Hello, just as a background I'm a Mechatronics bachelor student and not so good with the electrical bits.

Below is a schematic for my circuit. The main idea briefly is to control the speed of DC motor using PLC, so I went with the LM350 linear regulator. This interface shall have as shown 4 speed channels each at a specific voltage. My only problem is that even after using 2 extra transistors as master switch to ensure the motor wouldn't be running at input volt when none of the channels is active, there's still a 1.25-1.3V suppliued to the motor because of the LM350's minimum voltage output. I've been trying to find a way to cutoff voltage from the output pin, such as using a PNP transistor for example. But I don't know how this can be implemented. I'd be more than grateful for more efficient suggestions.

Thank you

 

Hi,
Have you tried any tests using this AC supply?
E

 

Hi,
Have you tried any tests using this AC supply?
E
View attachment 340780

No Actually, I don't use it for simulation.. just to design the circuit. I'm using a DC voltage suppply at the lab.

 

Hi mohamad,
Check out this link.
E
https://www.electronicdesign.com/te...5/drive-ldos-to-0-v-output-with-single-supply

 

Do you have a negative voltage available to do what is suggested in the posted link from eric?

 

I'd be more than grateful for more efficient suggestions.

Check out the LT3080 and 3081. They will regulate down to zero. They do not have the 1.25V problem.

 

I would try placing a P-Channel MOSFET on the input of the regulator, if you have a fifth control line.

Question:

Is the power supply to the regulator regulated?

If it is, you could create a simple motor power controller using a power transistor in follower mode, or you could even set it up on the output of the 350.

You would use the PLC to turn on and off the 222s just as you have shown but they would form a variable divider on the base instead of the adjust pin and the output could go all the way to zero.

 

Below is the LTspice sim of a circuit that uses a LM337 regulator to generate a 1.25V negative reference voltage (middle trace) which then allows the LM317 (or LM350) to have a minimum output voltage of essentially zero.

A 555 oscillator is configured as a negative voltage, capacitive-doubler type circuit to generate the negative voltage for the LM337 (top trace).
The 555 only has to generate about 10mA of current to provide the negative bias voltage.
That output voltage goes down to 0V for the zero setting of the pot (bottom green trace).

The circuit shows an LM317, but it operates the same as the LM350.