Transistor Approximations: NPN BC548 vs. 2N3904, PNP BC559 vs. 2N3906

In summary, the engineer is attempting to create a circuit that will let them control a computer using RS-232 signals. They are having trouble finding the two transistors they need, and they are looking for help finding the right voltages and specs for the transistors.
  • #1
nomizzz
2
0
Hello everybody,

I'm currently taking Electrical Engineering as senior physics major at Fordham University and have decided to take on a little side project to gain a better understanding.

What I'm attempting to create is my own Transistor Logic -> RS-232 (Rs-232 refers to the serial (com1) port on computers). I found a perfect simple circuit schematic which I've attached below, and I've purchased the breadboard, soldering iron and all the exact resistors, capacitors and diodes required. My problem is I couldn't find the two transistors, NPN BC548 and PNP BC559 at any of my nearby Radio Shacks.

After comparing the spec sheets online and doing a little bit of googling, I found that PNP transistor 2N3906's characteristics and specs seem to fit pretty well with BC559 and BC548 fits fairly well with 2N3904's. Unfortunately, we've only just got into transistors in EE right now and I have no equations to simplify them and determine if the voltage will be operable or even destructive for the computer that will be receiving the signal and sending it back.

Below is a fairly comprehensive diagram for the circuit that I retrieved from a website. My alterations, as I said, will be subbing in the 2N3906 for the BC559 and the 2N3904 for the BC548. What will this mean for my final voltages?



Xbox_360_lite-on_rs232_simple.jpg
 
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  • #2
It's really arbitrary for DC analysis. Beta of general purpose transistors is anywhere from 100 to 250. They are both silicon transistors with about 0.7v drop.
 
  • #3
You should use the recommended voltages unless you know what is on the other end. The PNP collector output is received by some comparator or transistor with a threshold which you do not know.
 
  • #4
nomizzz said:
Hello everybody,

I'm currently taking Electrical Engineering as senior physics major at Fordham University and have decided to take on a little side project to gain a better understanding.

What I'm attempting to create is my own Transistor Logic -> RS-232 (Rs-232 refers to the serial (com1) port on computers). I found a perfect simple circuit schematic which I've attached below, and I've purchased the breadboard, soldering iron and all the exact resistors, capacitors and diodes required. My problem is I couldn't find the two transistors, NPN BC548 and PNP BC559 at any of my nearby Radio Shacks.

After comparing the spec sheets online and doing a little bit of googling, I found that PNP transistor 2N3906's characteristics and specs seem to fit pretty well with BC559 and BC548 fits fairly well with 2N3904's. Unfortunately, we've only just got into transistors in EE right now and I have no equations to simplify them and determine if the voltage will be operable or even destructive for the computer that will be receiving the signal and sending it back.

Below is a fairly comprehensive diagram for the circuit that I retrieved from a website. My alterations, as I said, will be subbing in the 2N3906 for the BC559 and the 2N3904 for the BC548. What will this mean for my final voltages?



Xbox_360_lite-on_rs232_simple.jpg

Two things. First, the 3904 and 3906 jellybean trnsistors will work fine for this application. Second, you are not understanding how the RS-232 signalling works. It requires +/- voltages to work. A logical "0" is a voltage between 5V and 15V, and a logical "1" is a voltage between -5V and -15V.

That's why most uC circuits will use a MAX232 chip or similar, for the level conversion from TTL or CMOS logic levels to RS232 voltage levels. You can find more information about RS-232 signalling at Maxim's website:

http://www.maxim-ic.com/

.
 
  • #5
A big thanks to all for the quick and helpful replies.

I'm going to breadboard the circuit design and check to make sure the the voltage fall within the appropriate ranges that berkeman suggested. If that clears, I'm going to begin soldering the pieces on to circuit board I bought and solder the finished product to my input device.

I'll post an update when I'm done.
 

1. What is the difference between NPN BC548 and 2N3904 transistors?

NPN BC548 and 2N3904 transistors are both NPN type transistors, meaning they have a positively charged layer sandwiched between two negatively charged layers. However, the main difference between these two transistors is their maximum current and power ratings. The NPN BC548 has a maximum collector current of 100mA and a power dissipation of 500mW, while the 2N3904 has a maximum collector current of 200mA and a power dissipation of 625mW.

2. How do I know which transistor to use for my circuit?

The choice of transistor for a circuit depends on the specific requirements of the circuit. Factors such as maximum current and power ratings, voltage ratings, and frequency response should be considered. It is important to consult the datasheet of each transistor to ensure it meets the necessary specifications for the circuit.

3. What is the difference between PNP BC559 and 2N3906 transistors?

PNP BC559 and 2N3906 transistors are both PNP type transistors, meaning they have a negatively charged layer sandwiched between two positively charged layers. The main difference between these two transistors is their maximum current and power ratings. The PNP BC559 has a maximum collector current of 100mA and a power dissipation of 500mW, while the 2N3906 has a maximum collector current of 200mA and a power dissipation of 625mW.

4. Can I use a NPN transistor in place of a PNP transistor and vice versa?

No, NPN and PNP transistors cannot be used interchangeably as they have opposite polarity. This means that the current flows in opposite directions through the collector and emitter terminals in each type of transistor. Using the wrong type of transistor in a circuit can result in malfunction or damage to the circuit.

5. What other factors should I consider when choosing a transistor for my circuit?

In addition to the maximum current and power ratings, voltage ratings, and frequency response, other factors to consider when choosing a transistor for a circuit include gain (hFE), switching speed, and temperature ratings. It is also important to select a transistor with a suitable package size and type for the circuit and to consider the cost and availability of the transistor.

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