# Transistor design (Basic question)

Discussion in 'General Electronics Chat' started by Tahmod, Feb 23, 2013.

1. ### Tahmod Thread Starter New Member

Feb 23, 2013
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0
Hi everryone,
basically i want to design a circuit with a supply of 6volts, and a collector current having the value of 5ma. I want to use the transistor TIP31A 219 NPN.

-I dont know how to get the saturation current and the gain of the transistor from the datasheet ?
-In the design of a transistor what value you need and what value you need to take from the datasheet?

http://www.datasheetcatalog.org/datasheet/PowerInnovations/mXuqyry.pdf

2. ### #12 Expert

Nov 30, 2010
16,705
7,358
The saturation current is not on the datasheet because you can put the transistor into saturation at any current you desire, as long as it is less than the current which destroys the transistor.

In the design of a transistor circuit you start with the highest voltage the transistor can work with, then the highest current, then see if the gain is enough, then see about speed, and that is about as far as I have been.

Oh yeah...after you get done calculating and before you apply power, use Watts Law to be sure nothing will get too hot.

Last edited: Feb 23, 2013
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3. ### crutschow Expert

Mar 14, 2008
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What is the purpose of the circuit? What are the requirements?

4. ### ScottWang Moderator

Aug 23, 2012
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You can using the circuit as this.

Because your current is too low, so you may change the transistor to a small signal as 2SC945, 2SC1815, 2N3904, PN2222A, etc.

You may in series a resistor to limit the current from input side.

The zener change to 6.8 V 1/2W, if your power is need more precisely, then you need a power regular as 7806, but the input should be > 8V or more.

7806 datasheet.

5. ### Tahmod Thread Starter New Member

Feb 23, 2013
12
0
Saturation is when Vce = approximately 0V and collector current max voltage from the supply (the same as the supply).

-The highest the voltage of the transistor can work with and the highest the current, the higher the gain?
-Where can I find the maximum possible current from the transistor for base and collector, Im new with datasheet and dont know which one it is?

When you apply the power watts law, where do you use it, which current and voltage and how do you know if it will be too hot

As I said, I want the collector current from the transistor to change from 5ma to 2.5ma. With a power supply 6Volts. Using a transistor TIP31A.

6. ### #12 Expert

Nov 30, 2010
16,705
7,358
Saturation is when Vce is very small, but it can never be zero because no transistor is that perfect. Collector current is not "max voltage from supply" because current is not voltage. Collector current is always current from supply for that one transistor.

On the datasheet you provided, maximum voltage is on the first page. It is called, "Collector-emitter voltage".

The maximum continuous collector current and the maximum temporary collector current are also in that first chart on page 1.

The best way to find current gain is in Figure 1, page 3

You apply Watts Law to each part, one at a time. You consider the voltage on the part, the current through the part, and calculate with P=IE. To find if it is too much, you look on the datasheet for each part.

It was someone else that asked the purpose. I do not care the purpose unless you want me to make a drawing for you.

7. ### crutschow Expert

Mar 14, 2008
13,521
3,388
That tells me what you want the transistor to do. It doesn't tell me what the purpose of the circuit is or any requirements, such as what is the input you have or want to cause the change from 5mA to 2.5mA?

8. ### Tahmod Thread Starter New Member

Feb 23, 2013
12
0
I wanted to say, the Rc resistor take the full voltage of the supply and produce a collector current and Vce=0V (in ideal case), in pratical case Vce is not zero. 

-Using a switch, like the first picture in the page http://brunningsoftware.co.uk/Transisor.htm When the switch is on a 5V is going, when the switch is off a 2.5V is applying to the motor.

Question:
1. -The highest the voltage of the transistor can work with and the highest the current, the higher the gain?
2. Where can I find the gain Hfe (not the current gain)?
3. Where can I find the IcQ (the current at the Q point)
4. Where can I find the Vceq(The Vce voltage at the Q point)