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I have a PCB that is giving occasional problems.

The circuit is a remote receiver, based on the Heltec Cubcell Module. Basically, it will turn up to 3 relays on, depending on the signal it receives from the transmitters.

The circuit is powered from 12VDC, suppllied by a lead acid battery, which is charged by a stationary engine - simmilar to a rideon lawn mower setup.

2 of the 3 relays power up an electric clutch (max current draw 4.7A), while the 3rd relay powers up a small solenoid (~2A draw)

We tested some prototypes, and all appeared to work well, so we went ahead with a production run of about 30 boards.

Most boards appear to be working well, however, we have had a few boards that have blown up the DC-DC converter, and a couple of boards that a relay is sticking on.

I am a complete novice when it comes to electronic circuit design (I am a mechanical engineer), so am looking for any suggestions on what might be happening, or what could be improved in the circuit.

My only guess is that there could be a back EMF caused by the starter motor of the engine, or the electric clutches, that is potentially killing the DC-DC converter, or maybe the transistor.

In the case of DC-DC failure, sometimes the chip is visually blown, and other times, everything looks okay, but it does not output any voltage.

In the case of the relay sticking on, the only logical reason I can see is a faulty transistor, but I am unsure how to test the transistors.

Any help would be much appreciated, and apologies if this is in the wrong place

PCB Schematic

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  • \$\begingroup\$ The switched loads are inductive, which requires handling of the back EMF. Where is this done? The circuit looks OK to me, I would prefer to add a bulk capacitor parallel to C7. \$\endgroup\$
    – Jens
    Commented Oct 27, 2023 at 0:42
  • \$\begingroup\$ It has just occurred to me that I have a flyback diode on the coil of each relay, but didn't bother to do anything about the output of the relay. Would it be suitable to run a diode between Pin3 and GND of each relay? What purpose does the bulk capacitor serve? \$\endgroup\$
    – htdave
    Commented Oct 27, 2023 at 1:31
  • \$\begingroup\$ I would put a transient voltage suppressor (TVS) diode across C7 close to the regulator. Choose the clamp voltage to be between VCC (12V) and the maximum input voltage of the regulator (32V).. \$\endgroup\$
    – RussellH
    Commented Oct 27, 2023 at 3:07
  • \$\begingroup\$ Welcome! I'm missing decoupling capacitors on Vdd. What does your layout look like? \$\endgroup\$
    – winny
    Commented Oct 27, 2023 at 8:00
  • \$\begingroup\$ A very warm welcome to the site :-) Just on your schematic: recommend you never join 4 nets with a dot. The reason is that schematics with small junction dots or poor prints of schematics may leave it unclear if a junction dot is present or not. So a '+'-shaped junction may just be two nets crossing over or actually a 4-way junction. Whereas if you don't use 4-way junctions, the schematic connections will be unambiguous. It's a good policy and pretty easy to do when drawing them. \$\endgroup\$
    – TonyM
    Commented Oct 27, 2023 at 10:06

1 Answer 1

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From your schematic, it appears that the connection from C6 to L2 is not made (see below).

Connect this with a link wire on the board so that the regulator is operating correctly.

enter image description here

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