I recently bought a somewhat more expensive, good 7.5v wallmart for my Anushri. This one was not an ‘el cheapo’, but ‘made in Germany’ and ‘allowed for medical applications’. The idea of using a 7.5v power supply is that the regulators don’t get too hot, even though that does not have to be a problem (heatsinks etc.).
I already had some cheap 7.5v power supplies, which worked well, but this fancy one did not ! That is, it took about a minute or so before the Anushri started to become usable …
Out comes the multimeter, and the power supply turns out to be well made: it puts out 7.6 to 7.7v (loaded).
Measuring the ‘el cheapo’ 7.5v ones (with the Anushri connected, switched on, and working well) revealed (as has been said earlier in this forum) that these put out quite a bit more than promised: 10v !
But these 10v produce a nice +5v and -5v on the two rails inside the Anushri, whereas the 7.6-7.7v results in a +5v on the positive rail, and -4.2v on the negative one.
Hmmm. A little adding up of the voltage drops from the power connector to the rails is the answer, I think.
The 1N400x protection diode has a drop of around 0.7v, and the 7805/7905 regulators around 1.5v to 2v.
That means that the +5v for the positive rail is (barely) reached after these two components.
But for the negative rail the LT1054 has to be taken into account as well, which not only switches polarity, but looses almost a volt as well.
For my 7.7v (actual) power supply, the protection diode takes away 0.7, so the LT1054 gets fed with +7v, and produces about -6v (measured). The 7905 turns this into -4.2v …
So I guess one should not get a 7.5v power supply that actually turns out anything below 8.5v ! Most of the cheap 7.5v don’t do this anyway, so are fine, but for ‘good’ power supplies one should stay above 9v for a good negative rail voltage.