USB power adaptor question - HALP!!!

[jdogric12]

Hey gang. So I have this MIDI controller keyboard that gets its power from the USB connection. It also has a jack for a 9VDC 500mA pin-positive 5.5mm power adaptor, but I can't find one of those anywhere, and the gig is Saturday night, so shipping is probably not an option.

My Droid phone's power charger is a little wall wart with a USB jack. A cable with a regular USB plug goes into the wall wart, and the other end is a mini USB that goes into the phone.

Question: Can I use a regular USB-USB cable to connect my phone wallwart to the keyboard?

Note: The MIDI keyboard's "USB" jack is actually one of those fat square-with-two-corners-bitten-off jacks, not a regular USB jack.

Help! and thanks!!!

[beatlefreak]

You'll be fine. Power and ground is always dedicated to the same two pins on a USB plug (No matter what type of plug is used). Power is always low current on USB, so you don't have to worry about pulling too much curent.

[cjj]

The short answer is "probably." All USB "host" ports are required to supply power. This power is 5 volts DC. Now, here's where it starts to get tricky. A "unit load" is 100mA, a low power device uses one unit load. A high power device can use up to 5 unit loads, or 500mA (1/2 amp). Host ports do not have to support high power devices, and therefore might only supply 100mA. High power ports must supply 500mA.

So, it depends on what your cell phone charger can handle (Does it provide 500mA? Charging batteries may not require this so it may not), and what the keyboard needs. USB devices (your keyboard) do not have to be solely bus powered, so it might always need the 9V adapter along with the USB (have you ever run it just on USB?).

Now, "normal" USB ports are supposed to have overcurrent protection, so plugging something in that wants too much power won't blow them up. A cell phone charger may not actually qualify as a "normal" USB host port, so all bets are off as to whether it will fry if you try to draw too much current.

The fat square connector is a USB 'B' connector, the flat rectangular one is USB 'A'. USB A was originally supposed to be the host connector, and B was the device connector. But since stuff is always getting smaller, they keep changing what devices use and started using USB A, mini, and micro on the device end. In short, you can use USB adapters to convert pretty much any style of USB connector to another without causing problems...

Edit: OK, looking at my Droid's wall wart (HTC Incredible), it says it can supply 5V at 1Amp. So, if yours is the same, you should be fine.

[electrofaro]

As long as the Ampere amount is correct it doesn't matter what voltage it is, right?
Or should I have paid better attention in school?

[jdogric12]

Thanks gang! By the way, it's for an Alesis Q49, in case I forgot to mention that.

Fun side note... when I was in Guitar Center a few weeks ago purchasing this, the salesperson tried out a 9VAC adapter on a packaging-damaged duplicate item and we saw smoke... the Pro Audio section stunk for like five minutes.

[cjj]

As long as the Ampere amount is correct it doesn't matter what voltage it is, right?
Or should I have paid better attention in school?

You should have paid more attention in school...

[cjj]

Thanks gang! By the way, it's for an Alesis Q49, in case I forgot to mention that.

Fun side note... when I was in Guitar Center a few weeks ago purchasing this, the salesperson tried out a 9VAC adapter on a packaging-damaged duplicate item and we saw smoke... the Pro Audio section stunk for like five minutes.

Yes, if the device is expecting DC and you give it AC, there's a very good chance you'll let the "magic smoke" out...

[gibsonlp]

Jason, it's all a matter of current, if your keyboard can run off a single computer's USB then it needs no more than 500mA.
The plug you are describing is a Standard B-plug which is the same as most USB printers.

If your charger supplies 5v @ 500mA (or more...) then it should be fine, basically any ipod or iPhone charger would be fine (the original ones supply 1A)

[jdogric12]

Cool, thanks. And more mA on the charger is ok, since that's resistance, right? less mA would be a problem, right?

[cjj]

Yes, more mA is fine, the keyboard will only use what it needs. Less mA would be bad...

[jdogric12]

Cool. I can't wait to get home tonight and try it out. I am leaving work soon, going to my ice skating lesson, then practicing my skating some, then I'll go home and try it out. I'm pretty stoked, since this is my first gig with the new rig. I'm going from a very heavy Kurzweil 88-key and a borrowed Korg Triton LE, to an Alesis Micron and the Q49 with a Roland Mobile Studio Canvas. I should be able to fit all my gear in a backpack, literally! Of course, the drummer's usual ride's car is in the shop so I may have a car full of gear anyway!!! To quote Tony, it never ends...

[jingle_jangle]

I am leaving work soon, going to my ice skating lesson, then practicing my skating some,

Former Tampa boy...

[cjj]

It's getting to be spring here in Montana, which means the snow melts in the daytime, then freezes into ice over night. Here in Montana, we abhor ice, we don't play with it...

[jdogric12]

Well I fell tonight. Luckily all I hurt was my feelings!

The Roland Mobile Studio Canvas SD-50 arrived today too (whew!) and I've got it all hooked up... it works! Thanks Kris and everyone for your help, I love this place! Now I'm off to rock... if you're in Falls Church Saturday night come see us (Down Wilson) play at the State Theatre and you'll see and hear this new rig in action!

[jingle_jangle]

So we could call this the "System of a Down Wilson???"