3) There is
resistor in parallel with the regulator.
MITS did this routinely and it wouldn't surprise me if they did this
on the 680. In particular, I remember that the +5 supply for the 4K
Oh, right... The OP says he has the schematics, do they show any such
Yes. There is a 15 ohm, 2W resistor from the input to the output of VR-1,
a 7805 regulator.
OK. Let's analyse that circuit. A quick look at a 7805 data sheet will
convince you it can only pull the output toiwards the input voltage, not
towards ground. So if the output is already higher than 5V (in this
case), the 7805 wioll do nothing.
Consider the resistor on its own. The OP said there was an input voltage
of 11.5V. SO there's 6.5V (11.5-5) to be dropped between the input and
output -- without the 7805, that means the resistor is droping 6.5V and
carrying 6.5V/15R = 0.4333A.
If the load drwas less than 0.4333A, the output voltage will be higher
than 5V. If it drwas more, then part of that current will be carried by
the 7805, and the output will remain at 5V (without the 7805, of course,
the output votlage would be less than 5V in this case).
Light bulbs are an approximatley constant-current device. A 12V car bulb
that draws 0.433A (at 12V) is a 5.2W bulb. If the load is significantly
less than that, then of course the output voltage will be too high.
Danger Will Robinson.
I could understand mentioning Heath Robinson here...