Eeyore said:
The guy is totally clueless about hardware design.
Graham
Wot, Jamie or me Graham ??
Peter. As far as whether there is too much current being drawn, the absolute
maximum, which would take the original zener up to its 2 watt limit, is
400mA, which is obviously well within the capability of the 7812. However,
if I had been designing the thing, I wouldn't really want more than about
1.5 watts going west in that diode, (not that I would even have put it there
in the first place ...) so we're talking around 330mA. That would leave
around 4 watts or so in the 7812. Now that is basically nothing, especially
if a small heatsink is fitted, so I don't really see the point in having the
zener there to split the power dissipation. Maybe the 7812 is running
without a heatsink, and putting in a 2 cent zener was cheaper than a 3 cent
heatsink, but either way, unless we are missing something here, it seems
like a nonsense piece of design work to me.
I still don't really follow Jamies point about the zener being placed in a
"high resistive state". Once the zener 'knee' point on its curve has been
reached, that curve becomes pretty much vertical within a very small span of
reverse current. With the full rail current of 300mA or possibly more
passing through the one in this bit of kit, it should be in it's lowest
resistance fully on condition.
The point about a poor smoother, and excess ripple resulting in a low
average measured starting voltage is ok in principle, but if the cap was
really in that virtually open circuit condition, I would have expected to
have heard hum on the amp's outputs. The OP said that it was working ok.
I'm interested in the failures of these zeners, and how they were detected
by "smell". In the case of the one that had gone short, it can't have been
the diode itself that was smelling, as its dissipation must have dropped to
zero. That must mean that any power that was being 'lost' in it, was passed
on to the 7812 to lose, so was it this that was smelling to give the clue ?
If it was, then I am surprised, because if its dissipation had risen to the
point where it was smelling, I would have expected its SOA thermal foldback
to have kicked in. As for the zener that had gone 300 ohms (both ways)
leaky, it's hard to see how that circuit even carried on working, as all
voltage input for the 7812 would have disappeared across a 'resistor' of
this value placed in series with the regulator input, with just a few 10s of
mA being drawn.
It's all very curious. Given that the zeners have failed in normal service
in two cases, my inclination would be to short them out and let the regs
have the full 22v up 'em, and add a couple of heatsinks. Those circular
crinkly ones actually work very well, when the gap in the circle is forced
down over the devices's tab. I learnt that from a commercial board that I
repair, that has 3 TO220 Darlington motor driver transistors on it, heatsunk
in this way.
Arfa
