wrote in message
oups.com...
On Aug 24, 8:28 pm, "Arfa Daily" wrote:
"Franc Zabkar" wrote in message

...



On Fri, 24 Aug 2007 17:12:34 GMT, "Arfa Daily"
put finger to keyboard and composed:

wrote in message
roups.com...
On Aug 24, 4:11 am, "Arfa Daily" wrote:
I'm pretty sure that I did see a rectangular one once, but whatever,
if
it
is 0.1F at 5.5v, it is *not* just any old cap put there to stop the
regulator oscillating - it is definitely a goldcap for backup.

Yes - the output stability cap would be something like a 0.1uF
ceramic.

Are you sure that there is not a diode between the regulator's
output,
and the cap?

Hmm, I thought it was a direct connection (via jumper), but I'll
double
check that. There is a "+5V" silkscreen label in that general area,
which
brings up another possibility - perhaps that regulator is supposed to
be
a 7805. I'm working on getting the schematic, that should clear
things up.

TM

I guess it will. I have seen 7806's used with a diode and goldcap. The
diode
drops the rail to around 5.3v, and also ensures that there is no
reverse
leakage path from the backup cap through the regulator to ground. Most
system control micros are quite happy with a rail of just over 5v, and
lots
of hifis have got such a voltage on the 'nominal' 5v rail.

Arfa

A 7805/7806 in a TO220 package would be rated at 1A. How long would a
0.1F cap be able to supply such a load, or even a 100mA load?

Then again, maybe the 7806 regulator supplies the bulk of the 6V logic
while a diode-isolated gold cap powers some volatile RAM?

- Franc Zabkar

That's the usual arrangement, which is part of the reason that I asked
about
the diode. Often it's just the system control micro that has memory
maintenance power supplied to it from the backup cap via a separate pin.

Arfa

Spot on. I looked at the board again, and found two branches of the
circuit from the 7806 output pin. One goes through a 4.7ohm resistor,
then a diode, then to the positive pin of the 0.1F cap and a pin on
the connector to the front panel. The other branch goes through a
diode, then to a BA6208 (driver for the volume pot motor?) and perhaps
the front-end daughtercard.

Looking at DigiKey, it appears that Panasonic (Gold), CooperBussmann
(Aerogel), and Elna all have comparable supercap replacements. Any
recommendations? I'm considering paying extra for the Elna
DH-5R5D104T, since it's rated to 85C.

Thanks again,
Tony


Of those three, the only one that I've had personal experience of, is the
Panasonic, but they seem to be a component that might well be 'the more ya
pays, the better ya gets'. In any event, the higher temperature rating cap
is probably not a bad idea. I've never really looked at the temperature
rating of them, so I've no idea what the 'standard' is. 85 deg is certainly
the norm for 'bog standard' electrolytics, with 105 deg being 'high temp'
ones. When you do come to replace it, I might feel inclined to replace any
0.1uF and small electrolytic that might be glued across the output of the
7806, just in case they have failed, and the goldcap is working hard to
suppress any tendency to instability of the reg, as well as carrying out its
primary backup function. I don't imagine that they would be very good at
handling high frequency ripple, and they might well warm up, which might be
what you saw in the original being split open. It would be worth checking
the feed diode as well, to make sure that it has not failed short, or gone
leaky, resulting in greater than the max 5.5v rating of the cap, being
applied across it. I think as a final check before wrapping it up and
calling it a draw, I would just monitor the (power on) voltage across that
cap for a couple of hours, just in case there is any issue with the reg
output voltage, once it gets thoroughly warmed through.

Arfa