PWM output voltage problem (555 circuit)

Discussion in 'Digital Circuit Design' started by Nerat, Jun 10, 2016.

  1. Nerat

    Thread Starter New Member

    May 23, 2016
    5
    0
    Hello,

    I'm fairly new to electronics and i built myself a PWM circuit for fan speed control.

    I used below schematic and i built it on breadboard first. It worked fine.

    http://www.afrotechmods.com/groovy/PWM_tutorial/PWMcircuitwithmotor.jpg

    Only difference is that i added a capacitor (25V, 470uF) in parallel to motor and diode to reduce humming sound.

    So i decided to build it on perfboard (single copper holes). This was my first built btw.

    My problem is that output voltage is constant at 12V, no matter what the pot resistance is. I checked the connections they seem correct.

    As this is a very straightforward and aged circuit, i thought you might know what would cause this issue. Could you please help me troubleshoot the circuit? Could the transistor be damaged? It was very hot to touch while soldering and I had to use bit heat due to multiple single connections on perfboard.
     
  2. Marley

    Member

    Apr 4, 2016
    142
    40
    The capacitor is probably the problem. Remove it.
    The motor will make a whistling sound due to the PWM switching. This is normal.

    One way to reduce the sound is to make the PWM frequency higher so that it above the range of normal hearing. Most people can not hear much above 15kHz.
     
  3. Nerat

    Thread Starter New Member

    May 23, 2016
    5
    0
    I tested the capacitor on breadboard before putting it together on perfboard.

    I could adjust duty cycle with capacitor connected and it got rid of the sound. I don't see why it would be problem now.
     
  4. dannyf

    Well-Known Member

    Sep 13, 2015
    1,782
    360
    This is quite simple: if the connection is correct but the circuit isn't working, your circuit isn't correct or the parts are defective.

    No other possibilities.
     
  5. Nerat

    Thread Starter New Member

    May 23, 2016
    5
    0
    I already know that something is defective or shouldn't be there.

    Thing is, it's very messy to remove a component on a perfboard and I wanted to consult your opinions before disassembling every suspicious part with the assumption that it might be faulty.

    I know noone can pinpoint it for me, however as this is a fairly straightforward circuit maybe there was an obvious root cause for this kind of problem.

    I will try without capacitor, check resistor connections along with all connections and finally i will try to replace transistor. I'm sure that this circuit is functioning because I already tested it.

    Any additional insight is welcome.

    Thanks.
     
  6. dannyf

    Well-Known Member

    Sep 13, 2015
    1,782
    360
    Or the basic design is flawed.

    The circuits behavior is well understood so if you have a meter. Or a little scope it is straight forward to diagnose.
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    How much current does the motor draw?
    The transistor has a rating of 3 Ampere.
    You better not run the transistor at its maximum.

    Bertus
     
  8. Nerat

    Thread Starter New Member

    May 23, 2016
    5
    0
    Fan draws 100mA (measured it on BB) so it's safe.
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    One technique I use is to make a build with duplicate components, so that I can leave my breadboard alone until the build is done and working. Then you have a reference to test against and you can use the unit operations (oscillator, amplifier, etc) of one circuit to test the functions of the other.

    As you are seeing, a completed build that doesn't work can be a frustrating challenge to fix.
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    What happens when you reduce R2 to 470 Ohms?
    This will drive the transistor harder.

    Bertus
     
  11. madaboutears

    New Member

    Sep 29, 2015
    2
    0
    With PWM the output voltage remains constant (in your case 12v). The energy applied to the motor is determined by the on/off time which is achieved with the PW Modulation. Don't try to reduce switching noise in the motor with a capacitance. Change the oscillator frequency of the 555 to a value that gives you the least amount of noise in your motor. A potentiometer in place of the timing resistor is a practical way of adjusting the frequency while listening to the noise in the motor.
     
  12. Geirfinn Sirnes

    New Member

    Dec 16, 2015
    1
    0
    You may have blown the transistor through the capacitor - charging the capacitor may cause the collector current to be too high, momentarily.
    Geirfinn
     
  13. Nerat

    Thread Starter New Member

    May 23, 2016
    5
    0
    So afterall it all boiled down to two things:

    One of my 555 IC's were faulty.
    The pot i used was faulty.

    I build the circuit on a breadboard again to ease troubleshooting. Well none of the 555 PWM circuits i built worked (I think i tried every possible alternative). Even though i was SURE they worked last time i built.

    I have two breadboards. One of them is OK and it was the one on which i built my first PWM circuit. Well, the second one's power rails had a gap in the middle... It took me a looong time to realize this. I came to a point where i started to question my skills and intelligence and whether i should keep going with electronics. I doubted myself. Afterall, not all those circuits could be wrong...

    Anyway, with a 555 test circuit i found the faulty IC and then i measured potentiometer's resistance, it gave me readings as high as few Mohms. Also with a capacitor in parallel my output voltage was either 4-5V or 12V constant so i scrapped it.
     
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