Low Voltage Transistor and high Voltage transistor

[alphaai]

Hello,

I a newbie here. I've a question about transistors.

If I use a transistor to cause higher amount of currents to flow through the collector to the emitter by turning the transistor on,

1) how would I know how much input voltage can be applied to the collector and how much input voltage can be applied to the base? Is there a range? I see Vc

2) Would this have anything to do with choosing a Low Voltage Transistor or a High Voltage Transistor?
Lets say I have a motor which may require 12volts to run, does this mean I would
need a high voltage transistor? likewise, if I had a 5v motor, would I need a low voltage transistor to run it?

3) How would you know if your transistor would require a heatsink? Is there something to look out for in the specs that could determine how hot it could get?
I see 'PD @ 25 Degrees Celsius'.

Thanks heaps,

Ben

 

[ronv]

Specs.

1) how would I know how much input voltage can be applied to the collector and how much input voltage can be applied to the base? Is there a range? I see Vc

If you look at page1 of the data-sheet you will see Vceo and Vcbo. this is the maximum voltage the transistor can see.

2) Would this have anything to do with choosing a Low Voltage Transistor or a High Voltage Transistor?
Lets say I have a motor which may require 12volts to run, does this mean I would
need a high voltage transistor? likewise, if I had a 5v motor, would I need a low voltage transistor to run it?

Normally high voltage transistors are higher voltage. You can run a lower voltage on a higher voltage transistor.

3) How would you know if your transistor would require a heat-sink? Is there something to look out for in the specs that could determine how hot it could get?
I see 'PD @ 25 Degrees Celsius'.

Again on page 1 you will see a spec. for thermal resistance junction to ambient. For the attached transistor it is 200 degrees C per watt. You will also find maximum operating temperature (150C). So this transistor could dissipate .75 watt but it would be very very hot. Usually I try to keep the temperature below 50 or 60 C.

https://www.electro-tech-online.com/custompdfs/2011/05/PN2222A.pdf

 

[crutschow]

3) How would you know if your transistor would require a heatsink? Is there something to look out for in the specs that could determine how hot it could get?
I see 'PD @ 25 Degrees Celsius'.

Whether a transistor requires a heat sink is determined by the power rating of the transistor and the amount of heat the transistor dissipates in the circuit. The transistor power dissipation (heat) for DC switching applications is equal to the current going through the transistor times the collector-emitter voltage drop when the transistor is on (not the supply voltage).

For example, when a transistor is full on as a switch (saturated) then it's collector emitter voltage is typically less than a volt. Thus the power dissipated depends upon both the circuit and the transistor characteristics (such as it's saturated on voltage when carrying the circuit current).

If the power is too high for the transistor to dissipate by itself then you will need to add a heat sink. The size of the heat sink is determined by the amount of power the transistor is dissipating, the ambient temperature, and the thermal resistor of the transistor junction to case.

 

[alphaai]

Ok.

When you say 'You can run a lower voltage on a higher voltage transistor.'
Are you meaning that instead of choosing a low voltage transistor, I could choose a high voltage transistor which includes the low voltage range.

eg.
Low Voltage Transistor 2N3638 has a Vc of 25
High Voltage Transistor BC640 has a VC of 100
so with the BC640 I could still use a voltage of anywhere up to 25Volts and beyond?
It makes more sense to use that one.
Is that right?

Thanks,

Ben

 

[ronv]

Transistors

When you say 'You can run a lower voltage on a higher voltage transistor.'
Are you meaning that instead of choosing a low voltage transistor, I could choose a high voltage transistor which includes the low voltage range.

If it is a general purpose use that is true. If you are building an amplifier the 3638 has less noise so it might be better. It also has a higher gain, so if you are looking for a lot of amplification in a linear amplifier it might work better.

The BC640 is higher power so might be better for a transistor switch.

Just to have some around I would choose the BC640.