# +/-'ve Logic & Transformers

Discussion in 'General Electronics Chat' started by Electroenthusiast, Jun 9, 2010.

1. ### ElectroenthusiastMember

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How exactly can we design a IC to work in -'ve Logic ?
i.e., IC 7408 should work as High when input is -Vcc and Low when Input is 0v;

How can i make it work as High at +7V and Low only at -7v ? (Something like this.)

--------------------------------------------
I know that transformer work on 'change in magnetic flux principle'.
In the below diagram, there's a transformer with its primary windings connected to mains; and the secondary windings is connected to a load. The flow of current through the load is controlled by a switch. When switch is open, there's no current through load and when switch is closed, then there's current through the load.

http://static.electro-tech-online.com/imgcache/1841-43165d1275282701t-
transformer-trans.jpg

My question is:
>'Whether the Current through primary windings OFF/ON based on the the Switch connected to load?'
>On what factors does the maximum output current of transformer depend (Like: rated 500mA/ 1A) ?
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Last edited: Jun 9, 2010
2. ### colin55Well-Known Member

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Your first request is not clear.
Negative logic is slightly different to what you are surmising.
Negative logic still works within a 0v to 5v supply but the result(s) occur when one or more inputs go LOW.
For instance, all the inputs are HIGH and when one input goes LOW, the output changes.
You need to describe your requirement more-fully and preferably with a circuit diagram, to be able to get a reply.

Energy - the multiplication of volts and amps (watts) is transferred from the primary to the secondary via the magnetic path or circuit made up of the core (the laminations). It's a bit like someone filling a bucket at the beach with water from a small cup and someone else removing the water with a small cup.
When the secondary switch is open, the bucket gets filled and the person stops filling.
If there is a slight leak in the bucket, the person adds a little water every few minutes.
This is exactly like the transformer with the secondary switch open.
The incoming voltage very quickly fills the magnetic circuit with flux and this flux opposes the incoming flux so that current falls to a very small value.
This occurs on every positive and negative half-cycle.
When the switch is closed, the secondary circuit removes some of the flux and the incoming waveform delivers more current for a longer part of the half-cycle – in attempt to fully fill the magnetic circuit with energy - flux.
The magnetic circuit is a bit like this:
Imagine a bucket at the beach with a pipe halfway down the bucket and this small pipe going into a second bucket.
The first person fills the bucket and the second person empties the second bucket, but the small pipe between the two buckets limits the rate at which the water can be removed from the second bucket.
This is the limitation of the magnetic circuit.
It can only transfer a certain amount of energy and this limits the wattage of the transformer.

Last edited: Jun 9, 2010
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3. ### ElectroenthusiastMember

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Thanks For the answer!.. You brought me an idea.
Can you please suggest me a book for this(I've searched in many books)? Google Book would do...

My 1st Ques:
From a datasheet of any IC, the operating voltage range is specified? But when i was into some web,
i found that Logic systems are of types : Positive Logic/ Negative Logic
Suppose, i need to make it work in negative logic, What should i do then? How to make IC work so?
If i need to make an IC work as Low at +7V and High at -7V, then wat should i do?( IS THIS POSSIBLE)

This link will express my question POSITIVE AND NEGATIVE LOGIC

I want to know how to do so...

Last edited: Jun 9, 2010

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5. ### MikeMlWell-Known MemberMost Helpful Member

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That Link is confuses the concept of Negative logic by refering to -10V. Forget about that.

Think of a 7400 gate. Two inputs, one output. Single 5V power supply. Negative voltages not allowed. Inputs must be between 0V and 5V, output will be between ~0.4V and ~4.5V.

What does a 7400 gate do?

If both inputs are high (i.e. near 5V), the output is low. That describes an AND function for high inputs (actually AND followed by NOT) .

But a 7400 gate can be thought of another way. If either input is low, the output is high. That describes an OR function for low inputs (actually NOT followed by OR).

So a 7400 gate has a dual function, sometimes used to AND two signals; sometimes used to OR two signals.

The Duality concept is called DeMorgan's Thereom, and it describes the concept of positive and negative logic.

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6. ### ElectroenthusiastMember

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IC7400 is a 2 input NAND Gate
Let A,B be the Inputs: and Y be the Aoutput
Then,

A B Y
1 1 0
1 0 1
0 1 1

Thanks; I'll accept it in the above case.
What if it is somewhere else?
If negative Voltage is not possible then why is it specified like that in Link ?

Last edited: Jun 10, 2010
7. ### MikeMlWell-Known MemberMost Helpful Member

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In the truth table, you forgot A=0, B=0, Y=1.

A B Y
0 0 1
0 1 1
1 0 1
1 1 0

In words: the output is high if either input is low or the output is low only if both inputs are high

On the other question: just because something is posted on the net doesn't make it correct, useful, factual, good, etc. I think the author was trying to describe some very old logic techniques using resistors, diodes and transistors which used split power supplies and where the logic levels were 0V and -10V. Logic levels voltages have nothing to do with the Duality I describe above.

8. ### ElectroenthusiastMember

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Ok MikeML; -'ve Voltage Confused me though...

Transformers:
ANY BOOK???