Apple III Power Supply: Success (I think...)

[Dave McGuire]

On Jul 28, 2007, at 3:59 PM, Alexandre Souza wrote:
>> I opted to do a wholesale replacement of the electrolytics in the  
>> power supply, since I was likely going to be removing all of them  
>> to test them anyway and there's only a handful to take care of.   
>> $9 worth of parts later, the issue seems to be resolved -- the  
>> system powers up and runs diagnostics. (Though it keeps cycling,  
>> so I don't know yet if that's another problem or normal behavior :)).
>
>    In SPSUs it isn't always the wise way. Some of your old  
> capacitors surely are low-ESR types (you can spot them because they  
> are 105º capacitors and the high-esr ones are 88º) and it can mess  
> the regulation of your power supply. Have a multitester around?  
> Know how to use it? Measure the voltage of the outputs of the PSU  
> and see if everything is ok. If you still have the old capacitors,  
> take a look if any of them has 105º written on that.

   Whoa, hang on there Alexandre...I've seen no correlation  
whatsoever between ESR and maximum operating temperature ratings.

   It is definitely true that, in many parts of a switching regulator  
circuit, low-ESR capacitors are essential...Not just a nicety, but  
absolutely essential for the circuit to function.  This is  
particularly true of the output capacitor(s).  It is also true that  
many switching regulator circuits run hotter than some other types of  
circuitry, so the use of high-temperature-rated capacitors is often  
important in such designs.

   However, I respectfully submit that low-ESR and high temperature  
ratings of electrolytic capacitors for use in switching regulator  
circuits is essentially a coincidence.  While I've not specifically  
researched this, I have designed quite a few switching regulators,  
and during the component selection process I've not seen anything to  
suggest that a high temperature rating implies low ESR.

   Typical symptoms of excessive ESR in a switching regulator's  
output capacitor is excessive ripple and poor line regulation.  These  
problems can be severe (up to and including SMOKE) if "ordinary"  
capacitors are used without paying attention to their ESR.

            -Dave

-- 
Dave McGuire
Port Charlotte, FL
Farewell Ophelia, 9/22/1991 - 7/25/2007