On Wed, 09 Jun 2010 14:20:22 -0700, PlainBill47 wrote:

The TEA1522P incorporates the power FET within the IC. It also derives
startup power from the drain input. Given that it doesn't drain the
mains filter cap, I'd verify no output diodes are shorted,

Output diodes are all A-OK. All around 0.5V forward drop, according to my
DMM.

then replace
the TEA1522P before wasting time with the opto isolator, shunt
regulator, or other unlikely candidate.

Right, fair enough. I don't like blaming ICs for failures without some
proof-of-failure, but in this case I think you're right -- if the FET has
blown open, it's not going to be doing much...

If you MUST fiddle, use a pair
of 9 volt batteries in series to provide startup current via pin 1.
(Unless you have an isolation transformer, DON'T use a bench supply).

Actually, my plan was to test it with the AC supply disconnected, which
would allow me to push a small current into the 5V rail and check that
the shunt reg and opto were working.

Using an internal switching regulator to obtain startup power is
common, and those ICs are prone to failure. This design has gone one
step further, incorporating the switching FET in the package, this
sacrificing reliability in the quest for high efficiency and low cost.

One more note for the lab notebook!

It looks like the chip gets pretty hot too -- there's a lot of
discolouration of the PCB laminate around the pins; the diode pads show
similar discolouration. In hindsight, that was probably a Big Red Flag...

Thanks,
--
Phil.

http://www.philpem.me.uk/
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