Allow me to tag a "little bit" on to this thread.
Bear in mind that to charge a 3 volt battery, you would need to put more that 3 volts across it. Basically, best way to remove doubt is to measure what the official Sony PSU is putting into the player when under the load of charging. Yes, reasonably new units should be pretty well protected, though I can understand a reluctance to shove unknown quantities into expensive gear.
The newer Sony PSUs, as with most around now, appear to be switch-mode. That makes 'em smaller and cheaper than their transformer-based counterparts. Another advantage of that is that they are multi-region (I bought my NW-HD3 in the UK, and can use the same charger here in Canada. Provided I use an adapter to allow the UK plug to fit in the Canadian socket, of course). If you look on one of these PSUs, it even states "input: AC 100-240V~". Switch-mode has certain other implications to take into account.
Transformer-based PSUs rely on stepping down voltage according to a ratio of independent primary and secondary windings on an iron or ferrite core. For example, a transformer with 1000 primary turns and 200 secondary turns will produce an output at 200 divided by 1000, 20%. 100 volts in yields 20 volts out. Transformers drop power as the load increases because the windings themselves are not perfect conductors, and will be subject to Ohm's law, V=I*R. The current being pulled through the transformer, multiplied by the resistance of the windings, equals the voltage that will lost to that effect, which will be dissipated as heat. Okay, so it's a little more complicated than that, inductive loads, AC and so forth, but that's the jist of it. I shall ignore the bit of electronics responsible for AC-DC conversion for now, though it of course has an effect too (as someone here previously pointed out, a silicon diode, as used in rectification, has a 0.6V difference between anode and cathode).
The idea behind switch mode supplies is that the desired output is created electronically by taking the input voltage and switching it on and off really quickly through a whole bunch of coils, capacitors and other assorted crap. The controlling electronics monitor the output and vary the switching rate when required. If the load increases and the output voltage starts to dip, the 'tronics will pump more juice in, until the required voltage is once again restored. This is why they can generally handle multi region. They just switch the input at a different rate. The switching happens very quickly, which is why switch-mode supplies, like laptop charges, make a kinda squeaky, squealy noise, whereas transformers make that 50-60Hz buzzing sound.
So, where am I going with all this switch-mode talk, and how is it relevant to this, and what the hell am I talking about? Well essentially, switch-mode power supplies, by nature of how they work, are regulated. Not necessarily super-clean, but reasonably consistent under load (at least until the switching process is pushed to the point where it breaks down, or where the current limiter kicks in). The output voltage is rated at 6V on my NW-HD3 charger, and 6V is what ya get, whatever the load.
As for transformers, when they are rated at for example 12V @ 500mA, that means the voltage out of it is around 12V
when under load, ie when you are drawing half an amp from it. When not under load at all, the output is more likely to be around 14-15V. If a transformer-based Sony charger is rated at 5V @ 300mA, that means when under a 300mA load (ie charging the battery), the voltage from the supply should be at 5V (...not that Sony would ever deviate from an established rating convention like that, no sireee...).
As far as this case is concerned, PCs use switch-mode supplies, and the motherboard voltage regulators are also switch-mode, on account of better efficiency and lower heat when compared to linear regulators. If the minidisc is rated at 5 volts, in theory it should be absolutely fine to feed it 5 volts from the USB, provided the USB can supply the required current.
This is a somewhat crude breakdown of the theory, but hopefully may prove of some use to those exploring this avenue of alternative charging methods.
Edited by Loonie, 24 August 2005 - 09:04 PM.
:grin:
