# Fan switching circuit, which is better? [closed]

Beginner electronics hobbyist here. I was browsing through some test questions on circuit design and came across this one which asks which fan switching circuit is better, and why.

I'm leaning towards option A, because I can see a Schottky diode in there, but I'm not one hundred percent. Is someone able to explain these circuits a bit better to me and point me in a direction to learn more about these?

• Consider redrawing the schematics using the schematic tool - the included image is very difficult to read. Why does the presence of a Schottky diode cause you to lean toward the circuit containing it? Commented Sep 7, 2021 at 21:15
• What fan are we referring to? Commented Sep 7, 2021 at 21:16
• JYelton, I was actually thinking about redrawing them to help me learn and I also found them hard to read (possibly part of the source of my confusion). I'm leaning toward the schottky diode one because if i understand correctly, it will protect the fan against reverse polarity, and has a lower forward voltage (voltage drop). Is this on the right path? Commented Sep 7, 2021 at 21:29
• Modern computer fans have PWM input. Do you need a 2-pin fan with no PWM input? Commented Sep 7, 2021 at 21:48
• @Justme I don't see where he's talking about computer fans. This looks like just a simple on/off circuit for a fan. Commented Sep 7, 2021 at 22:22

##### About schematic A:
• It is more complete.
• It's easer to read.
• Component labelling is better, except for R101/R104.
• It contains a resistor (12kΩ) to hold the gate voltage low in the absence of an input connection.
• Resistor R101 (10kΩ) is way too large. It should probably be more like 100Ω. I would agree with 10kΩ if this were a biploar junction transistor. If the circuit is simply switching a fan on or off every few seconds, then this resistor isn't necessary at all.
• I would question the use of a schottkey diode. A regular rectifier diode would be fine.
• If such a thing is useful, it contains a correctly implemented indicator LED to show the switch state.
• It's really difficult to find information on the ES3400 MOSFET. I can't tell if it's appropriate in this application.
##### For schematic B:

I believe half the peripheral components are missing from this cutout, but in the spirit of "what you see is what you get":

• It's very difficult to read. I had to spend a lot of time to figure out where things like the fan should go.
• There's nothing to prevent the gate from floating.
• The MOSFET was easy to research, and well suited.
• I have no idea what SDDET means, but that LED has no companion resistor.
• There's no diode to protect the MOSFET (and everything else) from the motor. Perhaps that's what the LED does, there's no way to tell.
##### Verdict

The winner, for me, is A. At least I know it will work.

By the way, in response to your comment about the diode protecting the fan from reverse polarity, I have this to say: that diode is not to protect the fan/motor - it's to protect everything else from the fan/motor.

• Thanks heaps Simon, i've been doing some more research as i come across news things, and these points all make sense to me. Thank you for correcting me on the schottky, i knew what i meant to say but it came out backward. It's definitely not the clearest schematics i've found, but i figured if i can make sense of these, then proper ones should be a breeze! Commented Sep 8, 2021 at 2:32
• You're welcome. I wasn't pointing out any error on your part, the SS54 is actually a schottkey device. And to be clear, it would work well here. It's just that the chief defining characteristic of schottkey diodes is low forward voltage, which is irrelevant in this application. Commented Sep 8, 2021 at 2:40

Considering any computer 12V fan of any size, Option A is ideal for low side fan switch with "logic level FET that works from your Vdd level and RdsOn is < 1% of Fan Req=[V]^2/[W] to run cool. ... Unless you want to control speed..

• assuming the low current and inductance of the BLDC fan which has FETs, Hall sensors, IC and a Cap, any 100mA diode will do. Si or Sch,
• Thanks Tony, that's given me a few things to research. Commented Sep 7, 2021 at 22:07