# Two AC questions

Discussion in 'Electronic Basics' started by Jack, Mar 14, 2008.

1. ### JackGuest

Consider in the first case, I have a ground potential sticked on a simple AC
circuit, with a AC source and a linear resistor.
However, when the polarity changes (say the current reverses its direction
from going to the top of the circuit to going to the bottom of the circuit)
Would all current sink to the ground instead of going around thru the
resistor?
So wouldn't such circuits be not making any senses?

In the second case, a common-emitter transistor circuit, the dc supply are
connected to the AC source over the bottom of the diagram.
Would the ac voltage, during a negative cycle, reinforce the dc supply in
that transistor circuit?

Thanks
Jack

2. ### Andrew HolmeGuest

Simple answer? No. All the current flowing through the resistor returns to
the AC source, none of it flows to ground. The ground has no effect on the
circulating current in either direction.
I don't understand what you mean. Are the AC and DC sources in series or
parallel?

Try posting a schematic using http://www.tech-chat.de/aacircuit.html

3. ### JackGuest

Consider in the first case, I have a ground potential sticked on a simple
Does that mean the ac source (negative end during a negative cycle) has a
lower resistance than the ground?
I only know that the ground is a pond of charges with 0V potential. Do all
electrons replenish from negative end of the ac source and charges never
come from the ground?
/ |
------------ | |------| transistor ______
| cap \ __
DC
| | |
AC | |
|______________|__________________

4. ### JackGuest

Sorry my IE7 just gone crook....

5. ### Andrew HolmeGuest

Correct.

The not so simple answer is that the effect of the ground is vanishingly
negligible, except in physically large circuits or at high RF frequencies.

The total voltage across the two sources in series is their sum. The AC
component reinforces and opposes the DC voltage on alternate half cycles.
If the DC voltage is 5V and the AC voltage is 1V peak, the sum varies
between 4V and 6V.

BTW - That transistor would need some form of DC biasing in a practical
circuit.

6. ### JackGuest

Then I have a question. With ordinary circuit breakers (fuse) in an electric
circuit,
when the power surges/short-circuits , how come the electron can find its
path quickly, grounding themselves, without delay?
If nothing can "supersede" the path for the resistors?
Thanks
Jack

7. ### Andrew HolmeGuest

At the instant the ground connection is attached, there may be a brief flow
of charge, but this stops once the circuit node is anchored to 0V ground
potential.

8. ### JackGuest

At the instant the ground connection is attached, there may be a brief
Ummm Thanks... I would like to look into more of these topics.
Any books recommended?
Thanks
Jack

9. ### Andrew HolmeGuest

OK, so we're talking about the mains supply. There is a circuit breaker and
a load resistor. The mains electricity system is physically large. It has
capacitance to ground and is physically connected to ground at source.

Current flows to ground and the breaker trips if one side of the resistor is
suddenly grounded. Ground current ceases once the load resistor reaches 0V
ground potential. This may take a few microseconds, depending on the size
of the circuit. Current will briefly continue to flow through the resistor
also, but that too stops in microseconds.

10. ### neon

1,325
0
Oct 21, 2006
What is GROUND somebody mentioned 0v that is not so . GROUND is merely your, mine, somebody else reference point and a point where all voltages are measured from to . And ground can be set at any voltage above/ below from rails. Example ac converters may have battery power from battery set to +/- 600volts and ground set anywhere that is my design ground so that is my ground reference. i could use -600v as ground or +600v as ground it is design choice. I actualy work on logic where the -5v was the ground for interface purposes/