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

[efron]

Hi all,

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

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 DOES 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

[alm]

In your second circuit, imagine a diode connected from Vin- to ground (I believe the ICL7106 uses a slightly different scheme to bias the common input somewhere halfway the power supply voltage). This diode would draw additional current through RB, causing an error.

[efron]

Hi alm,

The LCD meter I'm talking about is like the one in the attached image.

No need for any additional controller like ICL7106.

If i connect a diode between Vin- and common ground, it is likely to connect Vin- to common ground and the LCD meter will probably be destroyed.

The connection between Vin- and common ground must be through high impendance like in my example.

[alm]

The diode was an example of what a controller, like the ICL7106 (this IC, or a clone, is most likely what powers the DPM), might use as a biasing scheme to bias Vin- within its common mode range. You can go through the datasheet, which given its age most likely contains a simplified internal schematic, to get a more accurate picture of what the biasing scheme looks like and what is connected between Vin- and V+/V-/Common.

[efron]

Hi Alm,

Finally I understood what you meant when you were making a reference to the generic LCD controller ICL7106.

In any case, I didn't succeed to make it work although I tried several connections.

At the end, I made an isolated 9V power supply found on the net on the basis of a square wave oscillator, a rectifier and a filter to make it work (http://www.discovercircuits.com/DJ-Circuits/9v-isolated%20supply.htm).

It works fine. I tested it in protoboard and I'm making now a small through-hole pcb to hang it to in the rear of the LCD.