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I'm using an ESP-32 to switch an AC load. My PCB has onboard

  • 12 V 3 A - From wall mount adapter
  • 5 V 800 mA - From the step-down regulator
  • 3.3 V - from ESP not being used anywhere.

I'm getting 5V at pin 5 of the relay but only drops to 4.9 V when triggered.

PCB switching schematic

My relay is JQC-3FC(T73)DC05V. Currently, I have not connected anything to the output of the relay.

I've also shorted R8, still no avail.

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  • \$\begingroup\$ What voltage are you measuring at the gate when it is triggered? \$\endgroup\$
    – Null ♦
    Commented Feb 24, 2022 at 12:36
  • \$\begingroup\$ @Null I'm measuring 4.9V \$\endgroup\$
    – Rohan
    Commented Feb 24, 2022 at 12:48

3 Answers 3

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You have a 70 Ω resistor (R8) in series with the coil and that will stop the relay operating. Given that the relay coil has about 70 Ω internal resistance, the external resistor will effectively prevent anything other than about 2.5 volts being applied to the relay coil. Hence, it doesn't work.

enter image description here

Picture from here.

The coil power is about 0.4 watts and, from 5 volts that means it needs to take about 80 mA hence, it's internal resistance will be about 5/0.08 Ω = 63 Ω.

The BS170 MOSFET will drop a little bit of voltage but, to me, the big show-stopper is R8.

enter image description here

OK, if the gate-source voltage is only 3.3 volts then the BS170 could also be a problem. The graph above from the data sheet shows that with a 4 volts gate-source drive voltage, it should be OK but, at 3.3 volts, it may fail to operate sufficiently.

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  • \$\begingroup\$ Hey, I saw that and tried even shorted R8 with a set of tweezers. Still no avail. \$\endgroup\$
    – Rohan
    Commented Feb 24, 2022 at 12:48
  • \$\begingroup\$ @RohanRege yes, I think you have two problems; I've looked at the data sheet for the BS170 and, it seems, that running with a 3.3 volt gate drive voltage is just not enough to turn it on. See the amendment to my answer. \$\endgroup\$
    – Andy aka
    Commented Feb 24, 2022 at 12:52
  • \$\begingroup\$ Thanks, @Andy aka I'll try replacing the mosfet. \$\endgroup\$
    – Rohan
    Commented Feb 24, 2022 at 13:10
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There is no question but you are right, the BS170 will be unable to switch any load with 3.3V gate voltage.

The FET barely turns on with the 3.3V signal from the ESP32 GPIO pin, it needs somewhat more like 5V to turn on enough.

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I assume that by "Nebuliser" you mean an output of your ESP-32. It would be useful if you included links to the data sheet of the devices you are using in your questions. That would greatly help us to understand exactly what might be going on.
I've found this for the relay https://pdf.voron.ua/files/pdf/relay/JQC-3F(T73).pdf
And this for the BS170 https://www.farnell.com/datasheets/2304244.pdf
But they may not be the models you are using
I think your problem probably lies with the BS170. According to the data sheet the gate threshold voltage can be as high as 3V (typically 2.1V)
According to page 43 of the ESP-32 datasheet https://www.espressif.com/sites/default/files/documentation/esp32_datasheet_en.pdf
Voh (High-level output voltage) is 0.8 x Vdd = 3.3*0.8 = 2.64V nominally, so you may have hardly turned the FET on at all. I suggest you look for a logic level FET which you can find by googling it.

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  • \$\begingroup\$ Yes, I'll be sure to follow it and will include the relevant datasheets next time. \$\endgroup\$
    – Rohan
    Commented Feb 24, 2022 at 14:28

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