Common Ground and Voltage In

spaine

Everything that I have read says that a motor should have its own power source, but I thought I read in another post that you still have to have a common ground. Could someone tell me that if I have three different ciruits interconnected, but using different Power source, do I tie all three into the same common ground?

Thanks in advance,
Stephen

Paul Baker

If they are interconnected at all yes. The ground of one power source may not be the same as the ground of another powersource. Rather than thinking of ground as 0 volts think of it as your reference point, so a 6V battery would mean your positive terminal is 6 volts greater than your negative terminal. This could mean 0 volts and 6 volts or 1 volt and 7 volts, the circuit sees the two as the same. But if you have two sources with two seperate reference values you may end up with a mismatch, say the reference terminals of the two batteries are off by 1 V, then there can be poblems with two circuits interfacing if one thinks ground is 1V and the other thinks ground is 0V. By tieing the two grounds together, the two circuits will be on the same page with what ground is. The same is true when combining more power sources into the mix.

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Chris Savage

·· One note for clarification is that if the Stamp is connected to the external device with a signal or control line, then the Stamp will need a reference to the other devices ground.· So in that sense, you need a common ground.

·· If the devices are isolated, either mechanically or optically, then the power supplies don't need a common ground.· An example of optical isolation would be if you're using an Opto-Coupler or an SSR (Solid State Relay).· Mechanical isolation would be if you're using a Mechanical Relay.· In these cases the device being controlled doesn't need a ground reference with the Stamp because the control circuitry isn't electrically connected to the controlled device circuitry.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Paul Baker

Thanks Chris, I neglected the point of electrically issolated circuits in communication with each other.

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spaine

Paul,

I believe I understood the explanation that you and Chris gave. I also came across a fairly good explanation in a robotics book that I picked up. I attached a schematic that they provided in the book, which shows the use of two different power supplies. Could you let me know if it (the schematic) is correct?

During my prototyping I'm going to use one of the Garmin handheld GPS units. I wasn't sure if anything special needs to be provided for it, as it has it's own power supply (and ground) via batteries. Since the BS2p24 will be receiving SERIALIN data from the GPS, I didn't know if that would cause any problems. I know that when I go with an OEM GPS board, I would just provide power via the electronics circuit (at least I believe that is right).

Thanks,
Stephen

Chris Savage

Stephen,

·· In that circuit, the motors would be getting the voltage from that connector, but apparently unregulated.· The electronics would be getting the power from the other connector, though through a 7805 regulator.· It looks okay.

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Chris Savage
Parallax Tech Support
csavage@parallax.com