power supplies

[H. Dixon]

I've got a couple of AC/DC power adapters that went to various things
that are no longer around. They have various output voltages and
currents. Would these be useful as a power supply into my breadboard?
One of these is 12VDC but 1.2Amps. The others are all in the mA range.
1.2A seems high for experimenting...

I read the voltage on the "12vdc" one and it was reading 15.5 vdc. Is
that typical - for the actual to be higher than what is listed? Also I
noticed that the measured voltage moved around a bit over time by a
few tenths of a volt. Does that mean it's shaky/going bad?

Sorry - lots o' questions but I'm without PS at the moment...the one I
was building from scratch is on hold as I popped the capacitors. Now I
know which axial end is which.

H. Dixon

 

[H. Dixon]

(snip)

Many "wall wart" type supplies are simple transformer,
rectifier capacitor filter designs that are completely
unregulated, so, at light load, they produce considerably
more than their full load rated voltage.  The output also
varies as the line voltage wanders around.  They are quite
useful for many tasks, as long as the lack of regulation is
okay for those tasks, and you don't expect them to protect
themselves from overloads by doing anything more than
failing.  Adding an external solid state regulator (i.e.
LM78xx series fixed voltage devices or the LM317 adjustable
type) can make them even more useful and add some overload
protection.

Something like this? --> http://www.instructables.com/id/How-to-Build-a-Bench-Top-Power-Supply/

 

[Ed]

Something like this? --> http://www.instructables.com/id/How-to-Build-a-Bench-Top-Power-Supply/

Yes, like that. Change R1 to 240 ohms instead of 560.

Ed

 

[H. Dixon]

The 1.2A is no problem -- For any given circuit at any given voltage and
condition, there'll be just one amount of current that it'll pull.  The
1.2A tells what it's _capable_ of supplying at 12V, not that it'll
_force_ your circuit to accept 1.2A at 12V (that would require
amendments to the law of physics).

Consider it this way.  Your car battery is _capable_ of delivering 100s
of amps (for a short while) to your starter.  Yet you can connect one
little old light bulb to that battery, and it'll a couple of 100
milliamps, no problem -- the battery can't _make_ the bulb take more
(but don't drop a wrench across those battery terminals!).

Your wall-wart is no different, except as John P. mentioned it'll tend
to deliver more than 12V at no load, then less and less as the current
draw goes up.  What the rating _really_ says is that if you are pulling
1.2A from it, it'll deliver at least 12V -- and that's just what it
says, we don't know if it's lying or not.

I save all my wall warts to reuse in stuff I build.  They're very useful
as long as you know their limitations.

--

Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says..
See details athttp://www.wescottdesign.com/actfes/actfes.html

Guys - thanks so much for the inputs. I'm glad I can go somewhere and
actually ASK something without
getting some super flamemaster saying RTFM. I eally try not to ask
until I've done some googling on my own for a few hours.
I've got the components and will build the above.

OT - I'm reading/perusing this newsgroup via web/linux. Any preferred
news reader that I can install and import some filters
others supply that would be beneficial to me? I'm not a fan of chinese
athletic shoes.....

H. Dixon