M7891-DC 2a fuse on +12? fixed!

From: Pete Turnbull <pete_at_dunnington.u-net.com>
Date: Thu Feb 5 12:32:18 2004

On Feb 5, 12:11, Brad Parker wrote:
> To follow up on my own post, replacing the fuse created a *hot* 12v
> regulator. My DVM indicated that something was shorting Vdd.
> Figuring it was a cap, I started pulling the + lines of the big caps.
> On the second one the short went away.
> Turns out one of the large Sangamo 47uf caps across Vdd was a dead
> short. I removed it and now the +12 is happy and the board works.
> (I'll replace it in my next digikey order)
> Do old electrolytic's short out? The MS11 parts list just say "AL
> which I assume is aluminum electrolytic. For some reason I was
> it would be a tantalum (based on it's silvery look) but now I'm
> not. I think I'll replace it with a nice axial lead tantalum.

What you described is the classic symptom of an aged electrolytic that
needs reformed. In an electrolytic capacitor, the aluminium foil forms
one electrode (the anode), the electrolyte (gel) is the other
(cathode), and the oxide layer on the foil is the dielectric
(insulator). Unless the capacitor is charged, the oxide dissolves into
the electrolyte over time, and no longer insulates. The cure is to
pass a small current to reform the oxide layer, and gradually build
this up until the capacitor can withstand its normal working voltage
without much leakage current.

Cylindrical caps in a metal can are more commonly aluminium rather than
tantalum; tantalum caps are more commonly made as beads of sintered
tantalum. The principle is the same, though: large surface area in a
small space, with a metal anode, oxide layer as dielectric, and
electrolyte as cathode.

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Thu Feb 05 2004 - 12:32:18 GMT

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