I bought a 2F capacitor from Fry's on sale. It's not a nano-carbon or aerogel cap, I think, just a really big electrolytic. It was listed as "2.0 FARAD DIGITAL CAPACITOR . Digital LED Voltage Meter CA170 #5477179 $29.99." The capacitor wouldn't hold a charge very long, and the voltmeter (the "digital" part) didn't work. Reverse polarity made an LED glow and a beep from the attached sounder, but nothing from the 7-segment display. I took it back and got another one, but it was the same. So I removed the DVM and the capacitor holds a charge pretty well.

The DVM has an "EM78P156ELPJ-G" 18-pin DIP, which appears to be an Elan OTP 8-bit microprocessor, a variation of this one. There's a 4Mhz crystal, and a scope shows no crystal activity; the oscillator is in the uP. The 7805 TO-220 shows 5V. I suspect the uP, even if it weren't programmed, would oscillate OK so there must be some other problem.

That's probably why they were on sale. A web search on "CAPI70" or "CAP170" shows nothing, nor does "DP Audio" (on the box) or "Divine Electronics" (on the can).

Update: In the intervening years, I've received a number of great comments and email messages on this post. One comment pointed out that the voltmeter needs to be powered explicitly. Craig AE6RR contacted me and said that he's tested that comment, and indeed that's the case. Here's what he said:

I tried connecting the + terminal to the small screw on the top that appears to just hold the plastic cover on and yes the relay clicks, the display lights and the voltmeter works. Maybe you are supposed to connect it to ignition switched voltage to avoid running your battery down? The package claims "relay included" but the manual doesn't say anything about it or the voltmeter. Also, the 1K resistor supplied is too high of a resistance to charge the capacitor due to the current draw of the voltmeter circuit. It will never get to 12V.

- posted by Leigh @ 20:34 z