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Convert 12V to 3V Without affecting Total Resistance

Discussion in 'General Electronics Discussion' started by KAD_Aragorn, May 23, 2018.

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  1. KAD_Aragorn


    May 23, 2018
    Hello Guys,
    This is my first thread here..

    I have a device that contains a circuit like this.

    The 1K Potentiometer RV1 is actually a Foot Pedal where the resistance changes when the pedal is pressed and outputs the Voltage to another Device through OUTPUT.
    however, The OUTPUT Voltage is set to a fixed Default Value of 1.2V when the pedal is Not pressed and It'll go upto 12V when the pedal is fully pressed.

    I need to tap into that output Voltage and connect it to a ESP8266 Analog Input. Since the ESP only works with 3V, I need to convert that output Voltage to 3V.

    I tried a voltage divider connected to the output like this.

    The Converted Out can be taken down to 3V. BUT the Voltage of the main OUTPUT that goes to the device changes when the Voltage Divider is connected. The Maximum 12V that is supposed to be achieved when the pedal is fully pressed is never reached because the Voltage Divider affects the Total Resistance.
    Note that the RV1 Resistor cannot be changed. It is fixed inside a device which is very hard to dismount AND there are 1000s of devices that needs this conversion.

    So what are my options? How can I get the Output Voltage of 0 - 12V range of the Device and Convert it into a 0 - 3 V Range ?

    Thank you for your support!
  2. AnalogKid


    Jun 10, 2015
    It is hard to answer questions with missing important information. What are the values of R3 and R4?

    You might have a problem satisfying two conflicting requirements. If you increase R3 and R4 both by 100x, then they will not load the output of the pot as much. But, the converted out source impedance now will be much higher, and the downstream device might not like that unless it has an even higher input impedance. If not, you might have to add a buffer stage like an opamp voltage follower.

    Is this for a guitar pedal?

  3. kellys_eye


    Jun 25, 2010
    I've read, but can't confirm by experience, the the analogue input impedance is around 20MΩ. But there is conflicting information everywhere!

    The only real answer would be to test it yourself.
  4. Alec_t


    Jul 7, 2015
    Many analogue inputs require a source impedance of less than 10k for accurate, reasonably fast A-to-D conversion. But a10k voltage divider across your 1k would drop the '12V' by around 10%, i.e too much. That points to you needing an op-amp to buffer the signal.
  5. AnalogKid


    Jun 10, 2015
    The "Converted Out" divider is connected to the wiper. If the input to RV1 has a zero ohm source impedance, the maximum source impedance for the output divider is 250 ohms. This can be used to calculate how much "Output" sags under load.

  6. KAD_Aragorn


    May 23, 2018
    Hello Guys,
    Thank you for your replies! I think everyone is kinda confused with the second circuit.

    Anyway let me simplify things.

    Lets say I have a simple Circuit like the attached image.
    There is a DEVICE that outputs a Voltage between 0V and 12V.
    I want to get the SAME Voltage but converted to a 0-3V without affecting the above 0-12V.

    So the OPERATION X would convert that 0-12V into 0-3V without changing the original Voltage.

    Thank you for all your contributions!


    Attached Files:

    Last edited: May 28, 2018
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    See the first post in this thread.
  8. AnalogKid


    Jun 10, 2015
    As above, it depends on what the 0-12 V is coming from and what the 0-3 V output is connected to. Without those two pieces of information, calculating a 2-resistor voltage divider is guesswork.

    You can use a high impedance buffer like an opamp voltage follower to isolate loading effects, but even a rail-to-rail opamp's output cannot swing all the way down to 0.0 V, so there sill be a small error in the output.

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