From: Humphreys, David (URB) (david.humphreys_at_honeywell.com)
Date: Thu Oct 16 2003 - 06:21:39 PDT
Mark:
> Voltage is the critical part. If that is correct you have a
> good start. The Current Rating is the maximum that the PSU
> can output safely (without melting) and should be at least
> the amount required, if not more. If it is above the rating
> required the PSU MUST have a Voltage Regulator to stop the
> voltage increasing under decreased load (Power in Watts being
> constant and a factor of Voltage and Current). This is where
> most cheap PSUs fall down, they are not reculated so a 12v
> 900mA PSU outputs 12v at 900mA load or as much as 20v at
> around 300mA. The vast majority of electronic computer
> equipment has a regulated power supply to prevent it being
> blown up when the load requirement drops.
>
> So there you have it :)
For the most part, you are correct. There are a number of factors to be considered when choosing a PSU for an
application.
Ordinary 'wall-wart' type adapters are no good. They are little more than a transformer, rectifier and large value
capacitor. The AC is stepped-down, rectified then crudely filtered by the capacitor. The transformer usually has very
poor regulation. This means that the voltage sags under load. In order to compensate for this, the transformer is wound
so that its' output is correct at the rated load, BUT smaller or no-load conditions have a much higher voltage.
If you plug one of these PSU's into a Newton, the initial voltage will be much higher than 7.5V and will cause the
transorb (a protective device) across the input to conduct. This shorts out the input, draws a big current and
(hopefully) blows the PSU fuse.
BUT, cheap-assed wall-warts seldom have a fuse (although some do) and now the PSU is free to push as much current as it
can through whatever it is connected to (i.e. your Newton).
Result: instant fry-up.
The Newton PSU, contrary to popular belief, is nothing special. It is simply built for purpose. It is a SMPS
(switched-mode power supply). I am not going to get into an explanation of how this beasty works, save to say that it
has a stable output under varying loads.
Specifications:
7.5V @ 1.2A
Tip +
Ring -
Barrel dia. .190"
Pin dia. .065"
It is not sufficient to 'just stick a regulator' on the output of an unregulated wall-wart. Unless you design it
carefully, you run the risk of the regulator running out of headroom as the supply is loaded. Basically, a regulator
needs a certain amount of voltage difference between its' input and output to work. Loading the PSU could drag the
voltage down to a point where it stops regulating and all sorts of crazy things happen.
> I'd have thought, from what my dad said, that a Laptop Power
> Supply woulkd have to be well regulated s othat when the
> battery charger stopped charging it didn't blow the mainboard
> or power board due to the increased voltage resulting from
> the drop in load.
Well this is a whole subject in and of itself. Laptop batteries are not just batteries but a combination of batteries
and electronics. Ever wonder why they are so expensive? The batteries themselves are a fraction of the cost. Disregard
the sales mark-up and the cost lies in the charging electronics that can be found inside most modern battery packs. In
reality, the battery charger circuit is separate from the other stuff to minimize problems.
> >> The connector is not a major issue, as long as I get the
> polarity right.
Actually, it's right on the label tip+, ring-.
> So is anyone willing to divulge this information (2 mins with
> a multimeter will do it) or am I gonna have to guess ;)
With a SMPS, this will not always work. You see, SMPS's like to see a load. That is to say, you need to connect
something to them to make them work correctly. The impedance of a multimeter may not be sufficient to get the switcher
working and so may give you erroneous readings. Connecting a 10k resistor across the output would help here.
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