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I'm using a level shifter (https://cdn-shop.adafruit.com/datasheets/txb0104.pdf) to interface with an LCD display (https://www.sparkfun.com/products/16398) that runs on 3.3V.

When connected to a DC power supply, everything works as expected (the screen turns on, the voltage coming from the level shifter to the screen is ~3.3V and the logic signal is ~2V.)

When I plug it in using an AC power supply, the output voltage to the screen drops from ~3.3V to ~2.8V, which is not enough to power the screen. I checked the level shifter's input voltage in both cases and it was the exact same (5.02V pwr 4.98V logic.)

I have included the relevant parts of my schematic below. (The input voltage for the entire circuit is 12V and I have a DC-DC converter that goes from 12V to 5V and the logic signal is coming from pin 7 on a 5V Arduino pro mini).

Why is the output voltage different and how can I get the screen to turn on using my AC power supply?

enter image description here

EDIT: Schematic edited to show 3.3V regulator Edited Schematic

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    \$\begingroup\$ Are you powering the LCD from the level shifter, or are you using the level shifter to translate signal levels? Level shifters are not intended to provide power. \$\endgroup\$
    – JRE
    Aug 19 '21 at 14:26
  • \$\begingroup\$ You're using a level shifter to power the LCD? \$\endgroup\$
    – hobbs
    Aug 19 '21 at 14:28
  • \$\begingroup\$ Sorry, no the level shifter just shifts the logic signal from the arduino down to 3.3 from 5 \$\endgroup\$ Aug 19 '21 at 14:29
  • \$\begingroup\$ How are you measuring the logic signal voltage? 2V sounds more like you are using a voltmeter than an oscilloscope. \$\endgroup\$
    – JRE
    Aug 19 '21 at 14:34
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    \$\begingroup\$ Are you turning the backlight on? The tiny 3.3V regulator in the Arduino may not have enough current capability to supply the backlight. The LCD itself should be okay. \$\endgroup\$ Aug 19 '21 at 14:50
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If your Bench-top-power-supply (i.e. your DC power supply) has current indicator, read that current. Or, measure the current, on the main power, 12V input. Put the AC power adapter (Wall Wart), that you are using now, in your reserve container, and get a new, at least 1.5x current rated, AC power adapter.
Meantime measure the 12V output, while loading, read AC, not DC. If you see more than 250mV, then put a large cap (200uF, 15V) in parallel with the power entry.
BTW, I meant "AC power" as DC Wall Wart, but not AC output. If your Wall Wart says "ouput: AC 12V", then come back with a new post.

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  • \$\begingroup\$ The benchtop power supply is reading 0.5A and the AC power adapter I am using is rated for 7.5A. Not sure what you mean by measure while loading though \$\endgroup\$ Aug 19 '21 at 15:13
  • \$\begingroup\$ @LilMamiChula, connect the Adafruit & Sparkfun, and then let it run. Measure the voltage (DC, AC) and the Current on the power supply terminal. It will tell if your power supply is sufficient enough. What else can be the problem?, Missing ground. Write down the readings, and post it here, please. \$\endgroup\$
    – jay
    Aug 19 '21 at 15:18
  • \$\begingroup\$ if the equipment mentioned is drawing 0.5A, something is not hooked up correctly; that's way too much current. \$\endgroup\$
    – dandavis
    Aug 19 '21 at 17:26

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