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How much current/voltage should be applied to the base of a 2N3904 NPN BJT? I'm trying to use it as a switch to allow the flow of 12v. I have 3v to work with for the base. A detailed explanation would be appreciated! Thanks
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Bipolar transistors are principally current-amplifiers - the current though the emitter-collector circuit is determined by the current through the emitter-base circuit.

Base-emitter voltage is in the range 0.5 to 0.7V or so, so with 3V to play you can set the current to the base by selecting a series-resistor for the base (10k would give about 250uA, 330 ohms would give about 7.6mA).

The DC current-gain of the transistor determines the ratio between collector and base currents IF THE DEVICE IS NOT SATURATED.

DC current gain does depend somewhat on current, tends to fall off at higher collector currents, note.

So if you want to carry 500mA via the collector, and the current gain is 50, then 10mA of base current will allow this, but the device won't saturate (it won't be fully on).  You need to increase the current through the base (a factor of 2 or 3 should be OK) - then the transistor is fully on and the collector voltage will fall close to the emitter voltage.

So the procedure is:

1) determine the DC current gain from the data sheet at the current levels you will be using.  Data sheets normally give "min", "typical" and "max" - only the "min" value is any use since you want your circuit to work with any particular instance of the device.

2) determine the base current needed to saturate the transistor when its on (several times the calculated base current).

3) pick a base resistor that will give this current
 
4) check the Arduino can supply that much current (30mA is about the limit).  Check the transistor Vsat, multiply by the current to check the power-dissipation in the on-state.  Do you need a heat sink?  A bigger transistor?  Check the transistor can handle the voltages (its Vceo is well above the supply voltage, perhaps a factor of 2).

Finally when the datasheet uses the phrase "Absolute Maximum Ratings", it does not mean "typical operating conditions"!!!
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Mark, I read your reply and have been Googling a lot of the words/concepts you have supplied me. So I have a few questions. What I would like to start with is the saturation of the transistor. Why is this needed and what is it for? To my understanding, the applied voltage at the base must be .7v. Not sure as to why tho. I have .3 at the base, reducing the Rce, leaving the transistor partially on.

What must I know and do to saturate the transistor? Reducing the 3v from the arduino to. 7 is not an issue.
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http://www.fairchildsemi.com/ds/2N/2N3904.pdf

From the datasheet, you can see the device has a max Ice current flow of 200mA.
You can see the Vce at that current flow is also given. Thus you can arrange a current limit resistor to stay below that. I would stay under 100mA, none of the graphs cover current greater than that.

You can also see these
VCE(sat) Collector-Emitter Saturation Voltage
VBE(sat) Base-Emitter Saturation Voltage

So assume a Vbe of 0.7, 10mA, should get you good & saturated.
(3V - 0.7V)/10mA = R = 230 ohm resistor, use a 220 for a touch more current.
Then the transistor will switch on and conduct as much current as your 100mA current limit resistor will allow.
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For switching application of 200ma or less using arduino output pins, I use these days almost exclusively the 2N7000 mosfet in a TO-92 case. You can get them for pennies a piece in bulk and they work without any series gate resistor.

http://www.onsemi.com/pub_link/Collateral/2N7000-D.PDF

http://www.ebay.com/itm/SALE-New-30X-2N7000-MOSFET-N-CHANNEL-60V-0-2A-TO-92-Transistor-Free-Shipping-/180753568927?pt=LH_DefaultDomain_0&hash=item2a15c0949f


http://en.wikipedia.org/wiki/2N7000
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Ugh, that has an awful Rds:
RDS(on) = 5 
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Ugh, that has an awful Rds:
RDS(on) = 5 


Ugh? It's not a power transistor, why would you expect it to spec like one? What's the emitter/collector saturation voltage drop on a 2n3904?
« Last Edit: March 04, 2012, 11:10:20 pm by retrolefty » Logged

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Ok, I can see that too.
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Ok now as far as saturation goes, when I apply the 3v to the 220ohm resistor that is at the base; it will reduce the voltage to .7v? That will drive the transistor into saturation I assume. So when the 3v are applied and saturation is achieved, I'm guessing the Ice will allow the flow of the needed 12v current? Or will I have to have to apply yet another resistor at the collector end as well? Also, where does the 10mA come into play?
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Sorry for the excessive amount of questions but I am trying to fully understand how the transistor works. It is a complicated concept for me, that I am finding a tad bit complicated. Thank you for your patients and replies.
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Quote
I'm guessing the Ice will allow the flow of the needed 12v current? Or will I have to have to apply yet another resistor at the collector end as well? Also, where does the 10mA come into play?

Yes, some form of external current limiting external to the emitter/collector junction will be required. This could be the resistance of the load itself (like a relay coil or DC lamp) or in the case of led then a resistor would be added in series of the collector.


Lefty

 
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