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Author Topic: Transistor Question  (Read 1145 times)

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Offline airman00Topic starter

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Transistor Question
« on: January 08, 2009, 05:40:30 PM »
In this schematic which uses a common NPN transistor


What would be the voltage at the OUTPUT?

Thanks,
Eric
« Last Edit: January 08, 2009, 06:20:28 PM by airman00 »
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Offline Webbot

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Re: Transistor Question
« Reply #1 on: January 08, 2009, 06:24:42 PM »
Not sure if thats a standard transistor or a MOSFET or some other complex beastie. But if its a normal transistor: then the answer is - a DEAD transistor. The transistor is a switch. Looking at your diagram: there are pins: the base (middle left), collector (top right), and emitter (bottom right). If the base is less than 0.6v above the emitter then the switch (between collector and emitter is open). So if you tied the base to 0V then the switch would be open: collector=12v, emitter=0v.  But you have connected the base to 12v which is much greater than 0.6v above the voltage on the emitter (which is 0v). This, in itself, would probably fry the device. If it doesn't then the switch will close and so you now have a short circuit between 12v and 0v and so the 'infinite' current flowing thru the transistor will kill it anyway. So you've probably killed it in two different ways !!!


That apart: the output will be 0v irrespective of what you do to the base. Once the transistor has died it will probably still be at 0v.

You need to have some sort of resitance between either: +12v and the collector, and/or the emitter and 0v, and/or the base and the input signal that is driving it.

But what are you trying to do?

« Last Edit: January 08, 2009, 06:25:51 PM by Webbot »
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Offline airman00Topic starter

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Re: Transistor Question
« Reply #2 on: January 08, 2009, 07:01:21 PM »
I didn't make myself clear

I know it will short circuit(I should have put some load between emitter and ground in the sample schematic....), I just wanted to know if a transistor can switch a 12V source if the base is getting 5V. I know it can do more current, but i'm not sure about more voltage.
Basically I want to switch a 12V supply from the 5V logic supply of the microcontroller- and I do not want to use a relay.
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Offline Webbot

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Re: Transistor Question
« Reply #3 on: January 08, 2009, 07:32:21 PM »
ok - so it will depend on the amount of current you want to flow through the collector-emitter (12v supply).

If you know that then first check the transistor can cope with that amount of current. At first glance it looks like the 2N706 can only deliver 10mA - which looks kinda small

Next look at the gain of the transistor. Typically they can be around 50. So this means that the current in the base is 1/50 th of the current required from your 12v supply.

Check the AVR datasheets, but off the top of my head, an output pin can supply about 40mA. So for 10mA divided by 50 gain = 0.2mA at the base - which is easy below 40mA so the mcu can supply the required current.

Then you need to place a resistor between your output pin and the base to limit the current. Using V = IxR then R=V/I = 5v/0.2mA = 25K ohms minimum.

You can calculate the wattage of the resistor via W=IV = 0.2mA*5 = 1mW

So then you have (sorry didn't have the time for a diagram):-

                                                  12v
                                                     |
                                                     |
                                                  Load
                                                     |
                                                     |
mcu pin -------- 25Kohm ----------- base

                                                     |
                                                     |
                                                     |
                                                     |
                                                     0v

So the main thing is to work out the minimum resistance, and maximum current, for your load.
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