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Looking for a certain transistor ...

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things

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hey guys, I am looking for a little transistor, that can handle "switching" about 3V @ around 300ma max. But one thing I need it to do, is if I give it a 0 to 5V signal to its base, I want it to pass an amount of current accordingly. Since I cant think of any way to describe it, like a signal controlled potentiometer. If I give th base 5V, it will have no resistance and let full current flow. if I give it nothing, it shuts off completely, if I give it 2.5V it lets halfpower through etc. Like a analog transistor.

Anyone know of any commonly available ones? I guess they'd be used in amplifiers too ...

Cheers
 
Hi things,

as you certainly know the collector current of a transistor depends on base current and the transistors current amplification factor Δβ (delta beta).

E.g. base current 500µA, Δβ 200, so at 500µA base current the collector current should be 100mA.

Independent of the base voltage if 500µA (which of course results in a certain base voltage) of base current are achieved it will result in a collector current of 100mA.

Calculating the base resistor on the base of that fact you can determine the load resistance. Using a fixed load resistance the transistor won't be able to drive the load from a certain point of decreased base current. You can measure that at the collector. The more the collector voltage approaches +UB the less current flow the transistor has from emitter to collector.

In the given example I used an NPN transistor.

Boncuk
 
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Hi, what kind of NPN transistor would you recommend me using for this? It doesnt have to drive much, as I said, a max load of about 3V @ 300ma
 
What are you really trying to do? We frequently get questions that we waste time answering, only to find out that the OP's proposed "solution" won't even do what he needs. Give us the big picture.
I wouldn't recommend a base-current controlled transistor, even if you are on the right track. Transistor beta (current gain) varies with current, collector-emitter voltage, temperature, from unit to unit, and with the phase of the moon.
 
Thats what I am trying to do, find a transistor or MOSFET, that would vary the amount of voltage or current, allowed through its collector/emitter, based on the voltage on the base pin, which would be varying between 0 and 5V. 5V would let all the current/voltage through, 0V would let nothing through, 2.5V would let half voltage/current through etc.
 
In that case don't use any transistor. Use a variable voltage regulator wired as constant current source instead. Vary the current with an adjustable feedback resistor and add a fixed value resistor at its output. (Feedback resistor between Vout and adj.)

The reference current will be Vref/Rfb (Reference voltage divided by feedback resistance)

The LM317 can supply up to 1.5A and the LM317L can handle up to 100mA.

The minimum current flow for the LM317 is 10mA and for the LM317L it is 1mA.

Depending on the burden resistor the output voltage can be varied using Ohm's law. E.g. a current flow of 4mA results in 2V across the burden resistor if its resistance is 500Ω (0.004*500), at 20mA it is 10V.

For max 5V output voltage choose the LM317L with a fixed resistor

Boncuk
 
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Thats what I am trying to do, find a transistor or MOSFET, that would vary the amount of voltage or current, allowed through its collector/emitter, based on the voltage on the base pin, which would be varying between 0 and 5V. 5V would let all the current/voltage through, 0V would let nothing through, 2.5V would let half voltage/current through etc.
You missed the point. What are you going to do with the output of this device? Where does the input come from? We can't be very helpful without knowing these things.
 
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