TC649

DS21449C-page 14

 2002 Microchip Technology Inc.

A base-current limiting resistor is required with bipolar

transistors (Figure 5-7). The correct value for this

resistor can be determined as follows:

sen transistor data sheet. It is now possible to solve for

EQUATION

Some applications benefit from the fan being powered

from a negative supply to keep motor noise out of the

positive supply rails. This can be accomplished as

on. Operation is otherwise the same as in the case of

fan operation from +12V.

FIGURE 5-7:

Circuit For Determining

FIGURE 5-8:

Powering the Fan from a -12V Supply.

Q1

GND

+ V

RBASE

–

(SAT)

–

+

SENSE

–

Fan

+

GND

+5V

-12V

Q1*

TC649

FAN

R2*

2.2 k

Ω

R3*

2.2

Ω

R4*

10 k

Ω

D1

12.0V

Zener

Note: * Value depends on the specific application and is shown for example only.

See Section 5.0, "Typical Applications", for more details.