Trying to Replace N-channel MOSFET with PNP Darlington Pair in 555 Circuit

Discussion in 'General Electronics Chat' started by steve_5a, Oct 30, 2015.

  1. steve_5a

    Thread Starter New Member

    Oct 30, 2015
    Hello all,

    I am using a 555 timer as part of a voltage regulator on a fan motor. The idea is that the pot can be used to control the speed of the fan. When I went to buy the parts, my local store didn't have the NTD4906N MOSFET called for. Instead I ended up with a TIP127 PNP darlington transistor. My (naive?) thinking was that I could change the circuit as follows:


    When I power this circuit on, the fan turns on, but the pot does not have any impact on the speed of the fan. I also get that 'hot' chip smell. When I turned off the power, the 555 chip seemed to be overheating.

    Where has this substitution idea gone off the rails? Is there any way to get this arrangement to work? Thank you for your help!
  2. dl324

    Distinguished Member

    Mar 30, 2015
    You need a current limiting resistor on the base of the Darlington. What is the current rating of your motor?

    Test the timer before trying to operate the motor.
    Lobo_79 likes this.
  3. steve_5a

    Thread Starter New Member

    Oct 30, 2015
    It's tiny - 80 mA. It's an CPU fan from an expired computer.

    How large of a resistor are we talking about?
  4. OBW0549

    Well-Known Member

    Mar 2, 2015
    10K ohms between pin 3 of the 555 and the base of Q1 should do the trick.
  5. dl324

    Distinguished Member

    Mar 30, 2015
    Not critical; something around 10K. To switch 80mA, you could have used a 5 cent transistor.
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    Maybe. A 555 (especially a bipolar 555) output does not swing to the positive rail, so a single-junction transistor might not turn off. The extra junction and two internal resistors in a power darlington should overcome this.

    You don't say what your target PWM freq is, but for noise control and fan happiness I'd add an electrolytic cap across the motor. At 80 mA and 500 Hz, a 100 uF cap would reduce the ripple to 1 V.

  7. steve_5a

    Thread Starter New Member

    Oct 30, 2015
    Worked like a charm! Thanks guys!

    I agree it was overkill on the transistor. It was strictly a question of availability and getting it immediately.

    Now for the harder part...trying to understand it better. What was causing this circuit to fail without that current limiting resistor? Why wasn't I seeing any speed control?
  8. Roderick Young


    Feb 22, 2015
    The base of the Darlington was being clamped to about 11 volts. Think of the base to emitter junction as being like 2 diodes.
  9. crutschow


    Mar 14, 2008
    As noted by Roderick, the base-emitter junction of a BJT looks like a diode, so you must add a resistor in series to limit the current.
    Otherwise it looks basically like a short circuit, which caused a high current into the output of the 555, basically limited by the 555's output current limit, and consequent overheating.

    For switching purposes a BJT acts like a current-controlled current source.

    A MOSFET has a very high input impedance and acts like a voltage-controlled current-source.
    Roderick Young likes this.