Help troubleshooting first PCB.  - Page 1
Forum section: electronics / projects

author: warpigs330
author_rank: Contributor
timestamp: January 22, 2021, 07:15:07 am
content: I recently had my first PCBs made for a prototype and a couple things aren't working. I have some transistor circuits that worked on the breadboard with equivalent through hole components. I also have a DAC circuit that is only outputting 0v on all the channels. I noticed one issue with my dac circuit compared to the one I was breadboarding with. One of the pins is missing a pull down resistor. I don't plan on using that pin so I did a bodge wire to ground to pull it down, and still nothing.The transistor circuits are meant to go high when a threshold voltage (around 1.5-3v, precision is not needed) is passed. I use an internal pull down resistor on my microcontroller to hold the pin low when the threshold is not reached. This circuit is a copy from a product whose circuits can be trusted. here is the schematic that I copied from.https://drive.google.com/file/d/0ByUu4ePJDmVnS3U1OUNiQ05heG8/viewAlso the script I am using is recognizing the DAC, but I'm not getting an output.I have double checked and the correct power voltages are going to the correct pins (for the dac and op amps that are connected to the dac's outputs)All of my buttons, switches, knob, and slider are working as intended.Any thoughts would be very much appreciated. I am still pretty new to this and learning quickly. Also any general tips for using JLCPBC for their SMD assembly service. I am assuming that any issues I have are due to my own mistakes and not theirs. I have included the KICAD project files for the board. Thanks for any help you can provide.EDIT: I forgot to mention that I am using a teensy 4.1 as my microcontroller.

author: thinkfat
author_rank: Unknown
timestamp: January 22, 2021, 09:01:23 am
content: Oh boy, where to begin...First of all, it's not the PCB that has a problem. The schematic itself is full of mistakes.Let's start with the obvious stuff:- Your transistors are incorrectly wired. They are NPN, you've connected the collector to ground and the emitter to your Teensy input which is also pulled down. Look at the original schematic again. You'll also need a pull-up at the Teensy input.- The outputs of the TL072 opamps are shorted to ground over a 100 Ohm resistor when nothing is plugged into the jacks. That is likely not what you intended.- The LP4OA1PBCT is wired weirdly. You have connected 3.3V to a pin that is marked "NC". Why?- The DAC needs LDAC pulled low, but I think you spotted that already.- Check the 3.3V supply. You have the Teensy providing this supply, be sure to not overload it. It's better to have a separate regulator for this.- The OpAmps should have decoupling capacitors on both supply rails close by.- Don't use 22µF MLCCs in 0805 size for bulk decoupling. If you apply DC, there will be effectively nothing left of their capacity (~70% loss). That you get them only with Y5V or X5R dielectric doesn't help either. If you absolutely need to use MLCCs, use larger sizes and use many of them. It's better to use electrolytic aluminium for that or solid polymer aluminium caps.Now for the obvious PCB problem:- The footprint for the OpAmps is wrong. The TL072 doesn't have a center pad. If you have JLCPCB assembling the boards, they will have printed solder paste on the center pad. As it cannot connect to anything on the bottom of the OpAmp, it might have squeezed out and shorted pins. You should check that.

author: warpigs330
author_rank: Contributor
timestamp: January 22, 2021, 09:30:36 am
content: Thanks a ton for the response. the LP4A1PCBT is wired that way because it made it output 3.3 v when pressed. I realize now that this is unimportant, and it would be better to just connect the other pin to ground and use the internal pull up.I now see the mistake I made with the transistors. I was looking too hard at the pin numbers of the schematic I was copying and not paying enough attention the the orientation of the symbol.I also learned about the op amp decoupling a little while after I got these boards made.This is all very useful information, and hopefully will lead to a functional prototype in a future!