New pcb design for S-100 prototype board available
ragooman at comcast.net
Sun Jun 3 11:46:53 CDT 2007
I was trying to convey that point about the filtering, which is one
reason why to have a higher output voltage on the transformer, eg. 10v
versus 8v. The 7805 is capable of running at 7V --provided that-- the
input is rock solid. When designing, it's not acceptable to let a system
run at the bare threshold, it is preferable to have an acceptable margin
to account for tolerances. The LDO voltage regulators (LM2940) are
capable of running at a lower threshold, offer lower dissipation and a
lower dropout voltage. This avoids having to replace the transformer in
most cases. I would still change the filter caps since they're 30yrs
old. As a result, it allows a greater margin for the power regulation
and better stability. They're a drop-in replacement for any of the
7805 and 7812 devices. I make it a point to replace them with these on
any old board I have.
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>> Wrong data. The 7805/lm309/lm323 reguire only 8V. All of the older
>> 3 terminal regulators must have a 2.7V differential to regulate.
>> The yabut, most of the S100 supplies were so poorly filtered that
>> at any load the ripple voltage was excessive and didn't meet the minimum
>> of 8V at the bottom of the ripple trough. A good example of that
>> was the early Altair 8800 before MITS upgraded the transformer.
>> With a 8A load the DC votage sat at 8.3V but the lowest voltage was
>> 7.1V due to the AC ripple. Needless to say that ripple fould it's
>> way to the 5V rail as the regualtor didn't have enough head room to
>> regulate. The fix was a higher voltage transformer or much heaftier
>> filter caps or both.
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