View Full Version : Tying Grounds Together?

Bill Chennault
12-06-2006, 08:35 PM

I am about to start experimenting with various motors (from Banebot (http://banebots.com/osc/support/gearmotor_comp.htm), mostly; I already have a few) and the latest version of the MotorMind Be. I am using the BoE Rev C. with the Parallax wall transformer to power the BoE. I communicate with the BOE via my tablet PC's USB connector to a Parallax USB to serial converter to the BoE. I will power the various motors with a variable DC power supply.

Question: How do I ensure all the grounds are tied together properly? (As I see it, the components are my tablet PC, the BoE driven by the wall transformer, and the variable power supply driven by 110Vac.

Is it as simple as the fact that everything is plugged in to the same circuit in my house, therefore the grounds are already tied together? (This doesn't right to me, but that is why they invented the word "newbie.")
Thanks for your help.


You are what you write.

Mike Green
12-06-2006, 09:47 PM
Actually, the fact that the various power supplies are connected to the same circuit in the house doesn't help much. Most AC powered supplies are isolated from the AC ground for safety reasons. The AC neutral wire sometimes isn't really ground which is the danger. If you have "grounded" outlets, sometimes the ground isn't properly connected.

Really what you want is a common logic supply ground. The USB converter provides a ground connection between the PC and the BoE through the USB cable. When you connect the MotorMind to the BoE, you will need to connect a ground, logic power (+5V), and logic signal(s). The big trick is to connect the ground of the variable DC supply for the motors to the MotorMind's ground via a relatively short, heavy wire. If the connection is short (a foot or two), and the peak current is only an ampere or two, you could use 22 or 20 gauge wire. If you need a couple of feet of distance, you should use 20 or 18 gauge (like speaker "zip" cord). If you're planning on using higher current motors, you need to be careful about wire length and thickness between the motors and the MotorMind and between the MotorMind and power supply.

12-06-2006, 09:53 PM
The important ground is a "signal ground". This means that the Vss (signal ground, and Vdd ground, and Vin ground) is tied to the ground of any servo's you are using. The Supply of the servo's can come from a separate power supply, as long as there is one thin wire from the Vss to the ground of the power supply -- preferably that thin wire connects to the Servo ground close to the servo. "Thin" because this wire doesn't have to carry much current compared to the Servo (or motor) ground wire.

The "wall ground" is a safety ground. Since most of the DC voltages you'll be using are quite low, you don't need (and probably shouldn't) tie Vss to the 'wall ground'.

The point being, you'll need a wire from Vss to the ground of your variable power supplies. But this also depends on how you want to control those motors. The MotorMind Be should also have some suggested ways to implement grounding.

The newbie mistake we're trying to protect you from is when newbies use Servo's with separate power supplies, they initially tend NOT to connect Vss to the ground wire of the servo -- since it's on a separate power supply. This means the BS2 has no 'ground reference' for its control signal, which then doesn't work to control the servo.

So no, it's not as simple as everything being plugged into your house circuit. That's the AC ground, and you don't want to mess with that. All these devices (wall transformer, variable power supply) have a DC ground, and THAT'S the one you want to use for your 'reference', or 'signal' ground.

Quite frankly, getting this right is almost trivially easy if you understand the voltage, current, power, and signal relationships. If you don't understand these relationships (like the BS2 control signal is 'referenced' to ITS ground, so to control something you must make sure the two are using the SAME ground, which you make sure of by connecting a wire to connect the two grounds) then there's no 'rule-of-thumb' that works in all circumstances.

Bill Chennault
12-06-2006, 10:24 PM
Mike Green and allanlane5 (http://forums.parallax.com/member.php?u=41268)--

Thank you very much!

Mike, I am using heavy gauge speaker wire on the variable ps right now! When I bought it, I wanted to test it (of course) and the only thing I had laying around were a few spools of very high-dollar speaker wire. It is satisfying to know that I put about four feet of it to good use!


You are what you write.