Flickering and current fluxuation - Page 1
Forum section: electronics / beginners

author: jrozner
author_rank: Contributor
timestamp: January 13, 2017, 08:38:33 am
content: I just got the first board I've designed back from Oshpark and got it assembled. Amazingly it seems to be working but I've noticed a bit of flickering on the LEDs and the LCD display. I've also noticed that the current is constantly jumping around within a few milliamps from the bench power supply that is powering it. Is this a sign that I need additional capacitors to keep the current more stable for those components? If not, what else might it be?

author: ataradov
author_rank: Super Contributor
timestamp: January 13, 2017, 09:02:54 am
content: That kind of depends on what else is on the board. Can you post any information? Schematic? Board layout?

author: jrozner
author_rank: Contributor
timestamp: January 13, 2017, 09:13:02 am
content: Sure

author: ataradov
author_rank: Super Contributor
timestamp: January 13, 2017, 09:18:20 am
content: Your LDO (LP2985) has no input and output capacitors. They are essential. They are not there for "smoothing" things, they are a part of voltage regulation loop. They values must match the recommended values as close as possible. It also appears to have incorrect symbol (wrong pin names).EDIT: it does not appear to be actually used.

author: jrozner
author_rank: Contributor
timestamp: January 13, 2017, 09:24:52 am
content: Thanks, I'll look at that and fix it in the next revision. That is used for regulating the voltage down to 3.3v to power an esp8266 on the 6 pin header above it. That currently is working and haven't noticed any issues yet.

author: ataradov
author_rank: Super Contributor
timestamp: January 13, 2017, 09:29:54 am
content: Have you actually checked with the scope how that power rail looks like?This LDO is also too weak. It has maximum output current of 150 mA , while ESP8266 alone consumes up to 200 mA in transmit modes. The module power consumption will be higher.What is the power consumption of the whole board?

author: ataradov
author_rank: Super Contributor
timestamp: January 13, 2017, 09:32:52 am
content: I'm also assuming that your 0 Ohm resistors are not actually installed. Otherwise you have reset connected to ground.Also, trigger button will connect VCC rail directly to the input. You really need to have a current limiting resistor there.

author: jrozner
author_rank: Contributor
timestamp: January 14, 2017, 01:31:39 am
content: Thanks for spotting that with the LDO, I'll look for an alternative. The 0ohm resistors are not placed, it was just easier than finding jumper pads.I haven't had a chance to look at the 3.3v or 5v rails on the scope, I'll take a look at that.For the button going straight to VCC will putting that after the 10k going to ground work or would I have to use another resistor? If so would using 10k work or would I need to use one with less resistance than the 10k?

author: danadak
author_rank: Super Contributor
timestamp: January 14, 2017, 02:35:43 am
content: I have worked with that LCD controller in past and there is a problemwith display "artifacts" appearing. It is an asynch device and if it is ina fast loop being updated/written to, often you can get a crappy appearance.Solution was in code to build a buffer in ram. When an update is neededthe ram is checked character for character for changes, and only that char(s)which are changed are written out to LCD and RAM. If there is no change fromone loop iteration to the next, no writes occur to the display or RAM. Thatcompletely cured the issue.Regards, Dana.

author: ataradov
author_rank: Super Contributor
timestamp: January 14, 2017, 02:48:15 am
content: Quote from: jrozner on January 14, 2017, 01:31:39 amYou will need another resistor in series with the button. Something like 330 Ohm should be sufficient.A better solution is to use active low level and have a pullup resistor that is shorted to ground by the button.