View Full Version : Supplying energy to servos from a PC Power adaptor...

09-25-2008, 07:48 PM
Hello everybody. Here I am asking again.
I’m trying to supply my servos (continuous servos) with 7Volts coming from a PC power supply (I tried with an AC-DC adaptor too). The problem is that the servo will always turn one way (clockwise) even when I use the 5V configuration. I can’t make it turn the other way. When I put a pulsout code (no matter which velocity, all velocities produce the same effect) on the BS and turn it on, looking at the movement of the servo, it seems that the BS makes the servo move slower into the same direction but kind of shaking. But it is still moving!.

Why is the servo turning not stopping?. I can’t even stop it using a screw driver.When I used the screw driver I had to turn it all the way to the right to make it move slower (Pretty fast, like 40RPM) but it still moves to the same direction (clockwise)
Any solutions?

Is the BS interfering in the power supply voltage?
Can I supply my servos with that voltage without burning them?

Thanks! 

Mike Green
09-25-2008, 08:15 PM
1) This is not the way servos normally behave. Stop what you're doing. Go back to a known power supply that worked before with the servos. If you've not used your servos successfully before, use a 4.8 to 7.2V battery pack or AC-DC adapter for one servo and the Stamp and try the servo calibration program from the "Robotics with the BoeBot" tutorial from Parallax.

2) The BS would not be interfering with the power supply. PC power supplies are sometimes problematic because they often have a minimum current drain for proper operation. If the 5V supply doesn't draw enough current, the regulator doesn't function properly. I'm a little concerned about the idea of 7V coming from the 5V output of the PC power supply. How do you have it connected? What voltages do you measure under what conditions? If the output voltage "spikes" a lot higher under light load, the servo theoretically could be damaged which could account for its behavior.

3) Servos are designed for operation with 4 to 5 alkaline cells in series or 4 to 6 NiMH cells in series. This is a voltage range of 4.8V to 7.5V. Towards the high end of that range, the motor brushes will tend to wear faster due to arcing. Above 7.5V, this arcing will increase significantly.

09-25-2008, 10:38 PM
You DID connect the Vss ground of the BS2 to the Servo ground and the PC supply ground, right?

Because if you don't, the Servo won't read a reliable BS2 control signal.

09-26-2008, 08:19 AM
It seems that i fixed the problem. I'll check it tonight.
My ground connection weren´t right. It seems that that was the problem. I realized it when i touched a metal bar that didn't have anything to do with the project. Then i realized that maybe I was a kind of "antena" with static electricity or something, so i re-checked the piece of iron I'm using as ground. After that i fixed it. BUT YOU HAVE NO IDEA HOW CRAZY I GOT WITH THIS!!. It was really strange....

About the 7V comming from the PC power supply...
I realized that energy is like water. If you have a dam of 12 "meters" and you connect it with a pipe to groud, you'll get water drop of 12 "meters". But if you connect that dam of 12 "meters" to another one of 5 "meters" you'll get a water drop of only 7 "meters". So i tried it with a voltimeter and it worked!. :)

09-26-2008, 10:19 PM
Yeap! it worked!! jeje.
But now i'm having another problem... i already made some projects with continuous rotation servos... but never did anything with the standard servos... i really dont understand the logic of using them. I know i use the pulsout command to controll them but as i can see they work different.
Plus, all the servos i've got (Futaba servos) stay stoped at different durations.
I'll re-read the "What's a microcontroler" text and the "robotics with the boe-bot" text to see if i can get some answears.
But, can someone tell me where i can get some good examples code with QUOTATIONS so i can learn different tricks?
I'm trying to control a robotic arm i've been doing for this passed months. ;)

Thanks for the answears!

Mike Green
09-27-2008, 01:14 AM
1) Read the texts

2) Servos are not precise things. Continuous motion servos do stop at different pulse widths. That's why they have a calibration potentiometer. "Robotics with the BoeBot" has a description of calibrating a continuous motion servo. Standard servos also vary. That's why you reposition the servo horn on the shaft. Sometimes even that doesn't align the way you want and you have to adjust the pulse length slightly.

09-27-2008, 01:40 AM
thanks Mike.
I just figured out what happened. The servos i've got says "standard servo" on the box, but they act as "continuous servos". Strange Uh?. That was wy i couldn't understand their logic.
I read both texts again and started over again. Experimenting and experimenting made me realized of the problem.
And even worst. All this servos can not be calibrated because they don't have a potentiomenter!! (remember: they came in "standard servos" boxes) so they will stop at very strange pulsewiths.

Servo 1 at: 273
Servo 2 at: 260.5
Servo 3 at: 392.5
Servo 4 at: 267.5
Servo 5 at: 460

I talked to technical department to tell them whats going on. so they can double check all the servos they have.
And i talked to sales department and they will send me new ones :)

So, as a good part of the story, i will get new servos for free. :)
and as a bad part, i'll have to wait 3 weeks to get them! :( I'm in Paraguay, South America.