PWM voltage divider

[mjwalker88]

hi i am trying to make a PWM voltage divider,

the idea is that when the fet is off i.e. PWM duty cycle is 0% then the second resistor should be out of circuit therfore the divider is not functioning then when the PWM is increased the second resistor is used but only for a percentage of the time until the PWM is 100% and then
the voltage divider should be used fully.

for example if both resistors are set to 2K with a 1volt input then at 50% PWM duty cycle for the fet then the output should be 3/4 of the input as max voltage divider would be 1/2 then 50% PWM means the divider is only used half of the time.

but no matter how i simulate or build it i cant get it to work any ideas ??

 

[BobK]

Hi mj, and welcome to the forum.

A voltage divider is not a a circuit you use to power a load. The load will affect the voltage of the divider, a varying load will make the voltage delivered vary, which is usually not the desired effect.

Depending on the type of the load you are using, PWM by itself acts to reduce the effective power delivered to the load, though not the maxium voltage. What type of load are you trying to driver?

Bob

 

[mjwalker88]

My diagram may have caused some confusion here, where it says load in and load out they do not refer to a load but rather "load in" is a signal from a load cell and load out should by the modified output. This circuit is to be used to alter the voltage from a load cell and decrease it depending a potentiometer input to a pic micro controller

 

[BobK]

Okay.

That will not work. The output would be a voltage that varies between the max and some other voltage at the PWM frequency. An digital potentiometer would be a better solution.

Bob

 

[mjwalker88]

That's the idea the micro controller will either give no pwm meaning no attenuation then vary the pwm to vary the amount of voltage reduction until it reaches the voltage dividers inherent output voltage so in total the overall voltage will vary from its original value down to that of the voltage divider and any value in between

 

[BobK]

PWM does not end up with a stable voltage, it is changing constantly. If you are reading the voltage, for instance with an A/D, you would get erratic readings. PWM with a low pass filter can put out a stable voltage. What is consuming the LOAD OUT signal?

A digital potentiometer does excatly what you want, gives you a variable voltage divider controllable from a uController, no PWM involved.

Bob

 

[mjwalker88]

Load out is connected to the load cell monitoring card that is used to display the amount of weight applied to the load cell

 

[BobK]

So it is an analog input? What is the range?

If I understand you, you want to take the output of the load cell and reduce the voltage by a factor controlled by a uController. If this is correct, I still say a digital pot is what you want.

Bob