Negative ground in series with Positive ground

[walters]

Does a circuit with Positive ground output negative voltages? negative DC offset? waveforms below the zero reference?

Does a circuit with Negative ground output Positive voltages? Positive DC offset? waveforms above the zero reference?

 

[akg]

Does a circuit with Positive ground output negative voltages? negative DC offset? waveforms below the zero reference?

Does a circuit with Negative ground output Positive voltages? Positive DC offset? waveforms above the zero reference?

in general yes.

 

[lord loh.]

well... I dont think it can be generalized...

The out put is with respect to the ground.

If the out put is supposed to be negetive with respect to the ground, output will be more negetive if the ground is negetive (with respect to some arbitraty ground)

This is similar to the LM317 which is a floating IC. The input and output difference should not be >40v.

even lm 78xx can be used as variable voltage sources if they are wired up as floating.... I saw this in the application hints sention of the datasheet.

 

[panic mode]

lord loh.

i think you missed the point. you are mixing floating and grounded systems. question was for grounded circuits only. akg is right.
unless output is isolated or larger than supply voltage, circuits with positive ground will be negative relative to ground.

 

[lord loh.]

Sorry I do not follow.

Can I get a sompel example circuit.... :? :?

 

[panic mode]

use simple 555 oscillator as a test circuit, make two of them and power each from separate 9V battery. connect batteries like dual voltage source (connect positive from second battery to negative of the first battery).
This will be the ground terminal. connect BLACK probe of your multimeter to it. connect RED probe to the output of the first 555 chip all readings will be positive. leave black probe connected to ground and touch with red probe output of second circuit. reading will be negative.

now remove first battery and 555 circuit but leave positive rail of second battery as ground reference. readings will still be negative.

things can get different in some cases like if the output of the circuit is isolated (it's a whole new circuit) or if the output signal is larger than supply source (you use output of 555 oscillator to drive charge pump to make voltage higher than battery from which is powered). for example if the output is 60VDC when measured to 555 ground (!), it will be some 51VDC when measured from ground because positive terminal of the 9V battery is grounded.

 

[bloki]

The question is too general and the answer must be of the same type.
Output signal polarity can be any, generally it does not depend on how supply voltage is grounded.

 

[walters]

So if i make a 555 timer with positive ground the Pulses polarity will be negative voltages like -5 volt peak?

If i make a 555 timer with negative ground the pulse polarity will be positive volts like +5 volt peak?

 

[panic mode]

bloki, that's true for loads. he just needs to understand difference so he can troubleshoot circuits. this is why he is asking signal level to ground reference. guys, please, stop confusing him...

walters,

you got it, the level will not really be rail to rail (I guess you had 5VDC source in mind) because there is some drop on output transistors.
see following example and note that a and b are not necessarily symetrical:

 

[bloki]

I am afraid he will ask answers for let's say 100 circuits and learn each particular circuit. Then he will imagine that he understand the problem.

 

[DigiTan]

...because nobody ever learns by asking questions; right, Bloki? [/sarcasm]

Walters, looks like you're getting the hang of this waveform stuff. 8)

 

[mstechca]

Does a circuit with Positive ground...

Does a circuit with Negative ground...

Think of a "ground" as a reference point. Let's ignore what it outputs.

The terms that are more commonly used are VCC, and Ground. Ground means a direct connection to the -ve side of the battery. VCC means a connection to the +ve side of the battery. VCC and Ground are two common terms used in digital circuits.

 

[akg]

...because nobody ever learns by asking questions; right, Bloki? [/sarcasm]

Walters, looks like you're getting the hang of this waveform stuff. 8)

yeah.. he is still surfing on the wave(form) .

 

[walters]

Thanks alot guys for the pictures and help

If ground is a reference then Negative ground would be Zero volts but with positive ground it will be VCC lets just say +5volts

So if ground is the reference and positive ground then ground is +5volts

If i have a input of +5 volt pulses going into a Positive ground circuit then it will take the +5 volt pulses and output a (-)5 volts because the ground is reversed like a mirror?

Or it takes the +5 volts and outputs zero to -5 volts?

