Dimemr Duty Cycle Problem

Discussion in 'The Projects Forum' started by tlcquest, May 22, 2012.

  1. tlcquest

    Thread Starter New Member

    Jan 26, 2012
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    0
    I am currently working on a Dimmer circuit that will go into a LED driver to run an LED but am facing duty cycle problems. Originally I was using a 50K analogue potentiometer and got a 1-99% duty cycle which I was happy but decided to try the new soft pot potentiometer (by spectrasymbol) which came only in 10K as i didn't want a long strip. The problem was that I could only get 67-99% duty cycle with the 10K which is not obvious to notice the dim effect. Could someone suggest a solution to my circuit diagram as I am totally lost. Is there a right calculation to my circuit to determine at least 33-99% duty cycle. Another problem I faced was having to come up with a sample and hold circuit to hold the resistance when i position the resistance on the soft pot as it floats to the maximum 10K when i lift my finger off. Currently am thinking of using a LF398 amplifier chip for sample and hold. Really would appreciate if someone can help. Thanks!!
     
    Last edited: May 22, 2012
  2. #12

    Expert

    Nov 30, 2010
    16,262
    6,769
    Changing the 1k resistor proportionately with the change in pot value would help.
    Connecting thresh and cont together seems to be a mistake. I hope that's only a drawing error.
    Sample and hold circuits sample and hold voltage, not resistance. You could design a circuit to represent a resistance but this all seems much easier to do by just using the correct potentiometer.
     
  3. tlcquest

    Thread Starter New Member

    Jan 26, 2012
    3
    0
    Thanks for the reply, apologies it should be to ground for control voltage. I must have used an old draft file. :p Apparently on the spectral symbol website, they did state something like having to use a sample and hold circuit to hold the last value of the resistance (although i do know traditionally sample and hold tracks voltage). I am really not sure on this but on the youtube sites I did see videos of soft pots arduino music projects which hold the last position of the resistance. I have not done programmed using arduino so have decided to stick with electronic components.

    In choosing R1, R2 and C .. I read that R1 and R2 should be in the range 1kΩ to 1MΩ after deciding on the Capacitance. I had to choose 0.1μF due to my specs sheet on the LED driver stating 200Hz max for linearity of duty cycle. R1 was recommended to be 1/10th of R2 (which i suppose to be my 10k potentiometer.) The formula is R2 = 0.7/ f x C1

    Maybe someone can direct me to a better calculation if I am wrong. I can't change my potentiometer as this is the only value spectrasymbol has for a short strip. Please can someone direct me to a solution. I have been trialling and error but to no avail in getting 33-99% duty cycle. I have posted the correct image.
     
  4. #12

    Expert

    Nov 30, 2010
    16,262
    6,769
    The formula is: time=1.1 RC
    I believe the "softpot" is not going to zero ohms. If it did, the discharge path would only have the impedance of a forward biased diode. That would only be a problem if you are trying to use a very low voltage, like 2 volts, to power the timer chip. I am also baffled by a pot that disconnects when you let go of it.

    It all comes down to using this softpot as the source of your problems.
     
  5. tlcquest

    Thread Starter New Member

    Jan 26, 2012
    3
    0
    Thanks again. I was going to say that I got 43.4% - 99.7% duty cycle on the waveform. I did get some spikes but easily solved with smoothing capacitors. You are right the softpot is a big problem because the minute i release my finger, the waveform disconnects on my oscilloscope so it does float to infinity but unfortunately I will have to use it. I wonder if anyone on the forum has had experience on using softpots.
     
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