Re: How much strain can a IIGS power supply handle?

[George Rentovich <grent@home.com>]

Hi,
I suspect after reading your post that the problem is right at the connector or
on the mother board. Some of the connector pins no longer have tight connection
do to repeated plugging and unplugging or age/heat. Even the crimp on the wire
may have oxidation over the years. The pins can be removed from the plastic
connector and resprung to make a tight connection. But it is not an easy task
that is for most people.

"Michael J. Mahon" wrote:

> Rubywand wrote:
>
> >     The main limitation on the original PS is the pissant leads that connect
> to
> >the motherboard. These limit the current you can draw to something less than
> about
> >2.5 Amps on the +5V line. You can dram more current; but, by then, you have
> >glitching problems due to noise and low motherboard voltages.
>
> The power leads in question are about 12" long, and are a single 18 AWG
> wire for +5 and two 18 AWG wires in parallel for the ground.  Since the +5
> carries the most current, and is a lower voltage, it is the most perturbed
> by wire resistance, so we can just look at it to simplify matters.
>
> The resistance of a foot of 18 AWG copper wire is 7.5 milliohms.  That
> means that the voltage drop in it is 7.5 millivolts per amp (or the delta-V
> is 7.5 mV per delta-I of 1 amp).
>
> Since the stock IIgs supply +5 is rated at 3A, and a heavy-duty one is
> rated at about 5A, it seems safe to assume that the IIgs load on +5
> is less than or equal to 5A (probably less for most, since the vast
> majority are getting along fine with a stock supply).
>
> This means that the voltage drop on the +5 wire is about 37.5mV and
> the drop on the ground wires (in parallel) is about 18.7mV, for a total
> drop of less than 56mV in the +5 supply.  (Of course, the +12 current
> returning through the ground wires will contribute another amp or two,
> but that will add only another 3.5mV-7mV drop.)
>
> So the total resistive drop from the power supply to +5V loads is
> about 60mV under maximum load.  If we assume that there is a
> delta-I of 2A (probably high), then there is a max delta-V (due to
> the power supply wires) of less than 25mV.
>
> TTL logic is spec'ed for 5V +/-10%, or +/-500mV.
>
> The IIgs board is bypassed with plenty of monolithic and electrolytic
> capacitors, and some inductive decoupling as well.
>
> Therefore, any problems with power supply impedance, at either
> high or low frequencies, is unlikely to be attributable to the wires
> connecting the board to the power supply.
>
> If the relatively small resistance of the power supply leads is sufficient
> to cause a problem, then there must be other factors which are out of
> spec--for example, bypassing capacitors on the main board may be
> defective, the power connector may be somewhat oxidized, the power
> supply +5 voltage may be low (near 4.5V), or the output filter capacitors
> in the power supply may have diminished capacitance.
>
> Replacing the power supply leads with heavier gauge wire is not an
> appropriate solution to any of these problems, though it may appear
> to solve or reduce them for a while--until whatever component(s) are
> actually degraded are slightly further degraded--whereupon the
> symptoms will recur.
>
> I view this as an aspect of the "monster cable" myth.  Electrons only
> know about resistance, and a certain amount of resistance is always
> allowed for by any electrical design, since it is unavoidable.  (Yes, I
> know about inductance and capacitance, and both are negligible issues
> for the wires we are discussing--that's why there are bypasses.)
>
> Reducing resistance below the nominal design level is not worth much.
> Eliminating failures that increase the resistance substantially above
> design tolerances is what is needed.
>
> -michael
>
>  Email:  mjmahon@aol.com
>  Home page:  http://members.aol.com/MJMahon/