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I recently got a freelancer to design a push-button latching circuit that powers a 12VDC motor from a 1S1P Li-po cell.

This is the schematic

I have soldered all the components but the board does not seem to work properly; only the push-button latching system works. When the push button is depressed, the LED lights up very brightly but only for an instant and goes back to a dimly lit state as if there was a surge (there are no burn marks observed). Input is around 3.8 V and output junction reads approximately 0.9 V.

The components' polarities are soldered as per the guidelines on the PCB. I soldered the second board the same way and it has the same issue. I took extra care in soldering the components for the second board and I am quite confident there was no overheating etc.

Can someone advise on how to troubleshoot the circuit further? Or does the circuit contains some design errors?

I have contacted the freelancer but he's not as responsive hence I am seeking help here. I still have enough components for three more boards to try.

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    \$\begingroup\$ Replace LED with a jumper .and expect 13V output. LED is overloaded and can go to output with 1k series To GND \$\endgroup\$ Dec 19, 2019 at 4:00
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    \$\begingroup\$ Err 12V out.... \$\endgroup\$ Dec 19, 2019 at 4:08
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    \$\begingroup\$ Is this the schematic the freelancer gave you? You got ripped off. He's trying to power the boost converter through the LED and an N-channel MOSFET wired as a source follower. There's no way this ever worked. \$\endgroup\$
    – Dave Tweed
    Dec 19, 2019 at 4:42
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 I removed the LED and bridged the pads, still doesnt work. \$\endgroup\$
    – Wb10
    Dec 19, 2019 at 4:59
  • \$\begingroup\$ @DaveTweed Yes, it's the original schematic he gave me. Are there any modifications I can do on my end to save this board? \$\endgroup\$
    – Wb10
    Dec 19, 2019 at 5:03

1 Answer 1

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This design was poorly done. The wrong FET polarity was chosen for a high side switch and is not even needed. There is an EN(able) logic level not being used .

  • remove FET and jumper across Drain to Source and 12V should then be working. If so , then
  • cut EN track and jumper to Q or Gate pad.
  • cut Vdd also on U1 and connect to Vcc track on U2 which is needed to make EN work.
  • keep U1- En,Boot & Vdd together but cut from Vout and connected to U2-Q out to enable if it ~5V.
  • as said in comment the LED is misplaced and never belongs there.

There may be other issues.

“The correct C3 capacitor for particular input and output voltage values cannot be calculated. Choosing the correct C3 capacitance is best done by trial and error, in conjunction with the checking of the inductor peak current to make sure your not too close to the current limit of the device. “

This is a perfect example of a time to scrap this effort which can be corrected but needs experience and order something from amazon or equiv.

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  • \$\begingroup\$ Appreciate the help. By remove FET and jumper across the drain to source, do you mean like this Also, how do I cut the tracks (EN and Vdd) if they are inside the board? \$\endgroup\$
    – Wb10
    Dec 19, 2019 at 5:35
  • \$\begingroup\$ Yes , you need to see a bare board or artwork to trace the connections to see how to disconnect Them and reconnect. This is what happens when mistakes are made while learning. I have made thousands of jumpers for my own and others designs. \$\endgroup\$ Dec 19, 2019 at 6:11
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 One concern about driving the boost switcher EN pin is that there will still be a leftover voltage at the motor when off, not great for reliability. The series LED is also in the way, it should be removed and placed in parallel with J2 pin 1 and with a correctly sized series resistor. \$\endgroup\$
    – eeintech
    Dec 19, 2019 at 13:24

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