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I have several 12VDC 1.05A pumps I am using in my hombrew system which I would like to drive via PWM on my BBB. Each one will have a hall effect flowmeter attached so that I can get flowrates, totals, and drive the pump to maintian a flowrate. I have seen how to control the motor using a mosfet (on the bldr website), but it drives from the negative side of the motor. Is there a way to control the motor from the positive side? The reason I ask is that I would like to use a common ground for the pump and flowmeter, and if I am driving the pump from ground, it will throw off the flowmeter readings.

Any thoughts?

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  • \$\begingroup\$ I'm not sure I understand how you think the flowmeter and pump would be wired with a low-side switch, and how that would affect the ground. Any chance you could craft a small schematic for us with your thinking? \$\endgroup\$
    – Majenko
    Commented Nov 5, 2014 at 15:36
  • \$\begingroup\$ I assume the flow meter is electrically parallel to the motor, in terms of connection to ground. There is no issue with making the motor's ground float using a low-side switch for PWM.. the flow meter's circuitry will be fine, it will still work and it will still detect the flow as you want \$\endgroup\$
    – KyranF
    Commented Nov 5, 2014 at 15:47
  • \$\begingroup\$ ... Unless you're thinking of having the MOSFET remote from the motor, and thus there would be a PWM switched ground connection going to both the motor and the flow meter? \$\endgroup\$
    – Majenko
    Commented Nov 5, 2014 at 16:14

1 Answer 1

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You can use a P channel mosfet to control the DC motor from the positive side: -

enter image description here

You've got to be careful though if you are using a common ground for both motor andf sensor - you could get false triggers in the sensor if sharing a common wire that is too long/ too flimsy.

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    \$\begingroup\$ When driving with PWM you may want to reduce the value of the P-channel's pull-up resistor (10K) as the switch-off time would be quite high - maybe longer than the period of the PWM - which would cause increased power dissipation in the FET. \$\endgroup\$
    – Majenko
    Commented Nov 5, 2014 at 16:26
  • \$\begingroup\$ @Majenko-notGoogle - good call dude - it depends on the PWM frequency for sure - 300Hz or less is probably OK with 10k ohm but, any higher frequency and gut feeling tells me I'd be thinking about lowering it to 1k ohm. \$\endgroup\$
    – Andy aka
    Commented Nov 5, 2014 at 16:29
  • \$\begingroup\$ Even at 300Hz, at high duty cycles, the off period could be much higher frequency. 300Hz at 95% duty would be like (from a switch-off PoV) 3kHz. \$\endgroup\$
    – Majenko
    Commented Nov 5, 2014 at 16:34

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