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

Thread Starter

steve_5a

Joined Oct 30, 2015
3
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!
 

dl324

Joined Mar 30, 2015
9,108
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.
 

AnalogKid

Joined Aug 1, 2013
8,135
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.

ak
 

Thread Starter

steve_5a

Joined Oct 30, 2015
3
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?
 

Roderick Young

Joined Feb 22, 2015
408
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?
The base of the Darlington was being clamped to about 11 volts. Think of the base to emitter junction as being like 2 diodes.
 

crutschow

Joined Mar 14, 2008
23,486
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.
 
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