I need to control 8 motors from Arduino.

speed control only with a single pwm output

I use a single channel at time

I have 74hc4051 analog/digital mux/demux ic

if I set mosfet drive circuit with gate pull down (40kOhm) and gate resistor 100Ohm on all output channels of 74hc4051

I need to know weather there is a possibility to generate floating noise on unused channels?

according to theory since I am using mosfet gate pull down on each output of mux channels floating noise will be pulled to ground is this correct?

enter image description here

analog mux output connects to mosfet gate

  • 2
    \$\begingroup\$ 40KOhm is not a strong pulldown. And 1,000 pF Cgate, with 40Kohm pulldown, is 40 microSeconds 1-tau decay, which may not be close enough to Ground to ensure FET turnoff. Can you provide a schematic of what you are planning? \$\endgroup\$ – analogsystemsrf Oct 7 '19 at 3:53
  • \$\begingroup\$ @analogsystemsrf i have attached mosfet driver \$\endgroup\$ – hsim Oct 7 '19 at 4:06
  • \$\begingroup\$ The switching R={RdsOn + ESR of Vdd bulk Caps) if < 10% of DCR coil resistances of all motors you wish to start up simultaneously then less chance of load regulation sag assuming 10x current is available from Vdd for start power > Tau=R*Ciss *8 which must be < T=Rise time of switched current. (<1us) Otherwise conduction self heating loses require heatsinks & minifan. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 7 '19 at 6:14
  • \$\begingroup\$ @Sunnyskyguy EE75 motor is 3V 70mA max current \$\endgroup\$ – hsim Oct 7 '19 at 6:46
  • \$\begingroup\$ Then start surge ~ 700mA and DCR ~ 43 Ohms mouser.ca/datasheet/2/308/NCV7708F-D-1522527.pdf \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 7 '19 at 7:15

Yes, that will work fine. I would use 4.7K pull downs. Only one (at most) will be drawing current at a time.

Put the pull-down at the 4051 output rather than the gate to avoid unnecessarily reducing the gate drive voltage.

Make sure your MOSFETs are logic-level drive (eg. 4.5V for a 5V MCU). With a couple hundred ohms in series with the gate they will not switch very quickly, so keep the PWM frequency reasonably low to avoid excessive switching losses.

The cross talk in the HC4051 will not be a problem.

The diode is necessary, but the capacitor may be counterproductive.

  • \$\begingroup\$ I would hope that the capacitor is for noise reduction - a low value as close to the motor as possible, to reduce brush noise. But OP doesn't say. \$\endgroup\$ – TonyM Oct 7 '19 at 8:22

Yes, it will work under selected conditions.

  • Be sure that the maximum 4051 drive current ability to drive the gate high or low is acceptable at the desired PWM rate.

  • You CAN use the 4051 as a "sample and hold" with due attention to drive levels and response times, and maybe PWM multiple channels at once if PWM is fast enough.

About 35+ years ago (!) I made a multiport X-Y switch with 4051s which allowed me to switch eg any 8 of N x 8 input channels to 8 output channels. I was switching analog signals but the principle is the same. (On a frame synchronised 'scope you could see the analog signals time multiplexed together.


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