what resister to use ?

[mark krawczuk]

hi, i `ve just made a pwm using a 555 to trigger a tipXXX transisitor thru
a 120 ohm resistor connected to the base , , to controll a12 volt electric
fan , but i want to use a 50 amp darlington instead , will the 120 ohm
still be ok to use with this ? ( mje11032)

thanks,
mark k

 

[ian field]

"mark krawczuk" <krawczuk@adam.com.au> wrote in message
newsqidnfKGcoTL9MTanZ2dnUVZ_jKdnZ2d@adnap.net.au...

hi, i `ve just made a pwm using a 555 to trigger a tipXXX transisitor
thru a 120 ohm resistor connected to the base , , to controll a12 volt
electric fan , but i want to use a 50 amp darlington instead , will the
120 ohm still be ok to use with this ? ( mje11032)

thanks,
mark k

Normally you would take the peak current and divide it by the gain to find
the required base current then calculate the resistor from the drive
voltage, but darlingtons usually have a pair of internal B/E resistors so
you need to look for their values in the data sheet and calculate what the
base drive voltage would be with an equivalent resistor in series with the
resistor previously calculated., usually the data sheet will quote an on
state base voltage - which can be as high as 2.5V, start with a divider
voltage about twice that and check that the transistor conducts fully during
on time.