PWM control through USB 6008/09

[johnsold]

The 1 W is the power rating of the resistor R3, not the diode.  The diode can be any rectifier diode which can handle the current of your motor.  The reverse voltage rating of the diode should be 4 times the motor voltage rating or more.

Lynn

[sufre]

Hi, John.

I have assemblied the components and tested it, (Due to unavailability of 1 W R3, I replaced with a 2 watts resistor of 220 ohms). I have connected the motor and switched the 12V supply, the fan was run which i think it is not suppose to be. The fan will only run when there is a signal from the 5v logic, correct me if I am wrong. If that is the case, is it because of the R3 values?

Kind regards

sufre

[johnsold]

sufre,

What 5 V logic?  The circuit is analog, not digital.  It is designed so that as the signal at USB6008 AO varies from 0 to 6 V, the motor output will vary from 1.25V to about 13.25 V (although the LM317 output may not go quite that high, depending on the amount of current).  What voltages do you have at the power supplu pins of the 741 (pins 4 and 7)?

Lynn

[sufre]

Hello John,

I have checked the voltage between pin 4 and 7, which read 12.Volts. What does it means? Basically, the fan motor will only rotates once there is signal from AO. Am I right?

In my case without connecting the AO, the fan is already rotates. !! Is anything wrong with my circuit? Can you please check my circuit diagram again? Your effort is highly appreciated.

Kind regards

Sufre

[johnsold]

Sufre,

What is the voltage at the output of the 741?  What is the voltage at the output of the LM317?  What is the input voltage to the circuit? It is somewhat difficult to troubleshoot a circuit I cannot see. So this may take a few questions and answers back and forth.

I just thought of something.  The 741 is not optimized for single supply operation. The input voltage range does not include the negative supply and the output cannot swing to the negative supply voltage. The worst case ratings have the input range and the output swing 3 V above the negative supply with typical about 2 V.  This means that with pin 4 connected to ground as shown on your circuit board layout, the output may never go below 1-3 V and the circuit will not work if the input is below 2-3 V. Do you have a -12 V or -15 V power supply available? Connect that to pin 4 of the op amp.  If you do not have a negative power supply available, it gets much more complicated.

Lynn