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Last drinks - And a tale of discovery

December's Numberwatch brought to mind a magnificent discovery that I mad in the 1990's.

As the proud owner of a Psion 3a and a Nokia mobile phone, I was frustrated at the need to carry two power adaptors to save battery use in both devices.

So I made an adapter which'd "near enough" step down the voltage from the 12V Nokia's adapter to near enough; the Psion's requisite 10V. Being a sneaky fellow; I thought I'd drop the voltage down using three silicon diodes (1N4004). No resistors.

Expecting 10V at the output on open circuit, I was surprised to discover the same voltage as at the input; zero voltage drop through 3 diodes. If there'd been anybody nearby, they have hear the gears turning between my ears. This was not what the text books said would happen. How dare the world throw these realizations at an innocent Mechanical Engineer?!

After a lot of thunking, I considered that the charge was migrating to the cathodes and, unable to be "lost" at the open circuit, simply sat there in an ticipation. Sure enough; when I loaded up the circuit with a several-kOhm resistor, the voltage dropped to the nominal level.

The gears were still whirring and I grabbed a green LED out of my bits box and soldered it in parallel with the diodes, providing a power-draw indicator that did indeed flicker as the Psion used more and less current during processing and communications. The heavier the current draw, the greater the voltage drop; all still well within that nominally available from the power adapter.

Lessons learnt: Diodes aren't just diodes in practice.

P.S. Have a wonderful New Year. Live long and prosper.

Re: Last drinks - And a tale of discovery

Interesting comment. The diode, of course is a non-linear device; therefore theorems such as Thévenin do not apply. Nevertheless, it is also not active, so at zero current the voltage drop is zero. You did the right thing by providing a current path through a by-pass resistor. However, do not leave it plugged in, as you will be contributing even more to dangerous global warming!

Re: Last drinks - And a tale of discovery

Ah! the good old days when a power supply was simply a power supply with certain defined performance characteristics. Simply assess the usual parameters and if nothing too different one power supply is much the same as another. I never had any trouble using whatever power supply came to hand (with matching adaptors, that is)for my laptops.

For some reason the modern computer power supply now includes a signal wire which identifies the power supply to the computer.
This may (or may not) have some justification if there are subtle but important power supply needs differing from one laptop to another and dangerous, in some way, if a perfect fit is not available.
But it seems the signal connection is fragile in some laptops/notebooks/etc. and users get messages telling them the power supply is not recognised and their battery will not be charged. A situation which gets progressively worse (see Dell forums for a measure of how big a problem this is for some users).

Whether this applies today to phones and IPads I wouldn't know but I wonder to what extent there is something devious being practised by the manufacturers e.g. as per the printer manufacturers.
Try using a refilled ink cartridge or a non-proprietary ink cartridge and many printers decide to get difficult and either not recognise the cartridge or refuse to report the contents. Perhaps this is intended to ensure we only use batteries and chargers supplied by the manufacturer and not purchase "compatible" equipment from China at rock bottom prices.