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I am trying to build an Arduino controlled piezoelectric humidifier. I am using two humidifier modules which have 5V dc operating voltage(they have usb connectors). When I connect one module to my cicuit it works fine, but with two weird things happen. If I try to switch on both modules simultaneously the cicuit works, but if only one is turned off the one that is supposed be on also turns off and an intergrated indicator led(in the module) only glows dimly. If replace one of the modules with another load(it doesn't matter which one) the circuit works fine so the modules must be producing some sort of interference. Is there any way filter out the inteference for example adding a capasitor somewhere or is the circuit somehow incorrect?I have measured that the gate voltage on fet that is supposed be on is 5 V. Circuit diagram is below. R1 and R3 have arduino digital outputpins connected to them and S1 and S2 represent the humidifier modules.

After further measurements I have found out that module that is supposed be on receives 5 V but only draws few mA of current. The issue must be within the modules themselves, but I still need a way to filter out the interference.

enter image description here

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  • \$\begingroup\$ The power source is rated 2,2 amps. It works with two emitters on but not with one turned off but connected to the circuit. Also the operating voltage stays at 5 V so I doubt it is an overcurrent issue. \$\endgroup\$
    – Jargo
    Dec 16, 2020 at 17:37
  • \$\begingroup\$ What voltage do you measure across the allegedly "off" FET? Could it be installed backwards? Could you have blown the gate oxide? \$\endgroup\$
    – Chris Stratton
    Dec 16, 2020 at 17:46

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I believe the circuit in these Chinese modules uses a boost converter to generate a relatively high voltage for the piezo, in addition to the MOSFET and oscillator. There are two drum inductors on the PCB, so one for the boost converter and one for the piezo. Image below is from this eBay listing but there are many others with the 113kHz frequency listed (not a very atomizing frequency, by the way).

enter image description here

Boost converters, especially those without a soft-start feature such as the antediluvian MC34063 can draw a massive surge when turned on. This surge, in turn, can interact with current limiting on your power source.

You can try adding a BIG capacitor across the supply, preferably 5,000-10,000uF. Be aware that the capacitor also may interact with the supply and keep it from starting.

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  • \$\begingroup\$ I dont know this could explain that it works with two emitters on but no one. \$\endgroup\$
    – Jargo
    Dec 16, 2020 at 17:59
  • \$\begingroup\$ There is also no bulk input filtering capacitance on the module so other effects could be at play. But try connecting them both without the MOSFETs and Arduino, just two SPST switches, two modules and the power supply and you'll quickly figure that out. \$\endgroup\$
    – Spehro Pefhany
    Dec 16, 2020 at 18:09
  • \$\begingroup\$ I added an big capacitor across the supply and it dint't help. I'll try bybassing the MOSFETs later. \$\endgroup\$
    – Jargo
    Dec 16, 2020 at 18:18
  • \$\begingroup\$ >not a very atomizing frequency What do you mean by this? \$\endgroup\$
    – Ben Longo
    Sep 6, 2021 at 1:12
  • \$\begingroup\$ @BenLongo “atomizing” implies a relatively small droplet size. There is a mathematical relationship between frequency and drop size, higher frequency-> smaller drop size. A typical humidifier transducer operates at something like 1.6MHz. \$\endgroup\$
    – Spehro Pefhany
    Sep 6, 2021 at 6:13

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