uses of transistors

[daniel mandala]

in some circuits, transistors are used as a variable resistor. how does this happen?

 

[Scubasteve]

Show me a circuit where a transistor is used as a variable resistor, and we will help you understand it. I have seen FETs used as variable resistors, but not transistors.

Steve

 

[bogdanfirst]

yes, i have seen FETs used to control volume in audio aplications, or connected to various type of sensors to create a linear variation, or even in variable power suplies. i have seen a LM317 regulator wich was controlled by a computer and the FET was used as the variable resistor.
but for bipolar transistors i don't think that you can unse ONE transistor as a variable resistor, maybe in some specific aplications where many conditions remain the same.

 

[stevez]

Since the conduction or resistance from E to C is variable, dependent on base current, going from a very high resistance to low resistance at full conduction, couldn't we look at many transistor applications as variable resistors?

 

[Scubasteve]

I'm not really sure about that, I am just sure I haven't seen one used as a resistor before. I think it might be something to do with its inability to conduct in both directions. Also, I am pretty sure you cannot just attach an ohmmeter across the CE junction and expect a reading variable to base current.. Someone with a little more experience, please help clear this one up

Steve

 

[mstechca]

Show me a circuit where a transistor is used as a variable resistor, and we will help you understand it. I have seen FETs used as variable resistors, but not transistors.

FET = Field Effect transistor LOL!

Anyways, I think all transistors can be used as variable resistors. Their resistance is dependant on temperature. Just make a multistage transistor amplifer and use an LED as the output. The brightness depends on the pressure applied to the "BASE" pin. The more powerful the amplifier, the less pressure you need to light up the LED completely.

 

[stevez]

I should have been more clear with my first post - for one direction of current flow a transistor behaves much like a variable resistor. As the base current is varied the conductivity of the CE connection varies. The net result is that the voltage drop across the power carrying leads of the transistor goes from very low (when it is full "on") to whatever the power supply voltage is when in the "off" state. Power is wasted across the transistor - the product of the voltage drop times the current. So, in some ways it appears to function as a variable resistor.

Note that a valve that controls the flow of water is a good visual model for a transistor - and works for a variable resistor too. "Valve" is/was a term for vacuum tubes for similar reasons.

So, while a transistor can't be dropped in place of a variable resistor in all applications the concept that they are similar helps to understand how they work.

 

[Scubasteve]

Show me a circuit where a transistor is used as a variable resistor, and we will help you understand it. I have seen FETs used as variable resistors, but not transistors.

FET = Field Effect transistor LOL!

Anyways, I think all transistors can be used as variable resistors. Their resistance is dependant on temperature. Just make a multistage transistor amplifer and use an LED as the output. The brightness depends on the pressure applied to the "BASE" pin. The more powerful the amplifier, the less pressure you need to light up the LED completely.

Okay smartie.. When someone says 'transistor' in electronics, it is an assumption they are talking about BJTs. I know what FET stands for, so please give it up.

In electronics, when someone says 'transistor', it is assumed they are speaking of a BJT. All that I know is that I have never seen the BJT intended to be used as a variable resistor, I have seen the FET used for this though. I didn't say the BJT could not be used, just that I haven't seen on used to vary resistance.

We know how to vary a LEDs intensity, talk about resistance if you want to try and poke fun at someone trying to help.

Steve

 

[bogdanfirst]

consider a simple transistor, a NPN. with E connected to GND and C connected via a resistor to Vcc. assume that the gain is 50 and Vcc is 5V. you apply a current of 1mA to the base. then you have 50mA going trough the C(aprox.) and let's say the resistor is 10R then the EC voltage will be 4.975. so the 'resistance' of the transistor is 90R. if you encrease the Vcc voltage to 10V then the 'resistance' will be 190R. so the resistance is not stable with the voltage applied.
but it is different with FET's. they conduct in both directions(if i am not mistaking'), and if you make a resistor with a FET, the resistance remains the same when the current trough it modfies. so in conclusion BJTs are not so adecvate for using as a resistor.
as in the example when you use it to drive a led. if you replace the led with a resisto and mesure the voltage drop on the transistor and the current troght the resistor and divide them, you will see that the value modifies when the power suply voltage modifies.