### Author Topic: 9v and 6v batteries with common ground?  (Read 6488 times)

0 Members and 1 Guest are viewing this topic.

#### Lucky75

• Beginner
• Posts: 3
##### 9v and 6v batteries with common ground?
« on: March 17, 2011, 01:29:56 AM »
Hi!

I have a toy car which I'm attempting to control with a microcontroller, but I'm running into a bit of a problem with the batteries.

The microcontroller takes 9v in and is dropped to 5v due to a voltage regulator. I also have a servo motor hooked up to a 6v battery originally from the RC car, and it's driven by a signal from the microcontroller.

How am I supposed to create a common ground? Won't I get current flowing through the ground wires due to a difference in potential voltage?

Thanks for the help! I'm pretty stuck.

Edit: I read the other posts about this, but I'm not quite sure how it applies in my situation. Also, I think I fried my servo today (started to smoke). I was measuring the current passing through the servo from the + and - terminals, and it seems to be pulling 2A even when not being driven by a signal from the microcontroller. I would assume that servos shouldn't do any work, and 2A*6V=12W of work for not doing anything seems a bit weird.

« Last Edit: March 17, 2011, 01:43:57 AM by Lucky75 »

#### Daanii

• Robot Overlord
• Posts: 138
##### Re: 9v and 6v batteries with common ground?
« Reply #1 on: March 17, 2011, 02:38:40 AM »
How am I supposed to create a common ground? Won't I get current flowing through the ground wires due to a difference in potential voltage?

Yes, your circuits should have a common ground. Just hook all your ground wires together. They will all be at the same potential, so you will not have a problem.

Just be sure not to connect the positive terminals of your two different batteries together. That is where you would have a problem.

#### Lucky75

• Beginner
• Posts: 3
##### Re: 9v and 6v batteries with common ground?
« Reply #2 on: March 17, 2011, 10:08:00 AM »
Two batteries with different voltages and different amounts of current have the same potential? I would have thought that I'd get current flowing through the negative/ground? Wouldn't I be causing a ground loop?
« Last Edit: March 17, 2011, 10:19:14 AM by Lucky75 »

#### mstacho

• Supreme Robot
• Posts: 376
##### Re: 9v and 6v batteries with common ground?
« Reply #3 on: March 17, 2011, 11:54:22 AM »
Think of it like two circuits in parrallel that happen to connect at one point -- the ground.  Current won't flow through the ground into the other circuit, since ground is the place of lowest potential for both.  If current were to flow anywhere but ground in either circuit (with the grounds connected), it would mean that the current would have to flow *against* the potential, which wouldn't happen.

It's an excellent question, though, I had the same problem trying to get logic power and motor power to work from different sources.  Turns out that unless you connect the ground, it will never work :-P

**EDIT: I think I see your confusion: you are thinking of "negative" and "ground" as the same thing.  In a circuit, the negative battery terminal will connect to the ground, and only then will they be the same thing.  This is why you can ground to the chassis of a vehicle, for instance, with a single wire.  It seems like that wire does nothing, but in reality it just makes sure what you call "negative" and what you call "0V" are the same thing.  I...hope that made sense.

MIKE
« Last Edit: March 17, 2011, 11:57:13 AM by mstacho »
Current project: tactile sensing systems for multifingered robot hands

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: 9v and 6v batteries with common ground?
« Reply #4 on: March 17, 2011, 01:51:16 PM »
Hi,

Won't I get current flowing through the ground wires due to a difference in potential voltage?
Just want to clear up a thing or two about "potentials".
A potential only exist when it has got a reference.
So, if you pick a single line, no matter it's potential within its own loop, it doesn't have a potential at all.
You could even connect eg. +9V from one circuit with 0V/Gnd/Common from the other circuit, as long as there's only this line connecting them. You shouldn't however, as you'd get in trouble if you then connected a second line.

Just connect the 0V side of both supplies and you have common ground.

Also, I think I fried my servo today (started to smoke). I was measuring the current passing through the servo from the + and - terminals, and it seems to be pulling 2A even when not being driven by a signal from the microcontroller. I would assume that servos shouldn't do any work, and 2A*6V=12W of work for not doing anything seems a bit weird.
Perhaps you put the positive supply to the servo signal line or something similar.

You should consider using the terms positive (+) and Gnd/Common for 0V, as negative (-) is only used for... negative voltages.
If you use the negative pole of a battery for reference (eg. when measuring its voltage), this will be your 0V reference. If you use the positive pole for reference, this will be your Gnd and your battery will give only out negative voltages.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### Lucky75

• Beginner
• Posts: 3
##### Re: 9v and 6v batteries with common ground?
« Reply #5 on: March 24, 2011, 07:31:45 PM »
Thanks for the responses guys, much appreciated