how much is "ample" current for hobby servos ?
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how much is "ample" current for hobby servos ? Pogo 01-02-2007
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Posted by Pogo on January 2, 2007, 2:02 am
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


Posted by Michael on January 2, 2007, 9:34 am

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



Posted by Gordon McComb on January 2, 2007, 11:13 am
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
>
> I'm trying to figure out if I have blown the servos, the board, or if there is
just not enough juice.

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

Posted by dan michaels on January 2, 2007, 11:36 am

Pogo wrote:
> 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

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.


Posted by John Nagle on January 2, 2007, 11:53 am
Pogo wrote:
> 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

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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