Transistor Question

Hi everyone,

I would like to know if a Transistor has a simple on/off state, or can a transistor control the amount of current/voltage flowing from the collector to the emitter.

Regards, J

Can be ON/OFF or can control current flow.

jgporteous: can a transistor control the amount of current/voltage flowing from the collector to the emitter.

Yes, but doing it accurately is very difficult...

Take a look at a datasheet for the BC327 (a common BJT transistor). My copy says that the gain is anywhere between 100 and 630. It doesn't even list a value for "typical".

If it were easy then audio amplifiers would just be a transistor in a box with a power supply instead of a huge mess of transistors, resistors and capacitors.

Negative feedback is usually employed to get accurate linear amplifier response out of a transistor (or any other non-linear ampifying device).

BJTs are fairly good current amplifiers if the collector is more than a volt or two above the emitter voltage (for NPN). For switching mode you never want to be in any state but saturated or off, for analog electronics you want to avoid both of those.

If you want to control current accurately, you would need to use a precision current control source or sink circuit

Examples: Precision Current Source: http://www.linear.com/solutions/1563 Precision Current Sink: http://www.linear.com/solutions/1562

It might be simpler to make use of the TL431 Adjustable Precision Shunt Regulator, look at fig. 28 and 29. http://www.ti.com/lit/ds/symlink/tl431.pdf

MarkT: Negative feedback is usually employed to get accurate linear amplifier response out of a transistor (or any other non-linear ampifying device).

But normally you'd use an op-amp for that, not a bare transistor.

Thank you everyone, this is very useful information and i will read up on all your comments/suggestions.

The reason I posted this: I created my first circuit with a BD241C Transistor, powering a fan. I noticed that when I checked the voltage on the pin and grnd, it was high or low, although my code looped through the values of 0 to 255.

I then thought it might be due to that, that I could only turn it on, or off, or maybe it could be the transistor.

I thought of something else, I'm using an arduino pro mini, with 5V in put. What if I used a 12V to supply the fan, via the transistor, but shared the ground of the 5V and 12V, would that damage my arduino? is that the preferred method.

Many thanks, J

fungus:

MarkT: Negative feedback is usually employed to get accurate linear amplifier response out of a transistor (or any other non-linear ampifying device).

But normally you'd use an op-amp for that, not a bare transistor.

High quality solid state hi-fi amplifiers have been using transistors for linear amplification for decades and I suspect some high end units still do as the 'golden ear' crowd still frown on the use of op-amps in the signal path (yes, foolish I know).

I noticed that when I checked the voltage on the pin and grnd, it was high or low, although my code looped through the values of 0 to 255.

How are you checking the voltage, a meter will not do as the values of 0 to 255 an an analog write give you a PWM signal not an analogue one. See:- http://www.thebox.myzen.co.uk/Tutorial/PWM.html

Thanks Grumpy_Mike, that makes sense.

Can I use the digital pin to control voltage and hence control the transistor?

Hi everyone,

I've managed to get my arduino to turn on and off a fan. The fan is on a 12V supply and the arduino on a 5V.

I'd like to size my transistor to allow a max of 10A, or a little higher to be safe. Other than reading each datasheet, is there an easy way to find or size a transistor? I'm currently using a BD241C (Complimentary Transistor) - NPN.

For the next part of my experiment, I'd like to figure out how to control the amount of current to the fan, but I see some of you mentioned a circuit I need for that, so I will read up on that now.

Thanks, J

I've managed to control the speed of the fan with pin 9 :) I can also see the voltage increase and decrease with the voltmeter.

Read up on how PWM works.

http://en.wikipedia.org/wiki/Pulse-width_modulation

MarkT:
for analog electronics you want to avoid both of those.

Except when you want clipping (guitar pedals, mainly)…

cr0sh:

MarkT:
for analog electronics you want to avoid both of those.

Except when you want clipping (guitar pedals, mainly)…

Ah, but discriminating guitar players will only use vacuum tube guitar amps, they distort better. :wink: