Re: Apple IIgs power leads (was: Re: 911 Fatal Error on TransWarp GS?)

[mjmahon@aol.com (Michael J. Mahon)]

Rubywand reported:

>     We have a spare IIgs (from relatives who didn't use it much); so, I
pulled
>the power supply to do some tests.
>
>     The spare IIgs is a little later model. Although the power supply is the
>original complete with the original leads, it comes in a brushed aluminum box
>(instead of the steel one used for earlier versions).  Otherwise, it seems to
>be the virtually the same. (The connector is slightly different.)
>
>     Direct measurement of the voltage drop across the +5V lead at 3 Amps gave
>just 0.025V. Figuring a corresponding drop (about 0.013V) across the two
Ground
>leads results in a total drop of roughly 0.04V. (For 4 Amps, the total was
>about 0.06V.)
>
>     This pretty well supports your assertion that there is no way the drop
>through the leads at 3 Amps could be anything like 0.3V. Instead, it must come
>from an error in regulation. Heavier leads do, mostly, eliminate the drop.

Actually, the lead resistance has only the IR drop effect on supply
regulation.  The "pure" supply regulation should be measured at the
terminals where the sense voltage is sampled--essentially the same
as the power terminals on the power supply circuit board.

At these terminals, the load regulation can be measured by observing
the voltage drop with increasing load current.  If the load regulation is
constant over the current range, then it can be characterized as an
effective power supply resistance, which is in series with the resistance
of any leads to the load that go beyond the sensed terminals.  Since
you observed only a 40 millivolt lead drop at 3 A, the power supply
load regulation is certain to dominate the equation.

(BTW, since the leads are a linear resistor, their resistance is not a
function of current (until they begin to get hot!  ;-), so the voltage
drop at 4A must be 4/3 times the voltage drop at 3A.  Any excess
voltage drop at the load end would be due to a regulation drop at
the power supply terminals.)

>     By the way, this particular power supply was very solid (at 4.9V) up to
>beyond 4 Amps. Since much of the noise in a system is in the RF range, it's
not
>clear that solid DC performance equates to significantly lower noise across
the
>PS and its leads.

The leads would have little effect on high frequency performance, since
their inductance would not be much different from heavier leads, and is
tiny in any case.  The microhenry or so of lead inductance would only
be an issue for frequencies over 50MHz, and even a 0.1 uF decoupling
cap near the power lead is sufficient to "short out" 50 MHz.

The same is true for lower frequencies, assuming that the on-board
electrolytic decoupling cap is good, and that the output filter cap(s) on
the power supply are good.  The lead inductance is a non-issue for
practical purposes (as scope measurements would confirm).

The conclusion, then, must be that heavier leads can only reduce
power supply voltage drop by 40 millivolts at 3A, even if made
of superconductors!  This level of voltage change is irrelevant if
the power supply is functioning correctly, for all reasonable loads.

The tiny capacitance of the leads can only help (insignificantly)
the high frequency decoupling, and the tiny inductance of the
leads is rendered insignificant by even minimal RF bypassing
on the main board (where ferrite beads and small decoupling
caps are extermemly effective at preventing 50MHz+
frequencies from propagating along the power supply leads.

>     It happened that, during the heavy loading, a capacitor blew-- POP!!!
with
>billows of sweet smelling smoke. Turned out to be one of the .22uF 200V caps
>used in the input filtering; so, the blow-out probably was going to happen
>whatever the load. (Replaced both caps with new .22uF 400V caps.)

Ahh...love that smell.  ;-)  But I think you mean 22 uF caps, not 0.22.

Since they are used in a voltage doubler circuit in a 120VAC supply,
the line current must flow through the series cap, heating it up as a
function of its ESR (Equivalent Series Resistance).  I'd bet that it was
this cap that blew under overload.  One of the best diagnostic tests
for an aging electrolytic capacitor is to measure its ESR--it invariably
goes up with age, making it less effective as a filter and making it
run hotter if AC current runs through it.

The excess heat can be sufficient to boil the electrolyte, causing
the cap to explode (slowly--pffffffft, or rapidly--BANG!), and
releasing the characteristic odor of boiled electrolyte.

>     Anyway, I've revised the FAQs. Thanks for your help!

Thank _you_ for doing the experiment.

-michael

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