# i need find the value of the Base resistor (Transistor Switching Circuit)

Discussion in 'General Electronics Chat' started by adnan20, Nov 28, 2009.

Dec 19, 2007
14
0
hi all

i need find the value of the Base resistor (Transistor Switching Circuit)

2. ### steveb Senior Member

Jul 3, 2008
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469
$I_b={{V_s}\over{R_c\; \beta_{min}$

where $R_c$ is the collector resistance and $\beta_{min}$ is the minimum possible value of the transistor current gain (beta) under all conditions, including temperature and manufacturing variations.

Once you calculate the base resistance, round down to the next lower standard value. It's better to have too much current rather than too little. For this reason, use a value of $V_{be}$ that is larger than the maximum you expect under all conditions. Also, this is why I did not subtract the transistor saturation voltage (about 0.2 V) from the supply voltage. Similarly, use the largest expected $V_s$ and smallest expected $V_{in}$.

3. ### Audioguru Expert

Dec 20, 2007
10,609
1,184
No.
Beta is not used for switching transistors. It is used when a transistor always has plenty of collector to emitter voltage when used as a linear amplifier.

The datasheet for nearly every transistor shows the max saturation voltage when the base current is 1/10th the collector current.

Dec 19, 2007
14
0
thank you steveb

But , i need calculate βmin

Dec 19, 2007
14
0
thank you Audioguru

ok

But , i need calculate Rb and Ib

6. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
OK, when used as a saturated switch, Ib is usually Ic/10.

So, if you need to sink 100mA current from the collector, you need 10mA base current.

The formula could be restated as:
Rb = (Vin-Vbe)/(Ic/10)

As a side note, 2N3904, 2N4401 transistors are rated for Ic=200mA, but 100mA is their practical limit
2N2222's are rated for Ic=800mA, but 400mA is a good practical limit.

If you are controlling a transistor from a uC, you're pretty limited as to the source/sink capability of the I/O pins. 10mA is likely safe for most uC's.

Last edited: Nov 28, 2009
7. ### steveb Senior Member

Jul 3, 2008
2,433
469
This is definitely a good point by both you and SgtWookie. My answer could be considered the minimum Ib that can be used safely. The application really dictates whether it is critical to operate deep into saturation.

8. ### Audioguru Expert

Dec 20, 2007
10,609
1,184
Datasheets have curves of typical transistors that saturate deeply and written spec's for the max saturation voltage for a transistor that is not as good as a typical transistor and its saturation voltage is not very "deep".

When you buy a transistor you don't know if it has excellent spec's, typical spec's or not so good spec's. Don't you want every circuit you make to work properly without testing the saturation voltage of every transistor?

9. ### lmartinez Active Member

Mar 8, 2009
224
6
I agree with you. Please provide the overall goal of your electrical circuit and what type of transistor are you reviewing to incorporate into the electrical circuit.

10. ### steveb Senior Member

Jul 3, 2008
2,433
469
Yes, of course I don't want to test every transistor. Basically, I agree with your comments which is why I said you had brought up a good point. If you notice, my post was very careful about using worst case numbers for everything in the calculation, including the minimum possible beta with manufacturing variations, temperature, current etc. In situations where low saturation voltage is not critical, I often use values of 20 or 30 rather than 10 (if the data supports this decision), but I agree that the saturation voltage will be higher if this is done. However, his simple example does not have any specifications about how saturated the transistor needs to be.

A simple way to modify my stated method to your satisfaction is to simply use $\beta_{min}$ set to 10. This is simple and straightforward. However, it's important to consider all of the worst case conditions (as I recommended) in order to guarantee that 10Ib>Ic . In other words, changing the current ratio, does not relieve you of doing the worst case analysis.

A final note. There is no rule that says 10 is the magic number. You can reduce the saturation even further if you use 5. So, I disagree with the idea that you need to be locked in at a ratio of 10. This is just a common standard to quote the specs. Different applications may cause you to choose 5 or 20 to get a particular benefit. Lower numbers reduce saturation voltage and higher numbers reduce power in your base driver circuit.

11. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
This discussion could go on for weeks.

Sure, you could use a number of approaches.

My point is pretty basic; this boards' membership has a rather high concentration of "electronic newbies", who are struggling valiantly to do something like getting their first LED to flash on and off.

In order to help such budding enthusiasts have very high probability of success with a minimum of confusion, it is a good idea to put forth the simple standard "rule of thumb" of Ib=Ic/10 when using a transistor as a saturated switch. If they follow that rule, they have a very high probability of good results.

12. ### steveb Senior Member

Jul 3, 2008
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469
Definitely a good point!

13. ### Audioguru Expert

Dec 20, 2007
10,609
1,184
Especially when the NOOBIEs have the collector and emitter of the transistor reversed.