alternative for powering LCD panel meters that don't support common ground

Discussion in 'General Electronics Chat' started by Efron, Jun 20, 2012.

  1. Efron

    Thread Starter Active Member

    Oct 10, 2010
    Hi all,

    I've got an LCD panel meter, very basic, 3 1/2 digits, decimal point and those basic staff.

    Regarding the power supply than can be applied, I understood there are mainly two kinds of such panel meters: those that support common ground connection and those that not.

    Mine is the one that DO NOT. In this kind of LCD panel it is forbidden to connect the Vin- pin of the LCD to the same ground that the one used for powering the LCD itself because this will probably kill the LCD internal circuit.

    The clue is then to ISOLATE the Vin- of the LCD panel from general ground.

    I've made research in the internet and I found interesting circuits to make its own 9Vdc isolated power supply from a common general power supply (the one that is used to power the circuit where the measure has to be done).

    However I'd like to consider the following alternative, based on the following assumptions:

    * The maximum differential input that is supported by these kind of LCD panel meters is limited to 200mV.
    * As a consequence, a voltage divider is mandatory if voltage to measure is greater (which generally it is).
    * The voltage divider uses two resistors, generally called RA and RB and the values are calculated so that 200mV appears in the input of the LCD when the voltage to be measured reaches its maximum value.

    This voltage divider is shown in figure 1, where an isolated 9Vdc has been used to power the LCD. The input voltage to the LCD is proportional to the real voltage to be measured and always under 200mV.

    However, I think that the schematic of figure 2 could work as in this case the
    the Vin- pin of the LCD is somehow isolated from common ground through RA, which value is big (some MEGA).

    I've not tested this configuration yet. Could you please give me your comments / advise before I do that?

    Many thanks