Author Topic: Calculate PNP base resistor value  (Read 4393 times)

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Offline scott216

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Calculate PNP base resistor value
« on: April 11, 2014, 02:21:04 am »
I want to confirm if i calculated a base resistor for a PNP transistor correctly.  Attached is the schematic

I have an 8550S transistor. 
I found a Data Sheet here: http://www.digchip.com/datasheets/parts/datasheet/670/8550S-D-T92-B-pdf.php
I don't know if it's the right manufacturer or not.

I'm assuming there are 20mA going through the diodes.  For the base current calculation, I assumed a beta of 100. So that give me 0.2mA (20ma/100) at the base.  I am going to have an Arduino control the transistor.  To turn on a PNP I bring the base to zero.  I'm assuming a 1 volt drop across the transistor - I got this VBE in the data sheet, but I'm not positive I picked the right number.  That means my resistor has to drop 4 volts at 0.2mA which equals 20k ohms. 

As you can see in the schematic, my diodes don't go to ground but to -9 volts.  I'm hoping that's not an issue with the circuit working properly.

This is a circuit that already exists and I'm trying to get an Arduino pro-mini  (3.3v version) to interface with it.  I guess that bring up another question, is 3.3 volts enough to turn the transistor off?

If this isn't going to work, I can desolder the transistor and put in something else in it's place, but I can't change any "wiring" around since it's on a PCB.

 

Online edavid

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Re: Calculate PNP base resistor value
« Reply #1 on: April 11, 2014, 03:09:24 am »
This is a circuit that already exists and I'm trying to get an Arduino pro-mini  (3.3v version) to interface with it.  I guess that bring up another question, is 3.3 volts enough to turn the transistor off?

If you are running the AVR from 3.3V, you need to connect the transistor emitter to 3.3V.

Otherwise you are doing the calculation properly.

You might want to use a lower beta value (50?) to make sure the circuit will work with any crappy transistor.
 

Offline scott216

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Re: Calculate PNP base resistor value
« Reply #2 on: April 11, 2014, 04:26:06 am »
This is a circuit that already exists and I'm trying to get an Arduino pro-mini  (3.3v version) to interface with it.  I guess that bring up another question, is 3.3 volts enough to turn the transistor off?

If you are running the AVR from 3.3V, you need to connect the transistor emitter to 3.3V.

Otherwise you are doing the calculation properly.

You might want to use a lower beta value (50?) to make sure the circuit will work with any crappy transistor.

I have 5 volts going into my AVR, but it's voltage regulator reduces it to 3.3 volts.  I could connect the PNP transistor to 3.3 volts.  I'd also have to swap out the 390 ohm resistor for a smaller one, but that's easy.
 

Offline ttp

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Re: Calculate PNP base resistor value
« Reply #3 on: April 11, 2014, 09:14:44 am »
As edavid mentioned you need to connect emitter to 3.3V. If you using the transistor as a switch it usually means you want to drive it into saturation - don't make base current just 0.2mA but 1-2mA to fully open it. Don't worry about LED current, the series resistor (390R or whatever you'll change it to) will control and limit it. Actually you can experiment with different base resistors and see how the collector-emitter voltage changes with different base currents.
 


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