Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Calculate PNP Transistor Resistor Signal Value

Status
Not open for further replies.

jack0987

Member
question.jpg


I need to calculate a resistor value for R1.
I do not know who made the transistor but close enough will most likely be ok.
This is what I did.

Maximum Current Required = 200MA
Supply Voltage = 5 Volts
R1 = Supply Voltage / ( Maximum Current Required / Minimum HFE * 1.3 )
R1 = 5 / (.1 / 200 * 1.3)
R1 = 13000.52 or 12K for nearest standard value.

Did I get it right?
 
The 2N3906 has a maximum allowed collector current of 200mA and some work poorly above 50mA. Use a 2N4403 transistor instead that has a maximum allowed collector current of 600mA and its datasheet shows all of them working very well at 150mA and not too bad at 500mA.

hFE is used only when a transistor is a linear amplifier with plenty of collector to emitter voltage so it is not saturated. You want the transistor to saturate so the datasheets show the saturation voltage loss only when the base current is 1/10th the collector current completely disregarding the hFE. Then your base current must be 20mA. The base resistor does not have 5V across it, it has 5V minus the base-emitter voltage that is about 0.85V at a base current of 20mA . Then the base resistor value should be (5V - 0.85V)/20mA= 208 ohms which is not a standard value, use 200 ohms which is a lot different to your calculated 12k ohms.
 
Thanks audioguru.

Most likely my max load will be 100ma or under.

I would like to use the 2n3906's I have on hand if I can.
I would consider the 2n4033.

Can you see the picture I uploaded as I can not?
 
Last edited:
The proper transistor that I recommended is a 2N4403, not what you wrote. I am posting your schematic as a thumbnail here:
 

Attachments

  • 2N3906.png
    2N3906.png
    11 KB · Views: 189
Thanks for your reply.

Ok. I will use the 2N4403.
If I understand correctly, the signal resistor (R1) for it will now be 200 ohms.

I can see the picture now. The problem apparently the IE8 browser. I am using the google chrome one now.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top