even if powered down. The way around this was to hold the mem switch down after a power down to discharge the 47 uF cap quicker.
With the PK-4 I eventually transitioned to a Tantalum 2.2 uF cap. It had the requisite Goldilocks ESR (not too low, not too high) but was 1/20th the capacitance so the power cycling problem was much less of a big deal.
Why didn't I use Tantalum caps to begin with ? They are more expensive than electrolytic caps. They are polarized but unlike electrolytics, Tantalum caps generally aren't as well marked. So there were reasons for using Aluminum electrolytics but I've never gotten any problem reports since switching to Tantalum caps. BTW, a Tantalum cap is also used in the Bug Descratcher III kit.
The polarity thing with Tantalums was a problem to the extent that 3 legged caps were sold, the center was negative and the outside pins were positive. It couldn't be put in the wrong way. Here is one removed from an old PC motherboard:
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| a Kemet 16 volt, 10 uF Tantalum 3 legged cap |
Another tidbit having nothing to do with Tantalum caps: a paddle seller was interested in using the PK-2 chip in an integrated keyer + paddle design. But for some reason he hated the fact that I started selling the PK-3 kit and discontinued the PK-2 kit (although I offered to keep selling the PK-2 chip to him). I think it had to do with the fact that the PK-3 was a more expensive chip and thus a more expensive kit (which is understandable considering that it was in effect a multi-chip module). Some times the fish wriggles off the line - that's the way it goes :)
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
