# Where does the current go?

Discussion in 'Homework Help' started by GARYN, Jun 19, 2010.

1. ### GARYN Thread Starter Member

Sep 4, 2009
47
0
Following on from an earlier post which cleared up a few things there is still a issue i cant quite understand.

On transformer secondary coils what happens to the returning current from the load to the 0v. i.e on the primary and other AC loads the current comes from the live through the load and returns down the Neutral where it is connected to Earth at source. But on a secondary coil where does this current go as it is isolated from the primary, is it induced back to the primary Neutral or does it flow continuously around the coil- if this is the case i only thought current flowed from a higher to a lower potential, this would explain the current flowing back to 0v from the load but how can it flow to high output end of the coil i.e 24v.

I am sorry if this is a bit confusing and i am probaly missing somthing obvious but i always thought AC current had to flow back to Earth where it is dissipated in the ground via the Neutral return, or in the case of the secondary coil does it just stop at 0v.

If someone could put me right in simple terms i would very much appreciate it. Thanks in advance.

2. ### Markd77 Senior Member

Sep 7, 2009
2,803
594
The current in the secondary just flows round the circuit connected to it. If nothing is connected no current flows in the secondary and the primary current is also low because it doesn't do much work.
If the secondary is isolated it can be at any voltage.

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3. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
The neutral ideally carries no current. Rather, the hot phases carry the load, and the neutral is left to carry only the unbalanced current. When designing electrical systems, much effort is placed on 'balancing' the phases so that the neutral carries little current. On a single phase circuit, the term 'neutral' no longer applies as it's intent is to carry full load. It is then termed the 'identified' conductor.

The primary is actually a secondary of the transmission system, which itself may be a sub of higher long distance systems. The currents in each subsystem circulate. Ideally, you don't want to loose any of that current to ground. If you can measure the current going out and compare that to the current coming in, then you can make some assumptions as to the integrity of your system. If the two become different, you can assume there is trouble, and you trip the system.

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4. ### Ghar Active Member

Mar 8, 2010
655
72
Disregard this post....

5. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
Regarding balancing, when you build a device.
If you have 1 amp of current flowing through a circuit, you will not want 1 amp leaving the circuit through the neutral. This is what we call a short. The components will convert some of that current to magnetic flux, or motion, heat, or light.

That way the power that is available, is used in the circuit. So in a well designed circuit or device, you will balance or use the power properly so there is not any left to go 'down the drain'.

A forward voltage, or voltage drop of, say...an led. is 3.3v and uses current of 20ma.

So if you have a 24v supply, after it passes through SEVEN LED's, the voltage into the 'GROUND' conductor (or 0v) will be:

Power Supply voltage - Total Voltage Drop = remaining voltage to GND
so
24v - 23.1 = .9 volts.

so when you choose a transformer, the amount of current available to the circuit, pulled from the mains, is correlated to the size of the core, number of turns, and wire gauge.

You know you will need 140ma amps (7 LEDs x 20ma per LED = 140ma)

When designing the device, there is no need to have a 200v 20Amp transformer, as this will waste a bunch of power as heat and money...as..money.

SO, when choosing a transformer to use in you device design, you go through this process to determine the proper size required (plus a little headroom) to keep from allowing juice to go down-the-drain.

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6. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,787
771
I think the OP is confused.
Primary side uses earth, even though it is not connected to the winding.
Earth acts as a protection by shielding the cover or by connecting to the core.

Since secondary does not use the earth, OP thinks there is no way for current to drain out.
What he does not understand is that current loops through live and neutral from the primary side and no current flows to earth, except in a fault condition

In secondary, the same happens. Current loops via secondary winding and load.

Earth is used for protection only, even though neutral is connected to earth at the power station.

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