 

[akg]

Thanks alot guys for the pictures and help

If ground is a reference then Negative ground would be Zero volts but with positive ground it will be VCC lets just say +5volts

So if ground is the reference and positive ground then ground is +5volts

If i have a input of +5 volt pulses going into a Positive ground circuit then it will take the +5 volt pulses and output a (-)5 volts because the ground is reversed like a mirror?

Or it takes the +5 volts and outputs zero to -5 volts?

even though u thanked all ...... the rest of the post makes me feel that u r in still doubt about the concept of ground. afterall this is not a difficult concept .


ok..oen more type of explanation .
consider a building with two floor viz ground(consider as electrical ground) and first floor(consider as vcc) ..
'A' is standing at the ground floor and 'B' is standing at the first floor .

A says that B is 2Mtr above(+v) me . but B says that A is 2Mtrs below(-v) me
and if C is standing on top of Everest , C says that A and B are below(-v) me ..
got the concept..??

It is just that from where we look (measure) the height (voltage).

 

[mstechca]

Walters, if ground is a difficult term for you to understand, then all you need to know are two terminals:

The POSITIVE battery terminal,
The NEGATIVE battery terminal.

The NEGATIVE terminal is usually the one that is designated as the reference point or GROUND.

 

[audioguru]

A circuit with a positive ground would have a negative supply and is for measuring negative voltages. Feeding +5V pulses to it would probably break it.
The same thing would happen if you fed -5V pulses to a circuit with only a positive supply, unless the circuit is inverting with negative feedback to cancel the opposing input voltage.

 

[walters]

THanks for the information


Most germanium PNP circuits use positive ground like (-) 9 volts so if the input signal is inputing a function generator with positive voltages the output of the circuit should output a mirror version of the input signal i would think its just flipped

Negative ground circuit:
If ground is Zero volts and the function generators is output a triangle waveform at (+)5volts p/p. On the oscilloscope the triangle would ramp up and down +2.5 down to -2.5

Positive ground circuit:
If ground is (-) 9 volts and the function generator is output a triangle
wavefrom at (+)5volts p/p. On the oscilloscope the triangle would ramp up and down -2.5 down to +2.5?


Does the Positive ground circuit reverse the input voltages? or mirror the input voltages? or change the DC offset to negative DC?

The height measurement to the positive ground is confussing me sorry

 

[audioguru]

Most germanium PNP circuits use positive ground like (-) 9 volts so if the input signal is inputing a function generator with positive voltages the output of the circuit should output a mirror version of the input signal i would think its just flipped

Germanium transistors have been obsolete for a long time.
Electronic circuits usually have the polarity if their input signals the same as their power supply. If you apply positive pulses to a PNP transistor that has its emitter grounded, The transistor would always be cutoff and wouldn't work. If the transistor is silicon and the pulse exceeds +5V then the reverse-biased junction would have its absolute max rating of 5V exceeded and it will have avalanche breakdown. It will be destroyed if nothing limits the current.
About the only way for a PNP transistor with a negative supply would work with positive input pulses is if its input had a negative offset voltage that cancels the positive input.

Negative ground circuit:
If ground is Zero volts and the function generators is output a triangle waveform at (+)5volts p/p. On the oscilloscope the triangle would ramp up and down +2.5 down to -2.5

Only if the circuit has a gain of one, a positive power supply and a negative power supply, is biased to ground, and the function generator is coupled to the circuit with a capacitor. If the circuit is DC coupled to the function generator then the circuit must have an input offset DC voltage of -2.5V to cancel half of the input DC level.

Positive ground circuit:
If ground is (-) 9 volts and the function generator is output a triangle
wavefrom at (+)5volts p/p. On the oscilloscope the triangle would ramp up and down -2.5 down to +2.5?

A Positive ground circuit doesn't have ground also at -9V.
A circuit without a positive power supply can't produce a positive output voltage.

Does the Positive ground circuit reverse the input voltages?

No. Positive-going is always positive-going unless something inverts it.

quote]or mirror the input voltages?[/quote]
An inverting circuit is needed to mirror signals.

or change the DC offset to negative DC?

No. A level-shifter or inverting circuit is needed. It would probably need positive and negative supplies.

The height measurement to the positive ground is confusing me sorry

About the only time you will see a modern circuit with a positive ground is with telephone systems in a central office.