Tough one.... power supply for LCD drive voltage question... + versus - ? - Page 1
Forum section: electronics / projects

author: Milacron
author_rank: Contributor
timestamp: August 11, 2011, 04:58:23 am
content: I have a aprox 5 inch LCD monitor on a 1996 CNC Router I need to replace (yes, it's ridiculously small for a CNC monitor, but that's all that will fit in the alloted space) It is a Hitachi LMG9520 RPCC-A. I can't find one anywhere, not even from the manuf of the router. Basically this is a 5 volt input monitor and the newer monitors of this type and size have gone to 3.3 volts for some reason.But, after much looking I found a 5 volt one that has nearly identical specs...an Optrex F-51167NCU-FW-AA.... except for one spec...and that is the power supply for LCD drive (not to be confused with power supply for logic, which is aprox 5 volts on both monitors). On the original monitor, the actual measurement from the control isnegative18 volts. (FWIW, the original monitor is still working, but has developed a strange halo that makes legibility extremely difficult)The data sheet on the Hitachi for this spec is confusing to me. Under "electrical characteristics", the min for this is negative 23 volts. But under "absolute maxium ratings" the spec is min. 0 and max +32 volts !Now to the Optrex....for it's "absolute maximum ratings" is listed minimum 0 and maxium +6 volts. There is no mention of "electrical characteristics" for Optrex.The bottom line is, there are 15 pins on the original Hitachi monitor and 14 of the 15 from the Optrex match (allbeit in different pinorder) in specs. But if the control is outputting -18 volts for a monitor whose spec calls for min 0, max 6...what will actually happen when I connect the new monitor ? And for that matter, what is the purpose of a negative voltage in the first place ?

author: amspire
author_rank: Super Contributor
timestamp: August 11, 2011, 05:46:49 am
content: You definitely cannot drive the Optrex LCD drive Vcc2 with a negative volts.Typically the LCDs need an AC pulse voltage to control the pixels. There is every chance that this display includes a voltage converter that changes Vcc2 into whatever voltages it needs for the display.I guess the reason they have taken this approach is that a display that needs -18V is just a pain, so they use a design that generates the needed LCD volts from plus 3v to 7V. A much easier spec to work with.As long as the 5V supply can handle 100MA for Optrex display, just connect the LCD drive Vcc2 to the 5v Vcc1 for the logic and you should be fine.Richard

author: Milacron
author_rank: Contributor
timestamp: August 23, 2011, 03:13:23 pm
content: Yesterday I connected the new Optrex LCD for the first time, with the cable I created. The result was the backlight worked well, but the screen is blank. I didn't use a trimpot on wire 17/5, as I hoped I would see something on the screen and then add that if needed. But with nothing on the screen at all, I now wonder if it is because there is no voltage at pin 18 on the new monitor. Pin 18 is listed as "Vadj" or "voltage level for LCD contrast adjustment".There was no corresponding pin equivalent on the old Hitachi monitor to the Optrex pin 18, so I just left pin/wire 18 off the grid. The stated relationship on pin 18 between brightest and darkest contrast adjustment is .8V (bright) to 2.7 V (dark). So, seeing as it's getting 0V, is it possible that is why I see absolutely nothing on the screen ?---------------------------------Below are pinouts for Hitachi (old monitor) and Optrex (new monitor)As mentioned earlier, the one spec that was different voltage was "power supply for LC" (Hitachi 7, Optrex 17) thus the suggestion to tap 5V off of 14/5.But in light of this new information about the existance of pin 18, which is apparently specifically for contrast adjustment (and note the trim pot diagram at the bottom of the Optrex specsheet, which is meant for Vadj, pin 18), does anyone think I still need a pot at 17/5 as well as on pin 18 ? (and where the heck to get the voltage for pin 18....splice it in with 17/5 as well ??)This is probably confusing as hell, so if you need further clarification I'll be glad to try and explain further.

author: Unknown
author_rank: Unknown
timestamp: August 23, 2011, 10:12:13 pm
content: Contrast adjustment is often quite touchy, so I'm not surprised that it doesn't work. Connecting a pot takes about 5 minutes, seems like a quick test to me.

author: need2gofaster
author_rank: Newbie
timestamp: February 01, 2012, 02:40:49 am
content: Hi,I saw this discussion on another forum but for some reason my account hasn't been activated so I figured I would just post here.I am trying to help my uncle who is in the same boat as you. He has a Biesse Rover 13 and the LCD is coming apart and he is having an equally hard time finding a replacement for a reasonable price.Have you gotten the Optrex display to work yet? If not I have a couple of ideas/questions, both of which you've already mentioned as being possible issues.1) The Hitachi datasheet doesn't specify MSB/LSB so I would have guessed that signals that had gone to the Upper Data bits, UD(3:0), would map to D(7:4) on the Optrex and LD(3:0) would map to D(3:0).For exampleUD3 -> D7..UD0 -> D4LD3 -> D3..LD0 -> D02) As you pointed out Vcc1 + Vcc2 nominal is 60ma on the Optrex versus the 6ma nom on the Hitachi. I would measure the voltage between pins 14 and 15 on the Optrex when powered on to make sure it is getting something close to 5V.Barry

author: Milacron
author_rank: Contributor
timestamp: September 29, 2012, 06:00:10 pm
content: Quote from: need2gofaster on February 01, 2012, 02:40:49 amI never got any substitute monitor to display text on the Rover 13. How about you ?