Electronics > Beginners
Help finding PCB design flaw
mrburns:
My first PCB design with SMD components pre-assembled in pcbway went south immediately. (Magic smoke...)
The pcb is basically a breakout board for a Pi Zero to connect 20 DHT22 sensors, but with 3-5V to 5V boost converter for external power and selectable 3.3V or 5V sensor voltage.
- It also has level shifters for data lines coming from sensors to gpio.
- Attiny10 smd microcontroller to have full system and sensor reboot possibility.
- Place for dual usb-a connector with power on-off possibility.
I ordered 5 boards from pcbway with smd components and usb-c and micro-usb connectors assembled. I want to solder through hole stuff my self but I did not solder those yet.
I just got the boards. Before powering on the first one I measured that nothing was not shorted and it looked good. Then I connected the usb-c connector to my phones usb charger.
1. First the power on led lit nicely and I was able to measure that the input was 5.125V and after the boost converter it was 4.999V. No problems. (The only thing consuming power at the pcb at this point was the small smd led.)
2. I then proceeded to carefully try to feel out if any component was overheating by just gently touching the top of the components. At this point the smoke came out of the feedback resistor of the boost circuit (Rfbt). (CRCW0402732KFKED CRCW Series 0402 0.063 W 732 kOhm ±1 % ±100 ppm/K SMT Thick Film Chip Resistor).
3. I disconnected the usb power immediately and started searching for shorts but found none.
4. I measured the second pcb for shorts and continuity on power traces.
5. I connected power to the second board thinking the probem with the first could have been a manufacturing flaw. -> Smoke from the input protection diode (D1) between the usb connector and the boost circuit.
6. I have been trying to find the flaw for two days now but cant find one. I already got smoke from third pcb and this time it was the p-mosfet (Q3) after the protection diode before the boost circuit. |O
All help is appreciated.
Pictures:
https://1drv.ms/f/s!AnfIOtcTpP6Bg4Njyl16cHLvhViyuw?e=eCatYS
Link to circuitmaker project:
https://365.altium.com/files/C33BBF2E-E81A-4FCC-8011-A663D419CE9A
Project files archive: (corrected)
https://d3mo.fi/storage/Pi_zero_20sensor_2024_03_25/
What tools is have for debugging:
Fluke multimeters, lab power supply, frequency generator, soldering equipment, internet. :bullshit:
ataradov:
First of all, stop powering things from USB power supplies. Use a lab power supply with a low current limit.
Then attach your actual schematic. The schematic in the archive does not seem to match the board. In the Schematic D1 just goes to micro-USB connector. On the board it goes to both.
Wait, schematic is for a completely different board.
mrburns:
--- Quote ---Wait, schematic is for a completely different board.
--- End quote ---
Thanks for noticing. The I fixed the link to point to correct project archive.
mariush:
Some oddities in your schematic ...
VBUS of both usb connectors seem to be connected together, with no protection ... what happens if you have let's say a power bank on one port and a phone charger on the other...
There's switch ICs like TPS21xx you could have used (TPS2121, TPS2211 to TPS2115) you could use to switch between inputs, there's ideal diodes you could use (ex LM66200 for dual ideal diode pack that auto selects highest input voltage, or single diodes like LM66100 that can be turned on or off with an enable pin to switch between two inputs)
Why use a tps61022 with maximum 8A switching current and an inductor good for 3-4A switching current, when the total power of your board is gonna be less than 500mA if you exclude the pi. Looking at max 2.5 mA for each sensor (if you read them all at same time), maybe 10mA per voltage translator chip, maybe 5-10mA for the microcontrollers...
The pi zero has it's own 3.3v buck converter on the board, you don't need to "stabilize" the 5v from usb, you're just doing a double conversion : https://datasheets.raspberrypi.com/rpizero/raspberry-pi-zero-reduced-schematics.pdf
Why the hell would you use possibly the shittiest version of 1117 regulator, that LM1117 needs capacitors with esr above 0.3 ohm.. you're using relatively expensive tantalum capacitors just to get this regulator working.
There's plenty of regulators that have very low dropout voltage and are stable with all kinds of capacitors ... some random examples just looking at highest stocked on digikey ...
AP7361C https://www.digikey.com/en/products/detail/diodes-incorporated/AP7361C-Y5-13/5638321 or https://www.digikey.com/en/products/detail/diodes-incorporated/AP7361C-FGE-7/5638318 ,
if 300mA is enough AP2210K https://www.digikey.com/en/products/detail/diodes-incorporated/AP2210K-ADJTRG1/4470821 or AP2127K would work https://www.digikey.com/en/products/detail/diodes-incorporated/AP2127K-ADJTRG1/4470788
for 500mA max, NCP8705 https://www.digikey.com/en/products/detail/onsemi/NCV8705MTADJTCG/5257920
You probably could have used a single buck regulator to convert whatever input voltage you have to something like 3.6v and run everything but the pi on 3.6v and use voltage translator if needed to change 3.6v signals to 3.3v for the pi)
mrburns:
--- Quote ---VBUS of both usb connectors seem to be connected together, with no protection ... what happens if you have let's say a power bank on one port and a phone charger on the other...
--- End quote ---
I was thinking this. But it would be good idea to add the protection if I ever order more of these. I just wanted both common port options so I would not need adapters with these.
--- Quote ---Why use a tps61022 with maximum 8A switching current and an inductor good for 3-4A switching current, ...
--- End quote ---
I was thinking that if i need to attach a 4G/GSM Modem USB dongle or similar devices i would have enough power for them. Also I want to have option to use sensors and/or devices that draw more current.
--- Quote ---Why the hell would you use possibly the shittiest version of 1117 regulator
--- End quote ---
I had no idea this is the shittiest version... :palm: It would certainly be possible to use the Pi:s 3.3V but I have had so many problems with devices drawing too much from the Pi so I wanted to avoid that if I ever use 3.3V sensors..
Did you get any ideas what might cause the power problems when nothing is even connected?
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