transistor

Thread Starter

electronics1

Joined Aug 21, 2009
42
I used transistor as switch In the basis I put a resistor
How I calculate the resistor so that don't burn the transistor
 

alitex

Joined Mar 5, 2007
140
I used transistor as switch In the basis I put a resistor
How I calculate the resistor so that don't burn the transistor
if u want to use it as switch,put 4.7 K ohm to collector and 1K to base
put 5V to collector through 4.7K resistor take output between collector and 4.7K ohm,that if you want take (not switch),
if you want opposite put 4.7k to emitter
 

mik3

Joined Feb 4, 2008
4,846
The current through the collector is limited by the 1K resistor.

To find R2 you need to know Ic.

Assuming Ic is 10mA

then

Ib=Ic/β

β depends on the transistor but for your application R2=2K is ok.

To find R2:

R2=(Vapplied-0.7)/Ib
 

ELECTRONERD

Joined May 26, 2009
1,146
β depends upon the transistor. In the transistor you provided in the specs, they say that it can go from a β of 120 to 700. You can choose the β within those values, and it represents the gain of the transistor.
 

alphacat

Joined Jun 6, 2009
186
β depends upon the transistor. In the transistor you provided in the specs, they say that it can go from a β of 120 to 700. You can choose the β within those values, and it represents the gain of the transistor.
But he wishes to operate the transistor in the saturation region.
In BJT's saturation region, IC doesnt equal to β*IB (Since in saturation region, both junctions - BC and BE - are forward biased).
So how can β be helpful in the saturation region?
 

alphacat

Joined Jun 6, 2009
186
But cant β be used only when the transistor is in the forward biased region, and not in the saturation region?

In order to reach ibmin=83uA, you used the formula ib = ic/β, which is valid only in the forward biased region.
In the case of using the BJT as a switch, you force it to be in the saturation region.
 

Jony130

Joined Feb 17, 2009
4,924
You suggest that the bjt will not be saturated for Rc=1K and RB=47K, Vcc=12V.
Maybe you build this circuit.
What then, cause bjt to saturted? Can BJT be saturate when Rc=0
 

alphacat

Joined Jun 6, 2009
186
Well,
I'm talking in general.

Which of the two following general notes isnt true?
- When using a BJT as a switch, you want to have the BJT saturated.
- When the BJT saturated, IB doesnt equal IC/β.

If they're both true, then how could you use β when using the BJT as a switch?
 

hobbyist

Joined Aug 10, 2008
887
If the transistor needs to be saturated, then according to the data sheet IB=10mA. @ IC = 100mA.

There using a forced B=10.

So your schem shows Ic=10mA. min. so a starting point for R2 could be choose IB=1mA.

(5V.-0.7V) / 1mA = 4.3K ohms.

Then work from there checking Vce, as well as IC, then adjust the R2 value as needed.
 

Jony130

Joined Feb 17, 2009
4,924
If they're both true, then how could you use β when using the BJT as a switch?
I simply assume that β will not drop below 120. And then for Rb<Rc/β_min BJT must be saturated.

Try the other way. Determine in which state the BJT (BC548) is for:
Rb=100K; Rc=1K; Vcc=10V
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,251
The beta (hFE) is used when the transistor is not saturated and is a linear amplifier with 6V or more between its collector and its emitter.
The datasheet shows that the transistor saturates pretty well as a switch when its base current is 1/10th its collector current.
 

alphacat

Joined Jun 6, 2009
186
Hey Audiogure,

Could you please give an example of a datasheet which shows that when IB = IC/10, the BJT is saturated?
 

Audioguru

Joined Dec 20, 2007
11,251
Hey Audioguru,

Could you please give an example of a datasheet which shows that when IB = IC/10, the BJT is saturated?
Almost every little American transistor like the 2N3904 and 2N4401 has a spec'd max Collector-Emitter Saturation Voltage when the base current is 1/10th the collector current. Its spec'd beta is much higher than only 10.

The European BC548 has a spec'd max Collector-Emitter Saturation Voltage when the base current is 1/20th the collector current which is also far from its spec'd beta.
 

zgmfx20a

Joined Sep 16, 2009
5
Very informative thread, thanks !

Is a npn transistor still a good choice if I'm trying to switch oscillating signals like a clock? Basically, I'm trying to short an oscillator clock to ground using the transistor here - http://digikey.com/scripts/partsearc...MMBT2369ATR-ND (its the only one I have available readily). Using the info from this thread the values would be:

Vb = 3.3V (need this fixed at 3.3V)
Rb = 2.6k
Rc = 26k (= βsat * Rb)

Is this correct ?
 
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