PWM control of a water pump with a MOSFET – can’t find the errror

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I have a 5V water pump and a 5V/2.4A power source. These work together. Now I want to control it with a PWM signal from an STM32 board using a FQP30N06L N-Channel MOSFET and that doesn’t work – nothing happens, the PWM signal goes out but the pump doesn’t seem to get the power and doesn’t turn on.

This is my circuit:

The diode looks dumb but it’s the only one I could find rated over 2A.

I would very much appreciate help in finding the error.

mosfetpwmShareCiteEditFollowFlagedited 12 hours agoasked 13 hours agoretiredgoblin3122 bronze badges New contributor

• 4Is your 5V supply Ground common with the SMT32 ground? Is it a 5V PWM signal (the MOSFET is only specified for 5V drive)? Is the PWM frequency low? (you don’t have a gate driver) – Spehro Pefhany 12 hours ago
• 2Need more info. What pump are you using, what does the PWM signal look like, etc. Also try connecting the PWM input to just 5V – does it function as expected for 100% duty cycle? – InBedded16 12 hours ago
• 1The ground is common, the PWM signal is set at 100% duty so 3.3V, the frequency is 1000Hz (should it be more/less?) – retiredgoblin 12 hours ago
• 1You should get something with 1000Hz. 3.3V is low for that MOSFET but I suspect you would see more like the MOSFET overheating than nothing at all. Is this connection actually solid (looks dodgy). – Spehro Pefhany 12 hours ago
• 1@SpehroPefhany Yeah, I just checked it again, it’s solid. What do you mean it’s low though? – retiredgoblin 12 hours ago
• 2Is the Vgsth low enough on the MOSFET to be fully on at 3.3 V? If not, choose another MOSFET. – winny 12 hours ago
• 1Okay, so if this is a wrong transistor for the job, could I ask for recommendation of a model that is actually fully open at 3.3V and will work in this circuit? Or can I use a voltage amplifier powered by the 5V that will amplify the 3.3V PWM signal? Would that make sense? – retiredgoblin 12 hours ago 
• 1You should use a MOSFET with Rds(on) specified at 3.3V or less in this application. Or you could use a gate driver IC that can accept 3.3V input and provide 5V at the MOSFET gate (Microchip and others make good inexpensive ones). – Spehro Pefhany 12 hours ago
• 2Measure the actual voltage on the gate relative to the source with a multimeter. If it is 3.3 then remove the PWM input connection and connect the gate to +5 (you can put a 1K or whatever in series) and see what happens. In the latter case, the pump should definitely run. – Spehro Pefhany 12 hours ago
• 1The MOSFET appears to be OK for the job. – Andy aka 12 hours ago
• 3Also, make sure your pump contains a BRUSHED DC motor. If it is a brushless one, then you can’t use PWM to control your pump (just turning on or off a few times per minute is OK) – Sandro 11 hours ago
• 1If you slow down the the PWM to say 0.1 Hz, what happens? – winny 10 hours ago
• Thanks everyone for trying to help, still no progress sadly. I rewired everything again and still no changes. I measured the output voltage from the transistor – 5V as should be (at 100% duty cycle), when I connected the pump, the voltage drop goes to like 3.8 mV. I can’t measure the current. I will try the other suggestions tomorrow. – retiredgoblin 9 hours ago
• (1) First thing first I would check the Vgs(th) of your FQP30N06L N-Channel Power MOSFET. Datasheet says the following: On Characteristics: VGS(th) Gate Threshold Voltage (Testing Condition: VDS = VGS, ID = 250 µA0) min =1.0V, max = 2.5V, (2) Ref: FQP30N06L 60V LOGIC N-Channel MOSFET Datasheet – Fairchild cdn.sparkfun.com/datasheets/Components/General/FQP30N06L.pdf, (3) Your power MOSFET is a “LOGIC” type one, I think it means that the Vgs(th) is of 3V/5V logic level. – tlfong01 1 hour ago    
• (4) So if your STM32’s PWM logic level is 3V3, it should work. You remind me once testing a similar power MOSFET IRF540N with Vgs(th) max = 4V and it didn’t work. So I tried the logic level trigger IRL540N with Vgs(th) max = 2V and everything works smoothly. (5) So I bought two lots of IRL540N, one lot three times more expensive that the other, then I found the expensive lot works OK, but all 20 something ones of the cheap guys did not work. I visually inspected the marking of the bad guys and found them being seemingly remarked, possibly IRF remarked as IRL. – tlfong01 1 hour ago    
• (6) Your pump of 5V power might have a problem. I would recommend a 12V pump, because your power MOSFET might not like to work under the hobbyist voltage of 5V. (7) You might like to, for trial and error, over drive your Vcc = 5V pump with 12V power, just for a couple of seconds, (so not heating up the motor and fry it). – tlfong01 1 hour ago    
• (8) Your diode HER308 is a rectifier, datasheet says “high speed”, but nowhere says it can be used as a flyback. I would recommend the popular signal type 1N4008, with “high forward surge current” or even better, Shottky one for flyback. – tlfong01 58 mins ago    
• (9) Your 3/5V PWM signal output resistance should be much much lower than 10k (470R should do), otherwise it would be voltage divided much too lower to switch on the power MOSFET. Suggestion: just remove the Rgs 10k. – tlfong01 48 mins ago    

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