Calculating a resistor value -Argh?!

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

wrote:

>--21128125b3a94ab298d56673a36c9717

>Content-Type: text/plain;

charset="utf-8"

>Content-Transfer-Encoding: 8bit

>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

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

wrote:

>--21128125b3a94ab298d56673a36c9717

>Content-Type: text/plain;

charset="utf-8"

>Content-Transfer-Encoding: 8bit

>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