Author Topic: Transistor question  (Read 1992 times)

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Offline h3roTopic starter

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Transistor question
« on: April 03, 2008, 10:01:28 AM »
Hallo, I have a very basic question about transistors.

If you simplify a transistor, it is really just a switch?

What I am trying to do use use a small voltage as an input to trigger something, but I am not sure how to do this.
The low voltage input triggers the transistor which makes the other part of the circut work(a small lamp and a 5v power source), I have tried connecting the different parts in many ways, but I always get the same result. Either the light is always shining (no matter if it gets an input from the small voltage or not) or it is never shining.

Could someone please help me with how to connect this? Or is it even possible?

Thanks

Offline szhang

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Re: Transistor question
« Reply #1 on: April 03, 2008, 10:13:16 AM »
No a transistor is not a switch.  Transistors are amplifiers.  If you apply a current at the base, you get a multiple of that current through the collector-emmitor junction.  You could wire them as a switch I suppose, but in that case a FET is a better choice.

If you just want a switch (that doesn't have to switch very fast), a relay might work better.

Offline h3roTopic starter

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Re: Transistor question
« Reply #2 on: April 03, 2008, 10:36:49 AM »
Thanks for your reply.

Do you know of anywhere I could look to see an example on how to wire a transistor as a kind of switch?

Thanks,

Offline szhang

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Re: Transistor question
« Reply #3 on: April 03, 2008, 10:37:57 AM »
*cough* google *cough*

http://en.wikipedia.org/wiki/Transistor#Usage

Check out the image to the right.

Offline BANE

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Re: Transistor question
« Reply #4 on: April 03, 2008, 11:03:47 AM »
you could also use a capacitor to filter out low signals if your using it as a (switch) if your using it the way i think you are.  (i could be wrong or misunderstood something but thats just my experience)

Bane

Offline alessio136

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Re: Transistor question
« Reply #5 on: April 03, 2008, 11:14:21 AM »
Hallo, I have a very basic question about transistors.

If you simplify a transistor, it is really just a switch?

What I am trying to do use use a small voltage as an input to trigger something, but I am not sure how to do this.
The low voltage input triggers the transistor which makes the other part of the circut work(a small lamp and a 5v power source), I have tried connecting the different parts in many ways, but I always get the same result. Either the light is always shining (no matter if it gets an input from the small voltage or not) or it is never shining.

Could someone please help me with how to connect this? Or is it even possible?

Thanks

you should use MOSFET if you wish to create a switch. moreover, "transistors as switches" is the base of CMOS logic, basic element of every digital processor.

take for example a NMOS. put the low power signal (TTL for example) on the gate, ground to the source and load between drain and power supply: if you pull gate up, a big current can flow in the transistor and in the load. otherwise, if you keep gate at 0V, no current can flow in the rest of the circuit.

ideally you can use a transistor just like a short-circuit or an open-circuit, just depending on the difference of potential between gate and source.

Offline szhang

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Re: Transistor question
« Reply #6 on: April 03, 2008, 11:23:53 AM »
ideally you can use a transistor just like a short-circuit or an open-circuit, just depending on the difference of potential between gate and source.

Yes, but only for FET transistors.  BJT's are a bit messier in that regard.  For BJT current WILL flow from the base to the emitter, so you must place your lamp on the collector side (the side without the arrow in the schematic) if you want a clean switch.

Offline alessio136

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Re: Transistor question
« Reply #7 on: April 03, 2008, 01:24:19 PM »
ideally you can use a transistor just like a short-circuit or an open-circuit, just depending on the difference of potential between gate and source.

Yes, but only for FET transistors.  BJT's are a bit messier in that regard.  For BJT current WILL flow from the base to the emitter, so you must place your lamp on the collector side (the side without the arrow in the schematic) if you want a clean switch.

of course I was talking about mosfet, indeed I mentioned gate and source.

Offline benji

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Re: Transistor question
« Reply #8 on: April 03, 2008, 03:30:45 PM »
BJTs as switches

1/ cut off  area  (the transistor is an open circuit)
2/saturation area (its a short)

make sure the two operating points are in these 2 areas and you got yourself a digital switch
good ol' BeNNy

Offline ALZ

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Re: Transistor question
« Reply #9 on: April 04, 2008, 01:15:18 AM »
Hi Hallo:

There are two kinds of transistors NPN & PNP. If you are useing neg. as ground then you should use the NPN type. It only takes .6 volts to start to turn on a  transistor. So small voltage doesn't tell us much. If your "small voltage" is always over .6 volts then your  transistor will always be on. Give me more info. and I can give you a better circuit. Just off the top of my head: Put lamp from +5 volts to collector, put a 1k resistor between your  "small voltage"  to the base and put the emitter to ground. If your "small voltage"  is always just a little over .6 volts when you want the lamp to be off then just add a diode with the 1k resistor to the transistor's base.  A FET is NOT really the way to go with the power supply only being 5 volts. You didn't say how much current the light will be taking. You can get a small transistor to give you 300 mil amps as a switch with no problem.  Cost of circuit: 1 cent for  resistor, 2 cents for transistor (if you buy 10 at a time).

Hallo, I have a very basic question about transistors.

If you simplify a transistor, it is really just a switch?

What I am trying to do use use a small voltage as an input to trigger something, but I am not sure how to do this.
The low voltage input triggers the transistor which makes the other part of the circut work(a small lamp and a 5v power source), I have tried connecting the different parts in many ways, but I always get the same result. Either the light is always shining (no matter if it gets an input from the small voltage or not) or it is never shining.

Could someone please help me with how to connect this? Or is it even possible?

Thanks

 


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