# Practical Application of Logic Level

Discussion in 'General Electronics Chat' started by Lineout, Nov 28, 2013.

1. ### Lineout Thread Starter Member

Nov 20, 2013
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What in general terms is logic level for things like transistors.

I looked it up and it appears there are different levels for different applications ?

Could you expect a typical 'logic level' to be
in the range of 2 to 5 volts ?

2. ### MrChips Moderator

Oct 2, 2009
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4,195
"Logic level for things like transistors" is a meaningless statement.
Transistors do not have logic levels.
A transistor is a circuit element.
The input threshold of a circuit depends on the circuit topology.
The output levels of a circuit depends on the circuit topology and the power supply voltages.

You can design a transistor circuit to output logic levels at -100V to +100V or any voltage you choose.
I had to design a transistor circuit with logic levels to 1kV.

3. ### Lineout Thread Starter Member

Nov 20, 2013
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Logic Level Mosfet.

4. ### MrChips Moderator

Oct 2, 2009
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That is a different question altogether.

Check the spec sheets of the specific logic level MOSFET.

The popular IRLZ44 MOSFET spec sheet states the Gate Threshold Voltage VGS(th)
as 1.0V min and 2.0V max.

Halfway in between would make it 1.5V typical.
That is the input threshold voltage between the gate and source pins.

So to answer your original question, 2V to 5V would work (assuming the source is at 0V potential).

5. ### Lineout Thread Starter Member

Nov 20, 2013
64
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Thanks,

Does 'threshold' in this case mean anywhere in between the 1 and 2 v and of course the 1.5 volt is dead center (and probably a good point to shoot for) ?

6. ### TheComet Member

Mar 11, 2013
88
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Absolutely NOT. 1.5V is undefined behavior.

In MrChips example, 1.0V and lower is interpreted as "low", 2.0V and higher is interpreted as "high". Anything in between these thresholds can result in undefined interpretations of the signal level, and in worst case scenarios cause some old ICs to oscillate and destroy themselves.

Are you talking about the transistor as a component, or are you talking about TTL logic levels (integrated circuits based on Transistor-Transistor-Logic)?

7. ### TheComet Member

Mar 11, 2013
88
12
Correct, except for one thing: Current will flow from drain to source, not the other way around (unless you're talking about a p-channel MOSFET?)

8. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Yes, any particular MOSFET you buy can have a threshold anywhere between 1 and 2 volts and still be in spec.

Sure, as long as you are OK with half your products not working.

MORE then 2V is the place to aim here.

9. ### MrChips Moderator

Oct 2, 2009
14,282
4,195
To confirm what Ernie is saying.
Read the specs carefully. VGS(th) is 1.0V min and 2.0V max.
Let us add to that a 0.5V margin of error.

What this means is that a logic low must be below 0.5V
and a logic high must be above 2.5V.

Hence you do not want to be anywhere between 0.5V and 2.5v for a valid logic signal.

Nov 20, 2013
64
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11. ### Lineout Thread Starter Member

Nov 20, 2013
64
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I hope I didn't say between .5 and 2.5 I thought you meant between 1 and 2 volts to gate would close the drain/source, and turn on a device that might be hooked up.

12. ### Lineout Thread Starter Member

Nov 20, 2013
64
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I said it wrong, and N channel is what I'm going to be using.

Side question: do they use the term source as a nod to electron flow theory as oppossed to conventional, the actual flow being neg to pos, or source as reference to o potential ?

13. ### MrChips Moderator

Oct 2, 2009
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What the min and max specs are saying:

1) that none of the MOSFETs of this type made by this manufacturer will turn ON if the VGS is below 1.0V

and

2) all the MOSFETs of this type made by this manufacturer will turn ON if the VGS is above 2.0V.

14. ### TheComet Member

Mar 11, 2013
88
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You'd be correct.

From Wikipedia: http://en.wikipedia.org/wiki/Field-effect_transistor

15. ### MrChips Moderator

Oct 2, 2009
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Source is the source of electrons.

16. ### Lineout Thread Starter Member

Nov 20, 2013
64
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How long before the rest of the world catches on , will it ever happen ?

Billions, trillions of components would need to be redisigned.

17. ### TheComet Member

Mar 11, 2013
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Not in a p-channel MOSFET it isn't.

18. ### tracecom AAC Fanatic!

Apr 16, 2010
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I just read that the terms drain and source are a throwback to the water analogy for current flow. The water comes from the source and goes out the drain. I don't know whether what I read is true or not, but it has a certain simple logic to it.

Remember that MOSFET gates are switched by voltage levels, not current levels. In my limited experience, MOSFETs will often switch reliably below their rated gate voltage. So if you are building a one-off circuit, you can often get it to work with gate voltages that are less than the minimum spec'd by the manufacture. However, the manufacturers don't guarantee that, thus doing so is certainly not good for products that are going to be commercially produced, but hobbyiests often do so.

19. ### TheComet Member

Mar 11, 2013
88
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Not in a p-channel MOSFET it isn't.

20. ### wayneh Expert

Sep 9, 2010
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I don't know if it's relevant to this discussion, but I think it's worth noting that a conducting MOSFET will conduct in either direction. A non-conducting MOSFET will conduct from source to drain via the body diode.