Adjust duty cycle by opamp

Thread Starter

aviotor1

Joined Dec 12, 2016
19
Hello everybody,
I need to change duty cycle for my project. First i have a op-amp to generate dc to square wave. (Because it is not allowed to use signal generator), but i can only change thi period of this square wave. I also need to change duty cycle. Here my circuit to generate dc to square wave. Any idea for adjusting duty cycle ? thanks...
 

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GopherT

Joined Nov 23, 2012
8,009
Hello everybody,
I need to change duty cycle for my project. First i have a op-amp to generate dc to square wave. (Because it is not allowed to use signal generator), but i can only change thi period of this square wave. I also need to change duty cycle. Here my circuit to generate dc to square wave. Any idea for adjusting duty cycle ? thanks...
So you need to charge and discharge the capacitor at different rates. A pair of diodes (one in each direction) will crate two current flow paths. You can add a potentiometer to shift the rate of current flow to/from the capacitor. Look up an example of how this is done on the 555 timer to make a PWM (pulse width modulator) circuit.
 

Thread Starter

aviotor1

Joined Dec 12, 2016
19
So you need to charge and discharge the capacitor at different rates. A pair of diodes (one in each direction) will crate two current flow paths. You can add a potentiometer to shift the rate of current flow to/from the capacitor. Look up an example of how this is done on the 555 timer to make a PWM (pulse width modulator) circuit.
Firstly, it is not allowed to use 555 or something like that. allowed to use opamps etc..
Secondly, if you have a answer ,do you mind send with a photo or schematic pls? Thanks
 

GopherT

Joined Nov 23, 2012
8,009
Firstly, it is not allowed to use 555 or something like that. allowed to use opamps etc..
Secondly, if you have a answer ,do you mind send with a photo or schematic pls? Thanks

I didn't say you should use a 555 instead of an op amp. I said you should research how PWM can be done with a 555 timer so you can more easily understand a strategy to solve your own problem.

I will not send a photo. You can easily google LM555 PWM yourself to see how it might work.
 

mtonge

Joined Apr 19, 2016
93
I'm curious. What is your output frequency, and which half of the wave do you want to increase the duration? By placing two resistors in series in place of 'R', and bypassing one of them with a diode, you can change the charge/discharge timing (eg. charge through two, discharge through one). More details would be helpful, like why a 555 can't be used.
 

Thread Starter

aviotor1

Joined Dec 12, 2016
19
I'm curious. What is your output frequency, and which half of the wave do you want to increase the duration? By placing two resistors in series in place of 'R', and bypassing one of them with a diode, you can change the charge/discharge timing (eg. charge through two, discharge through one). More details would be helpful, like why a 555 can't be used.
Well ,this topic just a little part of my project, we need to see wave duty cycle in oscilloscope.So that, we can adjust the duty cycle for desired temperature.And any signal generator not available to use.Only DC supplies :) So, i just need a solution to change duty cycle after 1st stage that i uploade photo.
 

GopherT

Joined Nov 23, 2012
8,009
Well ,this topic just a little part of my project, we need to see wave duty cycle in oscilloscope.So that, we can adjust the duty cycle for desired temperature.And any signal generator not available to use.Only DC supplies :) So, i just need a solution to change duty cycle after 1st stage that i uploade photo.
dont make it AFTER that stage... make it part of that stage
 

Dodgydave

Joined Jun 22, 2012
11,285
Here is the type of modified design you need, putting diodes in series with the charge resistor, you can use two separate presets one for charging and one for discharging the capacitor.


02673x01.png
 

Thread Starter

aviotor1

Joined Dec 12, 2016
19
ok those are can be diode(normal diode) but i am confused.Is this system will be able to change both frequency and Duty cycle? If it is yes ,which values should i use for best ? thanks
 

GopherT

Joined Nov 23, 2012
8,009
ok those are can be diode(normal diode) but i am confused.Is this system will be able to change both frequency and Duty cycle? If it is yes ,which values should i use for best ? thanks
For best what? What is this for? Motor controller, temperature controller, to amuse your friends and family, to show your professor that you can get strangers on the Internet to do your homework,
 

Thread Starter

aviotor1

Joined Dec 12, 2016
19
For best what? What is this for? Motor controller, temperature controller, to amuse your friends and family, to show your professor that you can get strangers on the Internet to do your homework,
If you don't want to help then doN't comment on .This topic just a part of my project. I am doin analog air conditioner system by only using opamp and transistor and Dc supplies.So , i handled other parts except for this duty cycle issue.They want to see desired temperature on oscilloscope .Thats why i try to create such a thing.!!!
 
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