Why is there a resistor in this circuit?

Question

Why there is a 220 ohm resistor in this circuit? Can't we just connect the thermistor directly to the base of transistor?

Answer 1

The 220 ohm pull down sets an operating point for the system. Without it, the fan would never turn off. With it, the fan will only turn on if the NTC resistance is low enough to pull the transistor base above 0.7V base-emitter threshold.

With a 220ohm pull-down, that transition will begin to happen when the NTC resistance gets below 2.6k ohms (about 55-57 deg. C). Some of the current will begin to flow in the transistor base, turning on the fan.

Adjusting the pull-down will shift the turn-on point:

• increase above 220 ohm, fan will turn on sooner
• decease below 220 ohm, fan will turn on later

It's slightly more complicated in the real world however.

First, the fan has some minimum turn-on voltage. A brushless 12V DC fan won’t light up with less than about 7V. So this will delay the turn-on point even more, to when enough drive is being put to the fan to satisfy this threshold.

Second, the transistor beta also influences this. A low-beta transistor will shift up the operating point compared to a high-beta one.

Answer 2

My answer is simple:

The transistor must be controlled by voltage, not by current, since it is used as a voltage comparator with 0.7 V threshold (the base-emitter voltage VBE). This voltage threshold can be thought of as an opposing reference voltage that is subtracted from the input voltage. If the difference is positive, a base current flows and the transistor turns on. So, the 220 ohm resistor is added to form a voltage divider that "produces" an input voltage.

An advice: Do not try to turn off a transistor by increasing the resistance of the base resistor (the thermistor here); this will not reduce the base current enough for the transistor to turn off. The base current must be diverted (steered) from the base by an additional element connected in parallel to the base-emitter junction (the 220 ohm resistor here).

See more explanations about the need of this resistor in my answer to a similar question.