I'm making a bench PSU. The Arduino runs off the purple 5v standby. The Mega also powers 2 LEDs (normal 5mm clear). The thing is, when I upload the code and watch the screen go through the process, it's bright and visible. But when I turn on the 5v standby (without USB) the text is visible if you try really really hard to see.

The 5v standby is rated at 2 amps MAX. I can't imagine it's the code but would anyone help? Why does USB power allow the LCD to power fully compared to the 5v standby?

  • 1
    Can you supply a schematic? And are you putting 5V into the barrel jack connector? If so, that's why you're getting a dim screen: the supply through the jack is supposed to be a minimum of 7V or so because it powers the onboard 5V regulator. 5V in will only give you about 3.75V out, maybe less. By "purple 5V standby" I guess you're using an ATX PSU? – CharlieHanson Jun 22 '18 at 15:50
  • get a barrel-jack to microUSB adapter for ~$1 online to by-pass the on-board voltage regulator dropout. Also, you might be able to adjust the contrast to work acceptably; i had one that i ran off 3.3v and at first it was super faint, but with tweaks it was more than acceptable for the viewing angle i needed. – dandavis Jun 22 '18 at 17:19
  • I do not know how to upload fritzing files to the comment. However, dandavis sounds like he is onto something. The problem i have is that, i made a slot on the side of the enclosure to allow updates and debugging easy. The 5v standby was soldered under the onboard barrel connector legs. can i power the arduino with the purple 5v standy via usb? – Icey Hood Jun 23 '18 at 19:35
  • You are not supposed to put fritzing files to comments. Instead edit your question and put a screenshot of the fritzing there. – chrisl Jun 25 '18 at 21:47
  • This really sounds, as if you were putting 5V to the barrel jack. That won't work, as CharlieHanson wrote. Connect it to the USB jack or directly to the Arduinos 5V pin. – chrisl Jun 25 '18 at 21:53

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