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AC and DC Power Adapters

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#1 wishmakingfairy


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Posted 27 February 2015 - 02:29 PM

I have no background in electronics whats so ever and am curious as to why this old electric piano is working only with a 9v AC adapter and not its 9v DC adapter which it calls for.


I know you should never switch them, but I have the general impression of "9v is 9v right?" Please don't get uber technical like people did when I worked at an auto parts store. A gentleman called in looking for a bolt whos thread was 38 pitch or some nonsense, like I went to bolt thread school.

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#2 Animal


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Posted 27 February 2015 - 03:31 PM

AC being alternating current like that comes from the wall. Stepped down from whatever your wall voltage is 9v AC. DC being direct current like that provided by batteries. Would be your household AC current stepped down to 9 v and converted to DC.

Thats the difference in the two adapters as you describe them. The piano requires DC current not AC and why it's not working with the AC adapter.

EDIT: In other words the piano needs battery type power not wall type power. And the AC to DC adapter provides that so you don't have to invest in truckloads of batteries.

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#3 Platypus


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Posted 27 February 2015 - 06:09 PM

The first possibility I see is that the 9V DC power supply has simply stopped working.


However, when it comes to AC and DC, no, 9V isn't necessarily 9V...


When 9V AC gets changed to DC, the resulting DC will normally be a bit over 12V.


So if you use a 9V AC plugpak on something that needs 9V DC power, and it seems happy to work that way, it could actually be being supplied too high a voltage. That might not matter, but in some cases it could damage the device.


The current rating of the power supply also matters.

Edited by Platypus, 27 February 2015 - 06:11 PM.

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#4 ddeerrff



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Posted 27 February 2015 - 10:17 PM

The AC adapter might be working because the piano has a reverse polarity protection diode on the power in, and that diode is acting as a rectifier.  It could be series or shunt.  I would expect you would get a bit more hum than usual using that AC adapter, and if the protection circuit is a shunt diode, you are probably overloading the adapter.


The DC adapter may not be working because A)  It is bad, or B ) it is wired with incorrect polarity, + and - reversed from what the piano needs.

Edited by ddeerrff, 27 February 2015 - 10:18 PM.


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