Another NON-OU question

[mondrasek]

Cheers all!  And Merry Christmas (belated).

Sorry for another off-forum "topic" question, but I'd like to pick the brains of you guys again. 

I do appreciate the knowledge and expertise that you all exhibit!  Especially after my Cree LED bulb question!  Hats off to TK for the links to the previous dissection of the bulb on that one!

I had a second purchase of a rechargeable appliance fail recently.  It has (had) a 9.6 V NiCad pack of 8, 1700 mAh cells inside.  The root cause of failure appeared to be failure of a DC motor due to water fouling of the brush connection area (visible water corrosion yet it would still run off of an 18V supply once given a spin.  It would not start up without the spin.  And it would change RPM while on the good voltage source once started by being spun by hand).

So here is the part I am confused about:  The plug in "wall-wart" AC-DC transformer did not appear to be giving the correct output voltage.  It was rated as an input 120 VAC unit with 11.8VDC, 200 mA output, which I thought was reasonable for the 9.6V NiCad battery in the appliance that it was intended to charge.  But when that transformer was measured by two separate DVMs the output (with no load) read ~20 VDC.

So I have to ask, WTF?  Is the transformer mislabeled, do I need the load (battery) in the circuit for it to measure correctly (doubtful), or is there a way for a simple AC/DC transformer to short internally and supply a stable and HIGHER DC output?

Thanks in advance for your advise.

M.

[MileHigh]

The answer is that you have what is called an unregulated power supply.  It is supposed to drop down to the approximate output voltage on the label when it is under load.  They are like this because they are cheap.  The more expensive type is a regulated power supply which will indeed show the "correct" voltage with no load.

[mscoffman]

Yes, there are about 5 different kinds of "Wall Wart" power supplies;

(a) there is the AC output type that is just a (small) transformer. @ Several amps AC
    maximum current.

(b) there is a Raw DC type that is just a bridge rectifier into some minimum
    filter capacitance. @ ~.5 Amps Maximum

(c) there is a DC regulated type that uses a series (resistive + heat sink) DC regulator.
    Often up to 1 Amp Maximum.

(d) there is a modern DC regulated type that uses a "switching" DC regulator.
     Often up to 2 Amps Maximum. "USB" output plug supplies are often of this type.

(e) there is a DC switching regulator type that has some sort of programming chip or
    resistor for certain cell phones. Cell phones use the resistor to set the charge/level
    rate for it's battery. Usually marked 5.1VDC @ 2 Amps. Often has miniture
    multicontact output plug at end of output cable.

The problem that you have is that the type b -> e above don't indicate which one is which
on there label. But generally you must get the correct one mated with each appliance
and also have the plug polarity be correct also.

The fact is that type b is often just raw DC so when it is unloaded it's voltage follows
peak to peak 18VoltsDC or so, but it pulls down to 12VDC when fully loaded at it's rated
maximum current. Peak to Peak 8Volts pull down to 5.+Volts when fully loaded. Portable
Computer Power supplies (non wall-wart type) were often rated at 18VDC regulated. These
power supplies expect current draw from the device to pull down the output voltage to the
correct operating levels.

So the first thing you should do is measure the voltage loaded and unloaded and if
voltage changes significantly, then you have a type b - unregulated
DC type. Sometimes these type go bad at the capacitor and you will see siginificant
AC hum on the output. Solution; replace output electrolytic capacitor.

If the regulation is marginally good then you probably have a series dc regulator. If
the current is one amp or less then it probably is a type c - series regulated DC.
(the most common). This type max generate hum if overloaded as it's output
capacitor is inexpensively set to a minimum value.

If the current is > one amp you proabably have a switching regulator supply and the
output voltage will stay in one spot loaded and unloaded. d & e. It is no problem for the
switcher to go above one amp. But the DC series regulater type will overheat when
it is force deviler more than 1 amp in it's confined space.

:S:MarkSCoffman

[TinselKoala]

Nothing to add to that excellent answer, except that you can take the ordinary non-regulated DC type and add a 7812 (for example) regulator and a couple of caps to the output and turn it into a cheapo voltage-regulated supply for a couple of dollars. I just did that a couple days ago to have a good PS for my arduino color organ projects, using a surplus wall-wart that indicated 18 volts unloaded but was labelled 12 VDC 1 Amp.

[mondrasek]

Thanks, guys!  That helped a lot.

M.