pulse width modulator

Discussion in 'The Projects Forum' started by swty_todd, Mar 25, 2009.

  1. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    We have been asked to make any circuit of our choice using op-amps on a breadboard or a PCB. We have found this circuit of PULSE WIDTH MODULATOR on the net and we have decided to go for it.But our teacher wants us to explain the designing of the circuit too namely why have we used the resiistors of that particular value only ,using equations and so on.
    I am uploading the circuit as an attachment
    please help.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    I have taken the schematic from the document.
    This way everyboby can see it directly.

    [​IMG]


    The circuit is an oscillator with R2 and C1 as time constants.
    R1 determents the dutycycle.
    When the input is 0 Volts the duty cycle is 50 %.
    R3 and R4 are setting the threshold level.

    Greetings,
    Bertus
     
    Last edited: Mar 25, 2009
  3. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
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    hey thanks
    i tried not to use an attachment and paste the circuit as you have done so that everyone can see it but i was unsucessful.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,646
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    Hello,

    In the FAQ there is a part about posting attachments and pictures.

    Greetings,
    Bertus
     
  5. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
    82
    0
    I tried finding a suitable circuit with a understandable theory and design but I couldn’t get one.
    So I decided to use allaboutcircuits simple bargraph driver circuit (where the component values were not shown) and decided to use my own values.
    View attachment cir.doc

    ckt.jpg

    For the opamp I have decided to use LM741 (since it is easily available). For the resistors which are connected in series with the diode I decided to use 1M ohm resistors ,so as not to harm the LEDs.
    I have decided to use the red LEDs.
    For the resistors I which are directly connected to the V+ I have decided to use three 47 ohm resistors.And I will take V+ as 15 volts. Now the problem is how to calculate V1 and V2 .Since V+ is getting divided into two parallel branches one into the three 47 ohm resistors and the other into the opamps, I wonder how to calculate the resistance of the opamp branch.
    I think I am stuck up. Experimenting in the lab is not a good option right now since my college is far away and its holidays for 2-3 days now.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    There are two mistakes.
    The three resistors in the voltage divider have a very low value.
    You can use resistors of 1 K over there.
    The leds will not burn as there is almost no current with 1 M resistors.
    Try resistors of 1 K over there. The current will be about 12 - 13 mA with that.

    Greetings,
    Bertus
     
  7. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
    82
    0
    thanks
    hopefully it will work
     
  8. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
    82
    0
    I tried the circuit in my lab and unfortunately it didn’t work, but hopefully after a bit of troubleshooting it will.
    My final circuit that I am using is below.

    [​IMG]

    Actually I am supplying +15 volts to pin 7 of IC741 and -15V to pin 4 of the IC.
    Even without giving any supply to the +V & Vin both the LEDs glow.

    Actually when pin 7 & pin 4 were connected as above, +V was at 5V & Vin was at zero, both the LED’s glowed. Using multimeter I noticed that the voltage across the LED2 was 7v .When I increased Vin to 0.1V the voltage across LED2 went to 13.3V & further increase in Vin didn’t lead to further changes.

    At the same time the voltage across LED1 was 9.5V(when Vin was at 0V) & 13.19 V (when Vin was at 0.1V).
    Increasing Vin further didn’t change the voltage drop across LED2.

    Increasing the values of resistors only decreses the intensity of light of the LEDs.
    I fail to understand why it’s reaching saturation so early, how should I modify it?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why don't you have V+ connected to 15v?
    That should set the threshold for 741-B to 5v, and 741-A at 10v.
    If Vin is below 5v, both LEDs should be off.
    If Vin is 5v to less than 10v, LED1 should be on, LED2 off.
    If Vin is 10v to Vcc, both LEDs should be on.

    You weren't measuring across the LED, you were measuring from the output of the opamp to ground; the total voltage drop across the LED and the 1k resistor.

    Note that the 741 has pretty low gain, even when used open-loop. You may notice that near the threshold voltages, the LEDs will glow dimly.

    Suggestion: your resistor divider network; increase the resistance so that current flow from Vcc is in the range of 0.1mA to 1mA.
     
    Last edited: Mar 31, 2009
  10. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
    82
    0
    Yes you are right I was measuring voltage across LED + resistor.
    Increasing the value of resistors just decreases the intensity of glow of LEDs.
    I noticed that keeping V+ at 5V or 15V didnt produce any difference.Seems to me that I need to add something more to the circuit.
    Thanks a lot!
     
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