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Calculating a resistor value -Argh?!

Former Member

Hi all. I am fairly new to calculating resistor values. I'm trying to

calculate the value required to drive the base of a BCV26 transistor from

5v. The BCV26 will be powering a relay drawing 150mA at 12v

 

 

This is the rule of formula I'm referring to from an example at

www.ermicro.com :-

 

IB = IC / hFE = 1 A / 1000 = 0.001 A

RB = (VPORT  VBE) / IB = (4.2  1.4) / 0.001 = 2800 Ohm, use 2K7 Ohm

resistor

P = (VPORT  VBE) x IB = (4.2  1.4) x 0.001 = 0.0028 watt, use 0.25 Watt

resistor

 

Here are specs of the BCV26

 

IC 1.2A

 

hFE   IC = 1.0 mA, VCE = 5.0 V  =4,000

      IC = 10 mA, VCE = 5.0 V   =10,000

      IC = 100 mA, VCE = 5.0 V  =20,000

 

VBE 1.5V

 

This might be a bit "left-field" for this forum but any help or referal to

a good tutorial would be appreciated.

 

 

thx, Darren

 

 

 

 

 

 

 

 

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  • On Sat, 24 Mar 2012 08:10:45 +1000, Darren <dgr81_05@hotmail.com>

    wrote:

     

    >--21128125b3a94ab298d56673a36c9717

    >Content-Type: text/plain;

    charset="utf-8"

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    >Hi all. I am fairly new to calculating resistor values. I'm trying to

    >calculate the value required to drive the base of a BCV26 transistor from

    >5v. The BCV26 will be powering a relay drawing 150mA at 12v

    >

    >This is the rule of formula I'm referring to from an example at

    >www.ermicro.com :-

    >IB = IC / hFE = 1 A / 1000 = 0.001 A

    >RB = (VPORT  VBE) / IB = (4.2  1.4) / 0.001 = 2800 Ohm, use 2K7 Ohm

    >resistor

    >P = (VPORT  VBE) x IB = (4.2  1.4) x 0.001 = 0.0028 watt, use 0.25 Watt

    >resistor

    >Here are specs of the BCV26

    >IC 1.2A

    >hFE   IC = 1.0 mA, VCE = 5.0 V  =4,000

         IC = 10 mA, VCE = 5.0 V   =10,000

         IC = 100 mA, VCE = 5.0 V  =20,000

     

    >VBE 1.5V

    >This might be a bit "left-field" for this forum but any help or referal to

    >a good tutorial would be appreciated.

    >

    >thx, Darren

     

    It looks like the hFE figure of 20,000 is the closest to your

    operating conditions, so for 150ma of collector current you would need

    only 7.5 uA of base current.  But why try to cut it so close?  The

    base is capable of being driven by up to almost 100 mA.  It is good

    practice to overdrive coil drivers anyway to make sure they saturate

    well.  So I would go as much over the hFE calculation I could

    comfortably go.  Can your PORTB source 1 ma?  If so, I would size the

    resistor to deliver 1 ma. into the base.  Assuming VBE of 1.5 volts

    and assuming PORTB sources 5 v, the base resistor has to drop 3.5 v.

    at 1 ma, hence it is 3.5K.  (Use 3.3K or 4.7K or whatever you have in

    that neighborhood).

     

    Robert Scott

    Ypsilanti, Michigan