AC/AC PSU Riddle ??? Outputs both AC & DC!

[Smithy]

Hope this is the correct forum.  :-\

im looking to buy a power suplyfor an alesis drum pad i ordered,

its shipped from the US with a 110v, 9v AC out, 830mA adapter,

but i need a 230V adapter, and want to avoid using a voltage converter (step down transformer.)

Anyhoo i spotted this item on ebay and immediatly spotted a mistake with the picture:

http://cgi.ebay.ie/ws/eBayISAPI.dll?ViewItem&rd=1&item=160227940293

Notice the DC Symbol in the pic! So i contacted him, nice friendly guy, and he wants to get to the bottom of this too!

Heres the replies i recieved:

"Hi,

I agree with you about the DC symbol, but I have tested the adapters on 2 calibrated Fluke multimeters & they only read as AC !! Yes I am confused as well.

cheers "

"Lets hope that you have a better understanding of electronics than I do. Admitidly I thought that mine wasn`t to bad but I am even more confused now. I did test with both meters & 100% sure that they did read correctly only on AC range. Then I tested a 12v DC brushless fan on the adapter & guess what? yes it ran. A friend of mine who is sitting an electronics degree came into my workshop & confirmed my results. now thats 3 of us confused. I have prob sold 200+ various adapters on Ebay & have never got it wrong. Maybe you can shed some light on this. :)"

So anyone can anyone here offer an explanation to this riddle?

I asked him i could post it up on here to try and find a solution and he was really enthusiastic about it.

Ill put down my idea of what it could be in the next post

[Smithy]

From wikipedia:

http://en.wikipedia.org/wiki/Switched-mode_power_supply#SMPS_and_linear_power_supply_comparison

"Electronic noise at the output terminals. Cheap PSUs may have a small amount of AC "riding on" the DC component at twice the main frequency (100-120 Hz). This can cause an audible mains hum in audio equipment or unexpected brightness ripples or other banded distortions in analog security cameras."

would be possible that there is so much "AC riding on DC" with this power supply

that it can only be read when a Multimeter is switched to AC, but can still power a DC Fan?

Or maybe its the opposite of that, let say this is an AC adapter with the wrong Symbol.....

Can "DC ride on an AC" current, thus it will be read as AC on the multimeter but can still power a DC Fan?

Its extremely confusing, any help would be greatly appreciated.

[Smithy]

Oh and please, i wrote this in confidence that you do not send the seller nasty messages.

He genuinely wants to get to the bottom of it, and his feedback shows that hes not lying when he said no ones had a problem!

[fluke]

It's possible that it's 9V DC with 9V AC of ripple, but surely that would kill the filter capacitors?  The only real way to be sure is to connect an oscilloscope up to it so you can see both the ripple and the DC offset.  Or cut one open and see if it has any diodes or bridge rectifiers in it.  An AC/AC adaptor would usually only have a transformer inside.

[doc]

I agree,

it's no riddle anymore if you measure it with a simple oszi...

greets

Doc

[Smithy]

It's possible that it's 9V DC with 9V AC of ripple, but surely that would kill the filter capacitors?  The only real way to be sure is to connect an oscilloscope up to it so you can see both the ripple and the DC offset.  Or cut one open and see if it has any diodes or bridge rectifiers in it.  An AC/AC adaptor would usually only have a transformer inside.

Thanks for the reply fluke!

will pass the info on. Ill have to ask him to try doing a DC test.

If anyone has any other ideas please post them.  :)

[lylehaze]

It is possible that the power suplpy is just a transformer and bridge, without any capacitor at all.

And the bridge may be no more than a single diode!

If that is the case, then it would be DC, as the current only flows one way, but it would be a very choppy DC that would have gaps of zero voltage. (if only one diode then those gaps would be more than 50% of the time!)

So, take two LEDs, connect them parallel (side-by-side) with one pointing each way, then add an appropriate resistor in series with the pair, and connect it to the output of the power supply.

If one lights, it's DC. If both light, it's AC.

Let's see if I can manage the drawing as ASCII art:

<----*-----|<|-----*----\/\/\/\/\------->

     |-----|>|-----|

You can use Google to find a resistor calculator

for your needs.

Have Fun,

Lyle

[edit] that would look better in a monospaced font.

The LED's are supposed to be together..

Oh well, I hope you can make sense of it.

[stryd_one]

<----*-----|<|-----*----\/\/\/\/\------->
     |-----|>|-----|

use [ code ] tags :)

[Smithy]

Got another repply and more info on the topic!

"I have got hold of a brand new 12v AC/AC adapter, it tests axactly the same as the 9v one my ebay auction but it does not run a DC fan where as the adapter on ebay does, which makes me think that it is DC output and not AC. I googled the part number on ebay and have found the manufacturers spec. Supprise supprise it says that it is AC/AC. I am so confused

Ps. here is the link http://www.mepos.ca/index.php?cmd=lin_ac_tuv"

[nILS]

which makes me think that it is DC output and not AC

Multimeter?

[Smithy]

Multimeter?

hes used 2 fluke multimeters to test it as was said earlier.