what is wrong with is this multi vibrator?

Motanache

Joined Mar 2, 2015
523
Can you set the initial conditions in LTSpice?
To condition the initially charged capacitor.Set the initial voltage for a node.
 

Motanache

Joined Mar 2, 2015
523
Instead of R you have to put two resistors in parallel, every one in series with one diode, the opposite sense. Capacitor C will load through a resistor and discharge through another.
 

Thread Starter

circuit29

Joined May 20, 2017
26
Instead of R you have to put two resistors in parallel, every one in series with one diode, the opposite sense. Capacitor C will load through a resistor and discharge through another.
ok, yes it is working like i have done in 555
 

Thread Starter

circuit29

Joined May 20, 2017
26
yes i have done this you need lt spice file?
and actually the pwm ouput from this is not so sharp so we need to create from long method i have made diploma project using L298 h bridges the speed control of dc motor.
 

Motanache

Joined Mar 2, 2015
523
Voltage comparator. Compared to a fixed voltage.
Depending on the value of this voltage, the duty cycle results.

Look for amplifiers class D. There that are explained.
This principle is also in drivers TL494, SG3535,.....It's not really a triangle, is a sawtooth.
 

crutschow

Joined Mar 14, 2008
27,017
Below is the LTspice simulation of a 741 PWM circuit using just two 741's.
It uses the exponential sawtooth from the multivibrator instead of a generated linear sawtooth for the PWM generation but that should be satisfactory for all but high fidelity applications.

upload_2017-5-20_12-57-1.png
 

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Motanache

Joined Mar 2, 2015
523
Looks like the schematic from post # 1, where you get a triangular signal from the node between R and C.
Basically it is a charging/discharging of a capacitor through a resistance, which is exponential, but can be approximated very well with a oblique line.

I joined this discussion to practice with LT Spice. I've loaded the previous file PWM 741 Pot Adj.asc.
The program gives an error, it does not know LM741. But another OA can be use.
 

crutschow

Joined Mar 14, 2008
27,017
............I've loaded the previous file PWM 741 Pot Adj.asc.
The program gives an error, it does not know LM741. But another OA can be use.
Yes, the 741 is a model I added to my directory.
I can post it here if you are interested.
As you noted, just about any basic op amp can be used in the circuit.
 
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