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I need to run 12v motor with using Raspberry Pi. I made a circuit which works but then I noticed a "little" problem. Pi's software PWM signal is only 3 volts so BUZ11 doesn't open enough.

I found this schematic:

enter image description here

Is it possible to somehow crank my PWM signal up? Ex. Using that schematic given with inverted pwm? Or can I just use transistor before FET to raise pulse voltage?

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  • \$\begingroup\$ How many amps do you need for your motor? \$\endgroup\$ – Fizz Nov 16 '15 at 19:23
  • \$\begingroup\$ Currently a few amps I think. Possibly more in the future \$\endgroup\$ – LizNet Nov 16 '15 at 19:26
  • \$\begingroup\$ Use charge pump \$\endgroup\$ – GR Tech Nov 16 '15 at 20:46
  • \$\begingroup\$ More directly related question I found in the meantime: electronics.stackexchange.com/questions/109128/… \$\endgroup\$ – Fizz Nov 18 '15 at 1:10
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If the MOSFET doesn't open at logic level of your MCU you either need:

  • a MOSFET that opens at lower VG (and gives you enough drain amps at that VG), or
  • you need to use a MOSFET gate driver that does the logic-level translation.

What you have there with that NPN is a workable albeit basic MOSFET driver.

Which of these two approaches is better depends on various factors... with the former, you need new MOSFET[s], with the latter you can use your existing ones, but the circuit becomes more complicated and driver isn't free either.

There is also catch with lower Vgs MOSFETs: those that are both low Vgs and high current tend to come in packages that are nasty to solder by hand, e.g. NVTFS4823N is similar enough to BUZ11 in Rds and max Id terms, but will give you more Id current at lower Vgs. The catch? Difficult to solder by hand. They aren't all that bad though; BUK9219-55A comes in DPAK and does 50A with 3V gate drive; also has good enough Rds on (compared to BUZ11):

enter image description here

Mosfet drivers can be realized in many, many ways. A bipolar transistor is used often enough because it is cheap. Sometimes a pair of push-pull transistors are used; these allow both transitions (on and off) to happen fast. You can find such transistor pairs as a single package e.g. MCH6541.

enter image description here

Additionally, if you need level shifting (and here you do) add another transistor in front) as in this answer:

schematic

simulate this circuit – Schematic created using CircuitLab

(The gate resistor depends whether you have any ringing problems with a given MOSFET.) However, for slow switching requirements, you'd be hard pressed to tell the difference between the 1-transistor (just level shiftier) and faster driver (i.e. with extra push-pull transistors):

enter image description here

Actually, if you also do level shifting, you can optimize away one of the transistors by using a twin-NPN totem pole instead of a push pull (but you need to add a diode):

schematic

simulate this circuit

There are also more sophisticated IC drivers that use custom-designed gate driving curves. These will usually reduce switching losses and prolong the life of the MOSFET. Some don't look much more than a transistor in size and connectivity requirements (4-pin devices) but basically have something like the above inside:

enter image description here

The catch? They cost more.

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Use a better MOSFET. The BUZ11 isn't specified to work as low as 3 volts: -

enter image description here

If you had 4.0 volts on the gate, a 2 amp motor (for instance) would probably drop about 0.3 volts and dissipate 1.2 watts. Not too bad but, to my mind the simplest solution is to get a better MOSFET like the SOT23 package AO3416: -

enter image description here

Volt drop looks about 0.1 volts to me and power dissipated would be about 0.4 watts. If this device is mounted on a small PCB with some PCB copper intentionally acting as a heatsink it should be good for a couple of amps.

But, as always, read the data sheets and think about stall currents and don't forget about the reverse diode across the motor to prevent back emfs destroying the MOSFET when the motor current is interrupted when the FET switches off.

If not this device then there will be others more than likely that will suit your application.

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  • \$\begingroup\$ That's good answer but is this suitable for bigger motors? And that schematic what I linked, will it work with PWM like RPM control for the motor? \$\endgroup\$ – LizNet Nov 16 '15 at 18:26
  • \$\begingroup\$ I would say not but there will be other devices that will work at greater currents. The circuit you linked will be OK but great at over a few kHz PWM. \$\endgroup\$ – Andy aka Nov 16 '15 at 18:29
  • \$\begingroup\$ I will try that schematic soon so can you suggest a good frequence? \$\endgroup\$ – LizNet Nov 16 '15 at 18:40
  • \$\begingroup\$ There's a 10kohm pull-up. This has to charge the gate capacitance in order to turn on the MOSFET. Look at the data sheet, find the gate capacitance and multiply it by 10k - this gives you a time duration and your operating frequency period should be at least to times this. For instance 1nF Cg and 10k gives t = 10us so go for ten times at 100us or 10kHz max. \$\endgroup\$ – Andy aka Nov 16 '15 at 20:06
  • \$\begingroup\$ I first used 10kHz but the motor felt a little "lazy", then switched to 1kHz and it worked better. Tried it also at 500Hz and it was best of all. Don't know is this healthy to the motor but it works \$\endgroup\$ – LizNet Nov 18 '15 at 19:49
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While the other answers are far better technically and more recommended on the long term, a cheap fast way to improve your circuit (so you can test it and decide what you need to do) is to use a smaller pullup resistor.

If you change R1 to say, 2K2~1K, you will notice a considerable improvement on turn on time (note that more current will flow through T1 also).

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  • \$\begingroup\$ I changed R1 to 1K and added also 10K trimmer so I can test and tune it. \$\endgroup\$ – LizNet Nov 18 '15 at 19:52

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