what resister to use ?

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

Reply to
mark krawczuk
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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.

Reply to
ian field

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