Help with Ground

[CuentaChocula]

What does "ground" mean in a battery powered circuit?

I'm working on a fairly simple circuit, but have a basic question. There are three batteries in the circuit with connections for the positive and the negative posts. Then elsewhere there are ground symbols. One of the ground symbols is connected between the positive post on one battery and the negative post on another battery, which make sense, but the other ground symbols are seemingly on their own with no battery around. What is the meaning of ground in a sealed battery powered device and what would I connect the wires to that are labeled ground? Is it just to indicate the points at which the circuit is electrically neutral and not an instruction to wire anything specifically there to "ground" that point?

 

[ericgibbs]

hi,
In my experience this can be little confusing.

Some engineers for example, refer to battery polarity as say, +V for one of the terminals and Gnd, -V, Common or 0v for the other.

I can give you an example on how to explain it:

Look at the battery on a car, you will see that one terminal of the battery
is connected the car chassis/metalwork. This is considered to be the
Ground or Common. The terminal thats connected to the car switch/fuse
box could be either the Neg or Pos terminal, it depends upon the car type.

Simply in this context, the ground is the terminal to which the other battery voltages are referenced to.

Hope this helps.

Regards
EricG

 

[fingaz]

Hi,
If you post the schematic, or a link to it, we will be able to advise you better. The two batteries with the ground connection between is used to provide a POSITIVE and NEGATIVE supply, this is quite often used to power OP AMPs.
Anyway, in order to give the best possible advice/guidance, we need to see the schematic.

 

[audioguru]

All points with a ground symbol on a schematic are connected together.
Ground symbols are used on schematics to avoid having many lines all over the place because many parts connect to ground.

Try it. Connect together with lines every ground symbol on your schematic. Lots of lines? A mess, isn't it?

 

[Hero999]

Please post the schematic.

I'm assuming, this circuit requires a bipolar power supply, it won't work if you just the positives and negatives.

Connect two batteries in series, connet the grounds in between the two batteries and the positives and negatives to either end of the battery.

 

[CuentaChocula]

Thanks

Thanks everyone. Audioguru's (and Hero999's) suggestion to just connect all the ground symbols together - which included the point between the positive and negative batteries connected in series - seems to have worked. There may be some error in the schematic because I had to swap a 1 megaohm pot and a 100k pot in the design because one was too sensitive and the other not sensitive enough.

I would like to post the schematic, but I don't have a digital camera or scanner so I'm waiting for the time a friend is here who can take a pic of it for me. It's simple enough to redraw on paper, but not so simple I can redraw it in some computer paint program. I even tried installing the free copy of Visio my school gave me but I forgot to get a serial number for it and I'm not enrolled this quarter so I don't have msdn student access to get it... Anyway. I still need to test the design some more and then move it off the bread board and into a box. I will post the schematic here asap.

 

[CuentaChocula]

Here, finally is the schematic. It's supposed to be a simple skin conductivity circuit. I found that it wasn't very responsive to anything but holding the sensors more tightly or less tightly. It's possible that I put it together wrong of course, but the tests for the circuit seemed to follow what they were supposed to do from the write-up. Sorry the picture is bad. The only digital camera I have is my phone.

 

[AllVol]

Pretty obvious. The op amp is bi-polar.

 

[CuentaChocula]

What's obvious? What is the significance of a bi-polar op-amp? Please explain.

 

[Sceadwian]

Bipolar opamps require low impedance signals. Skin conductivity is a very high impedance signal. Basically, in order to get it to work you'd have to pass enough current through the skin to cause the person touching it to pull away from the induced current in the nerves of the skin, if not from the heat it generated due to skin resistance.

 

[Nigel Goodwin]

What's obvious? What is the significance of a bi-polar op-amp? Please explain.

They are worrying unduly, even a bipolar opamp has a high input imedance, opamps have to have this to allow the formulas to work, essentially they 'should' provide:

Infinite gain.
Infinite input imedance.
Zero output impedance.

The formulas assume these are all correct, obviously in practice though none of them are - but they are near enough for the formulas to work satisfactorily.

However, it most be an incredibly old circuit - the 709 is a real antique from way back in 1965!.

 

[CuentaChocula]

Yes, the circuit is from a magazine published in the 1960's. I bought the parts in the 1980's and finally put it all together recently. I'm going to try a different circuit to see if it works better (there was a similar circuit posted here recently).

 

[AllVol]

What's obvious? What is the significance of a bi-polar op-amp? Please explain.

Sorry for being so brief, I was merely noting that the op amp requires a bi-polar power supply as many before had surmised. It was only after the schematic was submitted that it became obvious.

 

[audioguru]

The opamp looks like an old uA709. Remember them? It is even older than the old uA741 opamp.

Opamps don't need bipolar power supplies if their inputs are biased correctly near the center of a single supply voltage.

 

[chinsoon]

Opamps don't need bipolar power supplies if their inputs are biased correctly near the center of a single supply voltage.

huh? i thought the bipolar supply is to allow the output to have negative swing as well. What i saw from the previous thread was that if you only supply a positive supply, then the negative portion of the output signal will be cut off. so now you mean that even with only positive supply, that can also be done?

 

[CuentaChocula]

The opamp looks like an old uA709. Remember them? It is even older than the old uA741 opamp.

Opamps don't need bipolar power supplies if their inputs are biased correctly near the center of a single supply voltage.

It is a uA709. The design calls for a "Fairchild uA709C" and also cautions against getting a "dual, in-line, or flatpack" package and says "be sure you get it in the TO-5 package." I must not have read this at the time because I got the DIP one. Maybe it's all that was available in 1989 or so when I bought it at Hatry Electronics in New Haven, CT.

 

[audioguru]

I remember seeing old opamps in a TO-5 metal case.

 

[Hero999]

**broken link removed**

 

[audioguru]

He, hee. Bob Widlar who designed those old opamps retired before he turned 30 years old! That was 42 years ago.

 

[JTECH]

If the signal processed by the op-amp is ac, then you don't need a dual supply, provided the op-amp is biased properly and use is made of input and output coupling capacitors.