Transistor Question

[Nigel Goodwin]

I'm not an expert mind you, but everywhere I have read 1/10th is the rule of thumb.

A 'rule of thumb' depends on how big your thumb is!

Generally it's a question of stuffing as much current as you can down the base!.

 

[Sceadwian]

hrmm mmrmmrm m fff mmhrr mfmmffr!

 

[Blatman Bond]

The ratio of R3 to R2 determines at what minimum input voltage the LED turns on. Your values didn't work (the NPN never fully turned on).

Look at my values:

That you mean, my value? It is work in real (not in simulation).
In your simulation it is turned-on when input voltage only 2V. When input reach 12V it is reach somewhat ~2V hard saturation. My value is ~1volt Vbe normal saturation (enough for driving the LED).

This also my value for 5V input: 2K2//100pf Rb+C and 680 Rbe for fast ; 4K7 Rb and 1K Rbe for slow speed.
Try it with simulation then, might be wont work, but it is work in real.

 

[Blatman Bond]

Also don't forget to look manufacture graphs about saturation characteristics (might be included in other transistor spec that specially for switch).
Normally they used β=10 for saturation value.

 

[paul_ma7]

figuring out a transistor

I'm fairly inexperienced with electronics in general, but I've learned quite a bit over the last many months of self instruction (and some really good help from forums like these). I have one question on transistors whose answer has eluded my normally excellent googling skills, so I figure I would just throw out the question on this forum in hopes of getting an answer.

With a regular 2n3904 transistor, what happens if my base voltage is significantly higher than the voltage that I am switching on the Collector/Emitter? 12V on the base side, 5V on the collector/emitter side.

I've looked at a few javascript simulators I've found online and it says my Ve=11.3V (allowing for the forward voltage drop b to e.). Is this correct? I was worried that the simulators might not handle this "reverse scenario" correctly.

Here's an example scenario:

I have a garage opener button in my garage just outside the back door. It operates on 12V. Close the circuit by mashing the button and the door opens/stops/closes.

I would like to hook up a MCU (Atmel) running at 5V to the button to detect when the button is pressed. All the inputs are max 5V, so I was thinking of using a transistor as a switch. But I worry I'll send ~12V to my MCU inputs, which is grossly out of spec and generally a bad thing.

So, is a transistor just the wrong part for the job if my base voltage is higher? I really want to understand when I can and when I can't use a transistor for such things. What about PNP?

I know there are other solutions like a relay or a solid state relay/optoisolator, which would protect the 5V side from the 12V side. This is more about understanding how a transistor works than figuring out this particular example. I've run into this situation on more than one occasion.

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your transistor will switch without any problems,
relays will switch high voltages (125ac and above),
the maximum voltage a transistor can switch is in its datasheet, also the maximum wattage,
what you need to be concerned is that the current at the base of the transistor is small, how small?
everything it's state in the datasheet

 

[audioguru]

A 2N3904 transistor is an NPN type. When its emitter is at 0V then its base can never be higher than about 1.0V because the base-emitter is like a diode. The MCU applies up to 1V at a low current to the base to turn on the transistor.

A 2N3906 transistor is a PNP type. When its emitter is at +12V then its base can never be less than 11.0V. Because the base is at a voltage that is much higher than the 5V for an MCU then a special circuit is needed in between.

The collector's load voltage might go to the base of a transistor only when its load and the transistor blow up.