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Need help with potentiometers

  1. Sep 29, 2010 #1
    Hi,

    I have 2 x 3.7V (7.4V) batteries connected in series running through a TI PTR08100W chip. I need to regulate the output voltage so that it is variable, ideally between 3.7 and 6V. From the website listed below i have determined i need a potentiometer that ranges between 200 and 400ohms but i am not 100% sure of this value. Is this correct and is it possible to acquire a potentiometer of this range? Also where could i find one?

    What i am trying to do is regulate a 7.4V input voltage to a variable output voltage of 3.7V - 6V via a potentionmeter. If what i have said above is nonsense, please advise of the best solution to my dilema.

    Here is more info on the chip - “http://focus.ti.com/lit/ds/symlink/ptr08100w.pdf” [Broken]

    Thanks
     
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 29, 2010 #2
    That link didn't work for me, here is the one I used.
    http://focus.ti.com/lit/ds/slts284e/slts284e.pdf [Broken]

    Since the PTR08100W module can already adjust its output from 0.6 V to 5.5 V, why do you need the pot? It appears this module is capable of 10 amps output or whatever your batteries can supply. That will burn up most potentiometers.
     
    Last edited by a moderator: May 5, 2017
  4. Sep 29, 2010 #3
    we need something so that the user can manually adjust there desired voltage from 3.7-6V ideally via a knob or switch. how can we do this using PTR08100W ? Or is it not possible?

    Thanks for the reply
     
  5. Sep 29, 2010 #4

    berkeman

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    Staff: Mentor

    Table 2 in the datasheet shows how to adjust the "Adjust" resistor value to control the output voltage.
     
  6. Sep 29, 2010 #5
    Sorry if i dont understand you properly but what i think you are saying is we can change it via a resistor, based on the strength of resistance. We want it to be easily changable for the user via a knob or switch as opposed to swapping a resistor out of the circuit each time you want a different output voltage. I dont think you can do this with the chip but if i am wrong could you maybe post how to? Or anyother solution?

    Thanks for the reply
     
  7. Sep 29, 2010 #6

    berkeman

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    Staff: Mentor

    From the table, it looks like the resistance needs to vary between 131k Ohms (for 5.5V out) to 241 Ohms (for 0.6V out), you would use a 100k Ohm potentiometer in series with a 243 Ohm resistor (the closest 1% resistor value to 241) to get most of that range, or you could use a 200k Ohm pot in parallel with a resistor to get you closer to the 131k Ohm max value (and still add the 243 Ohm resistor in series with that to get you the low end value when the pot is adjusted to zero).

    Since you want the voltage to go up when you turn the pot in the CW direction (that's the intuitive standard anyway), you would connect the wiper of the pot to the CW end of the pot. That way, when the wiper is at full CW position, it is not shorting out any of the pot, and you get full resistance. When the pot is turned fully CCW, the pot is shorted out to zero resistance.
     
  8. Sep 29, 2010 #7
    Thanks for the detailed reply. I think we only need a max resistance of around 400 Ohms though as we dont need our output voltage to drop lower than 3.6V. Which would mean we would need a pot ranging from 400 to 0 Ohms (as your idea about the 243 Ohm resistor in series would work here). So my final question would be does this low level of resistance exist in a pot? and if so where could i purchase one?

    In relation to this, what does skeptic2 mean and will my pot burn up?

    Thanks again
     
    Last edited: Sep 29, 2010
  9. Sep 29, 2010 #8

    berkeman

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    Staff: Mentor

    You can buy pots at Radio Shack or other electronics stores, or online at places like Digikey or Mouser Electronics. It looks like 470 Ohms is the closest value to what you are looking for:

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=262965&k=potentiometer

    That was because he thought you were passing the current through a pot, not using it to adjust the regulator, I believe.
     
  10. Sep 29, 2010 #9
    Thanks for all your help i think thats all we need :)
     
  11. Sep 30, 2010 #10
    Clicking on your very useful link we have stumbled into a few more questions:

    Resistance In Ohms - 470
    Power (Watts) - ?
    Tolerance - ?

    Based on our calculations above what wattage and tolerance would we need?

    Also do you reckon an easier method would be to use a coiled resistor then tap the coil at pre determined resistances and use a multi switch to choose a desired tapped voltage output? This way the voltage could be fixed at 3 different settings which is ideally what we want.

    Thanks again for your much appreciated time.
     
  12. Sep 30, 2010 #11

    berkeman

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    Staff: Mentor

    I believe that the adjustment resistor is low power, but you can check that yourself in the datasheet. Does it say what the voltage is at the Adjust pin? Having the minimum resistance always present (in the extra series resistor) will help to keep the power dissipation low for the pot.

    And as for tolerance, as long as the maximum resistance of the pot plus the tolerance is okay for your adjustment range, you should be fine. It's all up to you in your design of the output voltage range.
     
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