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I want to use a small PWM driver PCB (that I hacked from a RC vibrator sex toy) to drive three other motors using a single 3.7V lithium-ion battery. (The motors are rated 3V.)

The PWM driver originally ran off 2xAAA batteries and drove a small motor connected between the + and the PCB output.

I replaced the original motor with an LED purely to indicate the state of the PWM, it will be removed from the circuit once working.

The three other motors draw about 900mA when connected directly to the 3.7V battery (no PWM) and I am afraid it will be too much for the original PCB to handle over a long period.

I tried to use a MOSFET to switch the higher load from the PWM driver, but I can't get it to switch on high enough to drive even a single motor. I can get it to flash an LED between the drain and +V, but as soon as I connect even a single motor it stops.

enter image description here

I have tried with IRF540N and NDP6020 (lower RDS), but neither seem to do the trick.

Can anyone make a suggestion?

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    \$\begingroup\$ IRF540N has too high a threshold voltage to drive it with 3.7V. NDP6020 is a P channel device. \$\endgroup\$
    – polwel
    Mar 4, 2022 at 7:43
  • \$\begingroup\$ I suspect the pcb can only sink current (I.e. pull the signal down to gnd). Due to the led, the gate voltage never gets high enough to turn the mosfet on. Remove the led or use a p channel mosfet. \$\endgroup\$
    – Kartman
    Mar 4, 2022 at 7:44
  • \$\begingroup\$ Use a FET where Rds(on) is specified for teh gate voltage your PWM can supply. \$\endgroup\$
    – user16324
    Mar 4, 2022 at 12:53
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    \$\begingroup\$ Please ALWAYS link to data sheets eg IRF540 1st graph on page 4 shows that at Vgs = 4V it might pull the skin off a rice pudding. At 3V it is just waking up. A lower Vgsth FET is advised. The cct seems OK enough. Add a gs reversed zener of say 4V7 rating to avoid inductive spike damage to gate. Add reverse diod across motors. \$\endgroup\$
    – Russell McMahon
    Mar 5, 2022 at 6:54
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    \$\begingroup\$ Look at the datasheet of an IRF540 and it says some of them conduct only 0.25ma when the Vgs is 4V. \$\endgroup\$
    – Audioguru
    Mar 24, 2022 at 23:10

2 Answers 2

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Thanks for the above comments! I ended up using the NDP6020 and getting rid of the LED's as they seemed to be causing the MOSFET to not turn on.

Using the following circuit I had no problem getting maximum current through all three motors. The PWM board output just needed to be pulled up to work and I thought the LED I had there would do it, but turned out just a simple resistor does the job! enter image description here

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You are missing two parts:

  1. A fast rectifier diode across the motors (cathode to 3.7 V) to carry the current when the MOSFET is off
  2. A large capacitor across the 3.7 V supply to supply the pulses of current
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  • \$\begingroup\$ Thanks. I can see the benefit if the capacity, unfortunately I dont have space for it in the current configuration! But it seems to work ok without anyway. Same goes for the diode. \$\endgroup\$
    – DOD
    Mar 28, 2022 at 1:01

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