I am using this BS170 MOSFET to drive a fan using a pwm signal generated by a 3.3V MCU according to the following schematics:

enter image description here

(I know about the wrong symbol it's just for the packaging on the PCB)

So my gate gets connected to ground using the pulldown resistor and to the PWM signal.

I have two questions:

  1. Should i still use the D1 diode for back EMF protection or i can relay on the internal one in my transistor.
  2. Is my transistor is suitable for a fan using 0.32A. It should handle 0.5A so it should work right?
  • \$\begingroup\$ You can draw schematics with the tool, learn how to use proper punctuation and grammar. \$\endgroup\$
    – Voltage Spike
    Jan 17, 2017 at 23:08
  • 3
    \$\begingroup\$ i pulled the schematics from a bigger design am really sorry about that. About grammer and punctuation am really sorry not beeing good at them but english is not my mother tang. \$\endgroup\$ Jan 17, 2017 at 23:13
  • \$\begingroup\$ The body diode doesn't protect from back emf.diode has to be there.I feel. \$\endgroup\$
    – User323693
    Jan 18, 2017 at 0:49

4 Answers 4


For your first question, yes keep the diode - if it isn't needed because of something special about the fan, it will do no harm. If it is needed then leaving it out will damage the reliability of your circuit. The diode in your MOSFET is not useful to do the same thing.

For the second question, personally, I wouldn't use this MOSFET for this job - it's a 'small signal' MOSFET, and you would almost certainly be much better with a power MOSFET of some kind:

  • The Rdson is too high - at 0.32A, you'd be dissipating 0.32*0.32*5 = 512mW which is more than the device is rated to dissipate. And that's before you start to think about switching losses caused by your PWM switching.

  • There are no characteristic graphs for Vgs as low as 3.3V - although you're above the threshold at 3.3V, you're clearly not operating the device in the way the designers intended.

Additionally you should almost always add a resistor in series with the gate of a MOSFET, to control the turn on speed/gate current.

Here's a post about choosing a FET: Selecting a MOSFET for driving load from logic

  • \$\begingroup\$ using a heat sink won't improve the situation ? \$\endgroup\$ Jan 17, 2017 at 23:30
  • \$\begingroup\$ I'm confused why you would want to put a resistor in series with the gate of a MOSFET. \$\endgroup\$
    – Bradman175
    Jan 17, 2017 at 23:30
  • \$\begingroup\$ it's a pulldown resistor to ensure that if the gate is unconnected it's pulled to 0 V \$\endgroup\$ Jan 17, 2017 at 23:34
  • \$\begingroup\$ @AhmadElbadri Sorry I wasn't referring to you. That 10k pulldown is correct. \$\endgroup\$
    – Bradman175
    Jan 17, 2017 at 23:36
  • \$\begingroup\$ @AhmadElbadri Using a heatsink won't make 512mW less than 500mW, which is the abs-max for the device. It's not sensible to use an unsuitable part and then try and keep it going with a heatsink. Just use something more suitable. \$\endgroup\$
    – user1844
    Jan 18, 2017 at 8:36

1) Depends on the fan, if it has inductance (A DC brushed motor) then you may want some kind of over voltage protection like D1. If its a brushless DC motor (like a PC fan) then it already has a circuit built in for overvoltage protection or it doesn't need it.

2) It says 500mA right in the datasheet, your fan can't be more than this. A 320mA fan will be fine. The mosfet also has a 5Ω RdsOn which means its like a 5Ω resistor when its on. At 320mV this will cause the voltage of the fan to be 1.6V, a fan with a controller may or may not be able to tolerate this.

  • \$\begingroup\$ 5 ohm Rdson would give a 1.6V drop @ 320mA, which would leave 10.4V for the motor. \$\endgroup\$
    – user1844
    Jan 17, 2017 at 23:11
  • \$\begingroup\$ should that be ok for a PC fan or the 1.6V drop won't let it on ? considering that am using it in PWM ?? \$\endgroup\$ Jan 17, 2017 at 23:15
  • \$\begingroup\$ A controller in BLDC fans may not like to run 1.6V above ground. If the current is ramping up\fluctuating in a starting condition, that 1.6V will also change, the controller may not like that. Better to go with a switch with lower resistance or at least be aware of it, depends on if this is for hobbyist work or a product. \$\endgroup\$
    – Voltage Spike
    Jan 17, 2017 at 23:16
  • \$\begingroup\$ Try it, see if it works. \$\endgroup\$
    – Voltage Spike
    Jan 17, 2017 at 23:19
  • 1
    \$\begingroup\$ How would the fan know it's 1.6V 'above ground'? What would be its reference for this 'ground'? \$\endgroup\$
    – user1844
    Jan 17, 2017 at 23:21

For the first question, I believe depending if your using a threshold voltage to manage the power to the fans, a diode will be useful.

For the second question, If I understand the question correctly, you want to manage the current to the fan to prevent overloading to the first Fan or the second actually. Seemingly, if the circuit is trying to maintain two fans that require different amounts of power, I suggest testing that Fan 1 can operate until a certain threshold is met and when the Fan 2 is needed to start.

Overall, according to the specifications, the maximum threshold voltage is 3.0V, so I would work around the voltage to determine the amount of current to each fan. Consider adding/removing resistors based on the 3.0v threshold.

  • \$\begingroup\$ thanks for the first answer in the second point i was asking simply if my fan is consuming 0.32A, would this transistor be sutiable ? \$\endgroup\$ Jan 17, 2017 at 23:33
  • \$\begingroup\$ I believe so, yes, as long as the circuit does not exceed 0.5A. \$\endgroup\$
    – JDavila
    Jan 18, 2017 at 18:11

Normally a low Vce(sat) saturated BJT needs 3% to 10% base current. Considering the fan is a BLDC motor the current is commutating and aliasing effects can be had with PWM. A 1 Ohm MOSTFET also works but costs more.

These things need to be tested to verify the assumptions.

When I tried this approach I was not satisfied with the results and went with a 3 terminal ADJ regulator with a thermistor controlled voltage to adjust the fan speed for smooth quiet operation with pot selected setpoint and 5'C range from 0 to 100% speed. ( all for under $2)

This Infinion BSS806N H6327 would be a better Nch FET choice for low 57mΩ RdsOn and low cost.


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