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I have a simple circuit board that breaks out an ESP32. It has some inputs and then multiple channels of outputs which are MOSFETs switching 12V. The board voltage is 12V and the ESP32 is powered via a buck. I had this board designed by someone and the brief was that each 12V channel just needed to switch 12V on or off and handle less than 2A in current. The loads vary but are mostly inductive loads only drawing mA. For example computer fans and small coils briefly when releasing latches.

On the bench the board works as intended. Each channel switches on and off and supplies 12V. However, when I apply a load i.e. a computer fan, the voltage drops to 0.7V. I don't know enough to know why this would be happening.

Below is the circuit diagram, where:

I have two boards with multiple channels/mosfets which all have the same behavour so I am inclinded to think not a faulty FET

I really need to finish this project and if I can't salvage this I will have to redsign a new board with a switch circuit I know works. But I am really hoping I can make this work. Is the circuit correct? Can I modify it within the existing footprint? 🙏

enter image description here

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  • \$\begingroup\$ What value is D1? Have you measured how much current flows into the fan when it is directly connected to a 12 volt supply? Is the fuse still OK? An inductive load requires that the drain of the pass-transistor is protected by a Zener diode to avoid damage. Does the circuit still work on a resistive 30 ohm (approx.) load? \$\endgroup\$
    – Andy aka
    Commented Apr 29 at 10:23
  • \$\begingroup\$ Functionally your circuit looks like it should work, and you know it works with no load, so we need more details about it in operation. Check Q1's gate-drain and source voltage to see if it's operating. Check the base collector and emitter of Q2. Maybe try a smaller load and see if you get different results. \$\endgroup\$
    – LordTeddy
    Commented Apr 29 at 10:43
  • \$\begingroup\$ @Andyaka Sorry I have edited the question and added D1 in. The fan draws 110mA. Fuse is fine and same behaviour across multiple identical circuits on the board. Is shorting the load with a 30ohm resistor a valid test? If so - same result voltage drops to almost 0V \$\endgroup\$
    – Phil Klitscher
    Commented Apr 29 at 11:28
  • \$\begingroup\$ What does the incoming 12 volts look like in terms of voltage both before the MOSFET is operated and when operated. Maybe your incoming supply is too weak? I didn't suggest shorting the load with 30 ohm I suggested only the 30 ohm load and no fan. \$\endgroup\$
    – Andy aka
    Commented Apr 29 at 11:34
  • \$\begingroup\$ @LordTeddy Q1 source is ~12V but drain voltage is not going very low, 11.6V when active. Base of transistor is 3.3V, emitter read 2.45V. All measured when active (3.3V applied to Channel 1). My knowledge is basic with this stuff so let me know if I am measuring something wrong. I tried just powering a LED and it could barely run it. Voltage dropped to 2.5V \$\endgroup\$
    – Phil Klitscher
    Commented Apr 29 at 11:42

1 Answer 1

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Q1 source is ~12V but drain voltage is not going very low, 11.6V when active.

Did you mean the collector of Q3 is not going very low? If that is the case, then that would explain your problems. Check D1, it may have been installed with reversed polarity (installed backwards).

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    \$\begingroup\$ YES. Thank you, this worked. I found that the diodes had been placed backwards. I reversed them and now the circuit is working as intended. Thank you 🙏🏼 \$\endgroup\$
    – Phil Klitscher
    Commented May 1 at 0:34

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