Is it possible to do this with LM385 fixed voltage reference diode

Discussion in 'General Electronics Chat' started by synaptic knob, Jun 24, 2016.

  1. synaptic knob

    Thread Starter New Member

    Jun 6, 2016

    I have two LM385 2.5v voltage reference diodes lying around. Before actually trying to do, I thought it would be a better idea to ask more knowledgeable & experienced members on the forum.

    So the question is:
    Is it possible to join the outputs(A & B) of the two separate LM385 2.5v diodes to get a 5v reference voltage like the figure below?


    I would highly appreciate member's advice, please looking forward to hear from the AAC forum members & moderators.

    Thanks and regards
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    No....the output will be -- guess -- 2.5V!

    You can series two of them for 5.0V.
    synaptic knob likes this.
  3. EM Fields


    Jun 8, 2016
    No. The outputs are in parallel so your output will be 2.5 volts.

    If you want a 5 volts reference, connect supply + to a current limiting resistor, then connect the free end of the resistor to the cathode of one of the LM385s, its anode to the cathode of the next LM385, and that LM385's anode to supply negative (ground). Take the 5 volt output from the junction of the resistor and the LM385.
    synaptic knob likes this.
  4. ian field

    Distinguished Member

    Oct 27, 2012
    Two Zeners/reference diodes in parallel will give you voltage equal to the lowest rated voltage. Series adds the respective voltages.

    My old favourite is the TL431 programmable Zener that you can adjust to what voltage you need - but its only as accurate as you can adjust it, and probably not as stable as your fixed reference device.
    synaptic knob likes this.
  5. synaptic knob

    Thread Starter New Member

    Jun 6, 2016
    First of all I am extremely sorry for the delayed response as I was out of station.

    I tried to wire LM385 2.5v in series and yes it works and now I am getting 5v.

    Thanks to joeyd999, ian field & special thanks to EM FIELDS for describing and making things very clear to me.

    Thanks again to the respected members of the forum for your guidance & I request you all to please keep up the good work as it means a lot for people like me.
  6. synaptic knob

    Thread Starter New Member

    Jun 6, 2016
    So here I am posting the final working circuit diagram.

  7. myntath

    New Member

    Jun 26, 2016
    You should probably decrease the resistor value a little. As it is limiting current just a little too much when there are two zeners in series.

    4V / 221kOhms = 18 uA - datasheet suggests a minimum of 20uA

    The example in the data sheet with one zener (where I assume you took the resistor value from) has a current of 30uA. So perhaps aim for the same. A 120kOhm resistor should be about right.
    synaptic knob likes this.
  8. synaptic knob

    Thread Starter New Member

    Jun 6, 2016
    Yes you are right. Thanks a lot for correcting me I highly appreciate.