Need Help regaring a 0-10V Variable Voltage Regulator

Discussion in 'The Projects Forum' started by buddyengineers, Apr 13, 2011.

  1. buddyengineers

    Thread Starter Member

    Mar 19, 2005
    26
    0
    I just designed a Variable Voltage Regulator for 0 to 10V dc.
    For tht I used a LM7812 with about 17V as the input and I get 12V at output.

    Now, to regulate the 12V o/p dc, I connected a Voltage Divider ckt with a Potentiometer.
    The pot is of 600Kohms and so I connnected a 120kOhms resistor in series to get the voltage divider output to 10v.

    Now at no load, everytime I move my pot, the output varies from 0v to 10v. but immediately as I connect a load (which is a standard Dimmable Balast for tubelights which takes 1-10v as input), the output voltage goes to 11.5v and on turning the pot, I dont get a linear response.

    The output remains at 11.5v for a long time and only after the resistance of the pot has decreased to less than 300kohms that the output starts reducing, but its not gradual and shows a exponential format...

    Am very confused on y is this happening??? Load rating of the dimmable ballast is 1-10v, less than 0.6mA.

    I know that this is not the ideal way to create a voltage regulator... but still am confused as to wat is the problem...
    Also suggest me a better way to make a 0-10v regulator please.

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That large resistance is most of your problem. Use a 2K fixed resistor and a 10K pot instead of the 720 Kohm string. The divider you are using will permit only 16.7 microamps current. That is a lot less than the 600 microamps you need.

    If you want a regulator for 0 - 10 volts, use an LM317. Tying the ground pin to -1.25 volts will allow adjustment to 0 volts.
     
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  3. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    With your present setup, the easiest way is to add a NPN transistor as shown, forming an emitter follower. Just about any small signal NPN will do nicely. I2 represent the 0.6mA loading.

    Due to the external loading, even with a 10K POT as suggested above, the voltage change is not linear when the variable resistor is near the bottom of the range.

    [​IMG]
     
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  4. buddyengineers

    Thread Starter Member

    Mar 19, 2005
    26
    0

    I tried using LM317.. but to no avail... I connected a 100k resistor as R1 and the 600k pot as R2. According to the formula, I should be getting about 8.75v... but my multimeter showed 13v... <snip> confused now!!!
     
    Last edited by a moderator: Apr 14, 2011
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Take a look at the data sheet. Pay attention to the values for the magnitudes of R1 & R2.
     
  6. buddyengineers

    Thread Starter Member

    Mar 19, 2005
    26
    0
    Thanks... This worked...
     
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