Capacitance

I have learned quite a bit about capacitors over the last few days...

The power supply for our Visioneer 8100 Scanner died a while ago.

I finally got around to cracking it open to see if I could fix it earlier this week.

After a quick examination, I surmised that the problem was that one of the capacitors had blown. The fix seemed simple enough - purchase a replacement capacitor, remove the failed component, install the replacement, and I'd be back in business.

Sometimes, things aren't quite so simple.

The capacitor in question turns out to be a 16 volt, 1200 uF electrolytic capacitor. No biggie, right? Sure enough, there are plenty of options for purchasing said capacitor, including my favorite supplier of electronic components, Mouser Electronics. Problem is, the capacitor in question is an 8x25. As I learned a while back, in addition to the type of capacitor (film, electrolytic, etc.), the type of leads (axial, radial, snap, etc.), the voltage, the temperature rating, and the capacitance (measured in Farads), one must also consider the size (width/length) of the component. This is important for two reasons. The first is that the spacing of the holes for the leads on the circuit board is related to the size of the package. The second is that sometimes (this being one of them) there physically is no room in the case or on the board for a larger component. Well, search as I might, I could not find a 16v, 1200uF, 8x25 electrolytic radial lead capacitor.

As I was looking at the components on the board, I noticed that there was also a blown fuse. Mouser also had a fuse that while not an exact replacement, would fit the bill. I had decided that I would go with a 10x25 cap, and perform some creative engineering with heat shrink tubing to allow me to reposition the capacitor to "fit." Unfortunately, while the total cost of the two components was $0.75, the cost of the shipping was around $6.00. For around $14.00 shipped, I can get a brand new power supply off of eBay, and not have to worry about making an oversize component fit, just to discover that there was another damaged component I had missed.

Oh well, in the process, I did learn some interesting facts about capacitors. The Wikipedia had a good entry on capacitors in general as did howstuffworks, and I found a few good references on using multiple capacitors to yield the desired capacitance when you can't find one capacitor that meets your needs: 1 2 3. I also learned that you can always use a higher voltage or higher temperature capacitor as a replacement.

The good news is that it turns out that the external hard drive (which I only turn on when making a backup) that sits right next to the scanner also uses a 12v DC center positive 1.25 amp power adapter, so, until I get around to purchasing a new one, I can just use that one to power the scanner when I need it.

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Posted by David at March 05, 2006 05:24 PM