Troubleshooting a PWM Circuit Design

[billy fok]

I need to design a pwm circuit, i using the circuit from internet that i attached it..
freq is 20khz which is my requirement and 15v to power up the tl494..
there is no response when i connect potentiometer to it...
waveform is attach

can anyone give me advice?

 

[Mike_In_Plano]

Is an old favorite...
Try removing the pot and typing PIN 3 to PIN 15 (output of op amp to - input). Next, attach one end of the pot to ground and the other to pin 14 (5V reference). Attach the wiper of the pot to pin 16 (+ input of op amp).
This done, the 0-V output of the pot should create 0-5V on the feedback pin 3.

Attach the two output transistors in parallel. The emitters (Pin 9 and Pin 10) tied together and the two collectors (pin 8 and Pin 11) tied together. The transistors will be most active when the wiper of the pot is at 0V, and turned off when the pot reaches 5V.

There are very real and significant limitations when applying this circuit to controlling a fan. For fans with logic level PWM inputs this circuit is well suited. For fans with permanent magnet motors, you will need to take precautions not to damage the transistors during start up (start with a low PWM), and include a freewheeling diode to protect the fan from recirculating currents.

This design is not suitable for driving a brush-less fan as the inputs had capacitors that will damage the 494 and the circuit uses electronic switching which will be uspest by having its power rapidly removed. In such a case a DC-DC converter probably the only option (Add inductor, capacitors, and a freewheeling diode.)

 

[billy fok]

hi.. thanks for the reply, it mean after i adjust the connection, it will create pulse signals and able to adjust the duty ratio using potentiometer??