Digitally Controled 500k resistor

Discussion in 'The Projects Forum' started by szymonm2, Mar 11, 2016.

  1. szymonm2

    Thread Starter New Member

    Jan 26, 2016
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    Hi guys.
    I'm trying to control a 500k ohm potentiometer using some sort of digital means in order to make a programmable buck/boost converter...

    How can this be achieved? The biggest digital potentiometer that I could find was a 10k one....
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Without a schematic it is impossible to say. Do you want to add a small motor to the 500K pot you have, or alter the circuit to use the 10K digital pot you can buy, or what?

    ak
     
    Last edited: Mar 11, 2016
  3. Alec_t

    AAC Fanatic!

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    Why does the buck/boost converter need such a high value pot?
     
  4. szymonm2

    Thread Starter New Member

    Jan 26, 2016
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    Its a ltc3780 buck boost converter. It has a 500k pot for voltage control and 250k pot for current limiting.... So any ideas how can I digitally achieve this. Thanks so much :')
     
  5. bertus

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  6. szymonm2

    Thread Starter New Member

    Jan 26, 2016
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    Thanks for the reply :)

    In this video the guy changes the potentiometers to external ones with a value of 500k and 250k. You can also see in this schematic that there is a 487k potentiometer..
     
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  7. szymonm2

    Thread Starter New Member

    Jan 26, 2016
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    I basically want to copy his variable power supply but replacing the potentiometer s with a keypad, arduino and LCD display.. I already figured how to measure voltage and current but I have no clue how to digitally control 500k,250k potentiometers...
     
  8. AnalogKid

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    I see a 487K fixed resistor in the voltage output sense feedback loop, but not a pot. Also, most digital pots cannot run between +48V and GND. What is the max output voltage you are trying for?

    ak
     
  9. szymonm2

    Thread Starter New Member

    Jan 26, 2016
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    Thanks for the reply :)
    If you watched the video the buck/ boost converter can output a max voltage of 30 volts. The module is rated at 120w so it technically can output 4 amps at 30 volts. I honestly cant find the 500k, 250k potentiometers in the schematic but I'm 100% sure that the module uses 250k/ 500k potentiometers....
     
  10. AnalogKid

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    That's because that is not the actual schematic for that board design. Linear Technology makes the control chip at the heart of the circuit, and the schematic is from their applications group, not the board vendor. It's probably very close, with pots substituted for fixed resistors. R5 is a 220K resistor that sets the current limit threshold, so that's where the 250K pot goes.

    Because the current sense adjustment resistor is so close to GND, it probably can be replaced with a digital pot. The problem for voltage adjust is that it touches the output. You might be able to leave R7 alone and replace R8 with the digital pot.

    HOWEVER - both the voltage and current set circuits have capacitors in them that are part of the control loop frequency compensation, and varying the resistors shifts the corner frequencies. Whether or not this will send the regulator into oscillation can be calculated by someone much better at this than me. I'd start by replacing R8 with a 10K pot and adjusting it all over the place to see what happens.

    ak
     
    Last edited: Mar 11, 2016
  11. dannyf

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    Sep 13, 2015
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    Between some unknown dudes video off the internet and the manufacturers data sheet , I would go with the data sheet every single time.
     
  12. MikeLogix

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    Mar 1, 2016
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  13. Alec_t

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    The VOS+ feed resistor R4 (10Ω) and C47 (0.1u) (as in the datasheet) would certainly affect the feedback loop frequency response, but I'd be surprised if reducing R8 and R7 by a factor of 100 even would make much difference. The reactance of C47 at the operating frequency is only a few Ohms, so if R7 were, say, 5k that shouldn't greatly affect the phase shift which R4 and C47 produce. That's theory, of course ;).
     
    Last edited: Mar 12, 2016
  14. szymonm2

    Thread Starter New Member

    Jan 26, 2016
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    So im all confused by all of this. So is it possible or not? From what i understand it is too complicated?
     
  15. Alec_t

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    My money says not, unless (a) you can source digital pots which can withstand 30V (or whatever your max output will be) and (b) the regulation will work with pot resistances ~10k or less.
     
  16. szymonm2

    Thread Starter New Member

    Jan 26, 2016
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    O
    Ok thanks :') is there a good guide to make a switching programmable variable psu with a max voltage of around 30v?
     
  17. dannyf

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    Sep 13, 2015
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    The pots values make zero sense to me.

    Having said that, if you still want to go down the path, attenuate the high voltage and then amplify it with an opamp whose gain is controlled by the pot.
     
  18. jpanhalt

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    Jan 18, 2008
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    Do you have a real schematic?

    John
     
  19. MikeLogix

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    Mar 1, 2016
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    I found this high voltage digital pot, it can handle 36 VDC.

    http://www.mouser.com/new/microchip/microchip-mcp41hvx1-potentiometers/
    I don't know if that part is a good solution or not, but thought I would mention it :)
     
  20. Alec_t

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    Haven't checked, but people here usually advise that it's cheaper/easier to buy one than build one.
    Ready-built ones normally have a fixed potential divider or trimmer feeding back from the output to set the ouput voltage. You might be able to hack that, using a digi-pot (30V rated?).
     
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