I'm using a 2x8 LCD display and it works fine, but I've discovered that it causes the 3.3V supply to rise to 4.2V. I suspect there may be some conversion going on inside the display. I'd like to connect a thermister circuit but it doesn't seem to work right under this circumstance. I tried connecting just power which was OK, then each pin one by one and discovered that when I got to the forth pin, the Register Select pin, the voltage jumped to 4.2V.
Any ideas?
I'm using Arduino uno R3, with Newhaven nhd-0208bz-fsw-gbw-33v3 display.
Thanks,
Dave
And how to you have your display wired to your Uno? Your Uno is a 5V device. If you are directly connecting some pins to the display, they will be at 5V as well. You will need a level shifter for each pin from Uno to the display.
blh64 is on the money. Driving the data pins with 5 V from the UNO will feed the 5 V back to the display Vcc via the protective diodes.
But why would you be using 3.3 V for a LCD character display?
Thank you. Yea...this was another thought that I had. I believe you're right. I screwed up. I guess I'll just have to buy another display that runs on 5V instead of 3.3V. Thanks again.
Thanks, Paul. Like I said to blh64, I screwed up. The display is meant to run on 3.3V only. I'll get another one.
Well, you can reasonably easily operate it at 3.3 V. If you do not need to read any information back from the display - as I presume you do not, you just connect each data/ control line (except pin 5 which is grounded) through a diode to the display, cathode to Arduino, and provide a 1k pull-up to 3.3 V on the display side of the diode.
Do not power the backlight from your 3.3 V pin. Connect it to 5 V via a 100 Ohm resistor.
Of course a 3.3 V display is quite inappropriate unless you wish to use a fully 3.3 V system. The module actually contains a charge pump to generate 5 V from the 3.3 V! 
And do not connect the contrast control potentiometer to (any) Vcc. It should connect to ground only. An interesting blunder in the Newhaven datasheet indicating they - as is so often the case - are selling a product of which they do simply not comprehend its workings. 
Your solution sounds reasonable. I wish the manufacturer had used a level converter for these situations. But that's too much to ask, I guess.Anyhow, I've ordered a 5v display. I suppose it's for the best, as I would like to isolate the 3.3v for use as a supply for a temperature probe, and now that have your attention, do you have any experience with meat cooking probes? Lot's of places sell these probes but none provide specs.
What situations? Why should the manufacturer include a level converter? A 3.3 V module is intended to be powered by 3.3 V with 3.3 V data levels. Correspondingly for a 5 V version. 
The 3.3 V from the Arduino should be used for the very minimum essential devices, sensors and not "effectors" (including displays). It is a different matter for the Arduino versions which specifically use 3.3 V processors and a regulator capable of actually supplying peripherals.
To be honest, no! 
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