Three stage 555 generated PWM

[Jtw11]

Hi guys,

First post on this forum - I had a search around but couldn't find what I was looking for.

I'm building a motor controller that consists of a 555 timer configured in astable mode to output a frequency around 6KHz, i'm then feeding this 50% duty cycle PWM signal through an RC integrator to convert it to a triangular waveform.

This is then fed as an input into a comparator, the other input being the output from a pot to allow me to adjust reference voltage, and thus PWM output. The reason I'm doing it in three stages like this is to give me 0-100% PWM as opposed to 10-90% PWM.

A few questions;

1. If i want to replace the pot with a 2 wire thermistor, how would I go about acheiving this? I can't quite work it out. The aim is to make one of the inputs to the comparator temperature dependant.
2. Secondly, looking at the outputs on a 'scope - the 50% duty cycle PWM signal from the 555 is perfect, as is the triangular waveform. However, the output from the comparator looks like PWM in that I have a high signal, but the low signal is replaced with wha tlooks like the charging of a capacitor? Would maybe adding a diode to the comparator inputs solve this?

I've attached some pictures.

Cheers in advance for any replys!

J

 

[Rleo6965]

What comparator ic was you using?

 

[Jtw11]

Hi there,

Using an LM311N comparator.

 

[mechtronics]

hi, u could change the pot to a 2 wire thermistor by putting on one wire of the thermistor to positive the second wire into the op amp inverting side, the same as where the pot is in the diagram and just run a resistor of approporiate value from the inverting side to ground.

 

[duke37]

Do you have a pull-up resistor on the output of the comparator?