how much is "ample" current for hobby servos ?

The Parallax USB Servo Controller ( http://www.parallax.com/detail.asp?product_id=28823 ) says to make sure the external power supply for the servossupplies "ample current" to power the servos. How much is "ample"? I am using a Radio Shack multi voltage wall wart adapter that supplies 800 mA and set to 7.5 volts dc output. I get a little jump in the servos when they are connected and that is all. Is 800mA enough ? (standard hobby RC servos ... )
I'm trying to figure out if I have blown the servos, the board, or if there is just not enough juice.
Thanks ! JCDeen
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The Parallax USB Servo Controller ( http://www.parallax.com/detail.asp?product_id(823 ) says to make sure the external power supply for the servossupplies "ample current" to power the servos. How much is "ample"? I am using a Radio Shack multi voltage wall wart adapter that supplies 800 mA and set to 7.5 volts dc output. I get a little jump in the servos when they are connected and that is all. Is 800mA enough ? (standard hobby RC servos ... )
I'm trying to figure out if I have blown the servos, the board, or if there is just not enough juice.
Thanks ! JCDeen
IIRC, hobby servos draw around 300mA max. each.
HTH,
Michael
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Pogo wrote:

) says to make sure the external power supply for the servossupplies "ample current" to power the servos. How much is "ample"? I am using a Radio Shack multi voltage wall wart adapter that supplies 800 mA and set to 7.5 volts dc output. I get a little jump in the servos whe

When it doubt, use a set of four AA alkalines. They provide enough voltage and current to run several servos simultaneously. They are also more likely the type of power source you will use in a robot.
7.5 volts directly to servos is a bit much. They are rated for 4.8 to 6 volts, though *some* tolerate 7.2. More than that and it's anyone's guess.
-- Gordon
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Pogo wrote:

http://www.parallax.com/detail.asp?product_id(823 ) says to make sure the external power supply for the servossupplies "ample current" to power the servos. How much is "ample"? I am using a Radio Shack multi voltage wall wart adapter that supplies 800 mA and set to 7.5 volts dc output. I get a little jump in the servos when they are connected and that is all. Is 800mA enough ? (standard hobby RC servos ... )

For standard size [44 oz-in] analog servos figure roughly 300-mA each, so your wallwart might be good for maybe 2 servos.
Also, realize a wallwart is a very poor supply for servos. Outputs are not regulated. If it says 7.5 vdc, then it actually outputs 10-12 vdc at no load.
As Gordon says, much better to use AA cells to power servos. I use NiMH AA cells with 2300-2500 mA-Hr capacity for all my robots. I usually use 5 or 6 in series, and these will easily power up to 16 servos for 30 minutes or more.
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Pogo wrote:

Actually, with that board, the USB port powers the board's electronics. The additional power supply line is just passed through to the servos. Your 800ma 7.5V supply should work for servos with no load on them.
" I get a little jump in the servos when they are connected and that is all."
That's what normally happens when you connect a servo to power but aren't feeding it a control signal. Is the board connected to a computer?
Plug the board into a USB port of a computer running the PSCI software, without the external power supply connected. See if it will talk to the computer. Try the command "File->Get PSC version", which should display the version number of the board. If you can get that far, the board is working and talking to the Windows computer.
Then connect a servo and apply the servo power. Now the servo should respond.
                John Nagle
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Thanks John. I reinstalled the FTDI usb drivers, and all I can get from trying "Get PSC Version" is this in the little green text box: "Version: ..."
On the PSC, I can see the green LED flash when "I talk to it" using the gui program and move the slider controls.
As for the wall wart power supply, I measured it & it does put out very close to exactly 7.5vdc. Although I did bump it down to 4.5 based on Gordon's input (no pun, honest!).
I haven't been able to use my Phidgets board for quite a while, and I figured I had either blown the board or burned out the motors or something. Now I'm starting to think it's something on the PC. It's a Vaio PCG-GRX550 laptop. I drug out the old faithful BoeBot serial version and can't get any response from it when I use the USB-Serial cable I got eons ago. I'm thinking the usb ports might be on the way out - possibly the whole machine.
The FTDI driver I used is from CDM 2.00.00.zip from the FTDI site.
I think the next step is to try the PSC and BoeBot on another computer. ( Got to dig up an old monitor to go with the 2 junkers I have in the closet.) Or try a USB PCMCIA card on the laptop.
Any other ideas ? And thanks ! JCD
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Pogo wrote:

Look for <usbview.exe> - available on the ftdi site - it tells you what all the USB ports are doing - if you cannot see your USB with this program, it is no longer in this world.
Dave
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Progress! I got the BOE to tell me the Stamp version! Just had to use a FRESH 9 vdc battery instead of the 4 celAA battery pack that was only giving 3.7 volts or so. Guess the Stamp was not getting enough juice. Now those batteries are in the charger. So, this gives me something to test the servos with to see if they work OK.
I will post more progress as I make it in hopes that it will help others with similar problems. JCDeen
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Hi, which shop do you buy the FTDI USB-Serial cable?at sim lim square or not? I see most USB-Serial cable is Prolific chip, FTDI chip not easy to find.
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