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What did I do wrong in my first project?

avner

Nov 19, 2023
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So I decided I was going to design and build my first PCB and do (yet another) irrigation controller and moisture sensors.
First and foremost, it was great fun. Learning KiCAD, designing everything.
I made the design based on AC24V input (as the pumps require) and, a fairly simple and straightforward diode bridge, and 2 VRegs (to 5V and then 3V3). Here is it: https://github.com/theDontKnowGuy/sprinklers
However, I is not working as expected; the LM2596S-5 (JLCPCB part# C347421) got hot in less than a minute to the level I couldn't touch it with my finger. Also a burn smell came out.
I quickly disconnected it and tried another board with AC12V. This time it got very hot but took longer. Like 2 minutes.

I would appreciate it if you could take a peek at the design https://github.com/theDontKnowGuy/sprinklers to figure out which mistake we made.


What I am doing wrong???
 

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kellys_eye

Jun 25, 2010
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24VAC input rectifies to around 35-36V DC after smoothing. Those regulators are right at their DC limit for input voltage and could be suffering as a consequence.

As above, no way am I opening an unknown zip file. Post the pics.
 

avner

Nov 19, 2023
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Thanks @kellys_eye! I am getting almost the same result when inputting AC12V. The LM2596S-5 (JLCPCB part# C347421) is getting hot very quickly. Not as hot and not as quickly as with AC24V but to the level I cut the power not to burn the chip.

Any help would be highly appreciated !
thanks!

Screenshot 2023-11-20 at 8.36.59.pngScreenshot 2023-11-20 at 8.41.22.pngScreenshot 2023-11-20 at 8.42.08.pngScreenshot 2023-11-20 at 8.45.32.png
 

Harald Kapp

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Nov 17, 2011
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Please supply more detaiul on L1 (make, type, value).
Make sure L1 is suitable for the current in your application. If L1 gets saturated due to high current, its inductance will decrease and the step-down regulator can't work as designed.
 

avner

Nov 19, 2023
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Thanks Harald Kapp!
Worth noting that, as far as I understand, the load on the power is limited to the ESP32 chip (after the 2nd Vreg).
I tested the circuit current consumption with DC12V and measured ~200ma.

The L1 is this:

YPRH1207-330M​

Manufacturer: YJYCOINMFR.:
Part #YPRH1207-330MJLCPCB :
Part #C339951
Package SMD,12x12mm
Description: 3A 33uH ±20% SMD,12x12mm Power Inductors ROHS
datasheet: https://datasheet.lcsc.com/lcsc/2304140030_YJYCOIN-YPRH1207-330M_C339951.pdf
 

Harald Kapp

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This inductor is rated for max. 3 A, more than enough for your application.

The layout is not very suitable for this application: your traces are way too thin, especially in the powre section. Have a look at the datasheet and this app hint:
1700465525741.png
For a quick check whether your issue lies here, you can reduce teh resistance of the green and blue wiring (post #4, second image) by soldering some wires in parallel.
I also notice that you have not observed the hint about the feedback trace to be kept away from the inductor flux. On the contrary, you feed teh trace frpm pin 4 of the chip right under L1 to C6:
1700465807600.png
You can try and cut this trace at pin 4 of the chip and connect via a strand of wire to C6 instead, making sure it bypasses L1.
 

avner

Nov 19, 2023
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Thanks much @Harald Kapp !!
That all sounds very reasonable and I will try it today.
As I will not be able to clone all the wires in the power section, which ones are the most critical to overcome the over-hitting?
 

danadak

Feb 19, 2021
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D5 mounted on board with wrong direction ?

You met the input C ratings ?

1700482726410.png



Your layout does not seem to reflect the datasheet guidline layout ....?

Input C precautions, datasheet overall has a lot of precautions ....

Use caution when using a high dielectric constant ceramic capacitor for input bypassing, because it can cause severe ringing at the VIN pin.


Regards, Dana.
 
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avner

Nov 19, 2023
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So with all the good advice here (Thank you!!) I made some further adjustments and measurements and have the following hints
and still need help.... :):

1. As Harald Kapp suggested, I cut the feedback wire that is close to L1 and connected it with a strand to C6.
2. I made sure D5 is assembled in the right direction as danadak suggested.
3. I used another version of the same setup that has thicker wiring (the controller instead of the sensor, same setup, just with relays and wider wires).
Still having the exact same issue: LM2596S-5 (JLCPCB part# C347421) is getting very hot very quickly.

The main hint to what is going on I have is this:
1. Instead of inputting AC 24V I used regulated DC 12V to regulated DC 30V. Also in these cases, the LM2596S is getting hot. Not as quick as the AC 24V, but still. (Does it suggest that the problem isn't in the rectifying part / voltage peaks issues, but downstream?)
2. When supplying DC 12-30V, I measured a current of 120mA from the regulated power supply, regardless of the voltage provided. (Isn't it weird?)


What am I missing??
Thanks for helping.
 

Harald Kapp

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Have you ever measured the output voltage of the LM2596?
Try another regulator chip. The one you have may be broken by now.

Check the polarity of caps C5 and C6.

While you're at it: check power dissipation /current drawn while the old chip is out of the circuit to ensure the issue is not with the other components.
Also while the chip is out of the circuit apply 5 V (DC) from your regulated power supply across C6 and check the current drawn by the rest of the circuit (3.3 V regulator plus ESP32).
 

avner

Nov 19, 2023
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So following all the good recommendations here by Harald Kapp, danadak and others, I performed the measurements to the best of my ability.
Here are some results.
Bottom line: Even with regulated 15VDC the VReg LM2596-S getting hot very quickly. with 30V you can boil the ocean in less then 30 seconds
and I can't figure out why ....
I assume that the PCB board is assembled correctly as JLCPBC did it and it looks right.
 

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kellys_eye

Jun 25, 2010
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What device are you using for D5 (flyback diode)? You can't use an 'ordinary' diode, it must be of the schottky type (fast recovery). Try changing it.

Can you wire that device up off-board (flying leads or breadboard will do). You might have gotten unlucky with two dud devices and/or been the victim of 'fake' semiconductors.
 
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