transistor bias

youngbill

I am using a NPN transistor to turn on a 100 ma LED string. I have the hook up with common emitter.

I understand this biases is 180 degree out of phase,,,I can understand that,,with a AC signal...but when I use this as a switch,,when base is on,,,load is on...Am I missing some general understanding here..

So Is this circuit OK for this purpose?

Phil Pilgrim (PhiPi)

No, you're not missing a thing. When the base goes hgh, the collector is pulled low (i.e. 180-degrees out of phase), and that's what turns on the load.

-Phil

youngbill

Phil Pilgrim (PhiPi) wrote: »

No, you're not missing a thing. When the base goes hgh, the collector is pulled low (i.e. 180-degrees out of phase), and that's what turns on the load.

-Phil

Ahhh,,,Yea thats a better way of saying it...thanks.
...One other question...my load is 100 ma....say 12 volts...( the voltage don't matter as such)...but here is the question,,,I am using a tip 122,,,( gain 1000)..and good for 3 amps,,,,,I am using a 1 k resistor on the base( its driven from a 74hc595 shift register,,,,Is there any problem with going to such over kill,,,,I mean its saturated,,( 1000 gain it don't take much to saturate ),,and good for 3 amps when i am using 100ma,,,,Is there anything wrong,,,it seems to work,,,I want to be sure it will work....THANKS for the help

kwinn

No problem with that setup. When the transistor is saturated the current is determined by the load.

youngbill

Ok...I am testing and learning about a pn2222 transistor. All the many calulations i do allways come up with aprox 7 or 8 k ohms for saturation of the transistor.
The load is 100 ma,,with 5 volt load and signal. i use a 50 ohm as a test load.
I was adusting a variable resistor on the base and watching when Vc-e gets low,,( .2 volts seems saturated),,,and no more Ic for more base current.
But then when I measure my base resistor...I get aprox 3.3 k NOT the 7.7K I calulate....Do I need to plot on a graph to actually see ..or is what I am doing with watching the meters acurate enought?
If i plot do I use a variable voltage on the base?
I want to find the base resistor,,,,so I know what Rb to use to saturate
usually I use common emitter bias..
And my math is like this,,,Ib =Ic /gain or 100ma/ 200= .5ma
So Rb is 4.4V/.5ma = 8.8 k ohm any help

kwinn

My data sheet for a 2N2222A shows a min Hfe of 100 at Ic = 150mA and Vce of 10V, so a 3.3K base resistor is reasonable. For use as a switching transistor I might even go a bit lower to be sure it is saturated.

youngbill

But,,why is my math wrong? why can I not calculate the real life saturation level of 2 or 3 k,,,why is the math at 8.8k

Tracy Allen

1) The gain of the transistor decreases as it enters saturation. On your 5V power supply the gain of 100 applies when the voltage across your 50Ω resistor is 1V and Vce across the transistor is 4V. The deeper you want to force it into saturation, the lower is the effective gain. Looking at the MMBT3904 data sheet, it specifies Vce-sat as 0.3V when Ic=50mA and Ib=5mA, so the effective gain at that point is x10. If you only need to drive Vce down to 1V, it is not yet quite in saturation, but the data sheet says that the gain at that point is hFE=30.

2) The gain hFE is not well controlled in manufacture of transistors. The MMBT3904 data sheet specifies a gain range of 100 to 300, and again, that applies only to the non-saturated measurement conditions.

bill190

Here is a nifty transistor simulator...
http://transistor.20m.com/transistor2.html

And the free Qucs circuit simulator...
(Windows Setup.exe type install 3/4 way down page)
http://qucs.sourceforge.net/index.html

Best instructions for Qucs here (Workbook)...
http://qucs.sourceforge.net/docs/workbook.pdf