IoT fan speed control with PWM and IRF830

Thread Starter

rkrasowski

Joined Jul 10, 2012
10
Simple project, I am trying to control the speed of the fan using PWM. The fan is 12V, 1.5A at max speed. I am using IRF830 to control it as per project from:
https://www.digikey.com/en/maker/pr...raspberry-pi/c5061a5c6cf646b69a2ff6d698298422
The problem: I am not able to achieve full fan speed, the max voltage that I measure with the fan connected is about 6V not 12V as expected, without the load fan) it is 12V. Also, without IRF830 controlling unit current, is 1.5A, with the unit drops to 0.8A and again, speed is only half what should be.
What should be a solution?
Thanks in advance
Robert
 

AlbertHall

Joined Jun 4, 2014
11,530
The circuit should work so if it doesn't then I suspect that you have something connected incorrectly. Check the datasheets for the transistor and MOSFET pinout.
 

Ian0

Joined Aug 7, 2020
3,511
IRF830 is a poor choice of MOSFET, because Rds(on) is 1.5 ohm. That means you will lose 2.25V at 1.5A, and dissipate 3.3W. I suspect it is running rather warm, which would mean the Rds(on) is even higher.
Choose a more suitable MOSFET (you don't need a 500V voltage rating!) with low Rds(on) and a voltage rating of about 30V.
 

Thread Starter

rkrasowski

Joined Jul 10, 2012
10
Aha, now I understand, and yes, it was getting very hot. I ordered some sets of MOSFETs, will get it tomorrow. The best I can see is IRF3205 with Rds(on) of 8 mOhms, another one is IRFZ44N with Rds(on) of 17.5 mOhms.
Thanks for your help, will post here the results of the testing. I will test 9 different MOSFETS.
 

Ian0

Joined Aug 7, 2020
3,511
A TO220 package standing up with no heatsink will dissipate 1W.
Design for half that. So, 0.22 ohm will do the job.
If you buy a MOSFET with a REALLY low Rds, then it will have a high gate capacitance, and your drive circuit will struggle to switch it.
 

Thread Starter

rkrasowski

Joined Jul 10, 2012
10
I will get IRF 520N with Rds of 0.2 ohms, will try that as well. Thanks again
Ok, yes, that was the problem, too high Rds. I tried IRFZ44N (Rds only 17.5 mOhms) - I get the highest speed of the fan and the current is 1.4A. With IRF520N (RDs 0.2 Ohms), the slightly slower speed with amperage of 1.38A. Now is the time to play with speed control using PWM, so far I did not notice any difference between IRFZ44N and IRF520N. Will do more test.
Thanks so much for the help
Robert
 

Ian0

Joined Aug 7, 2020
3,511
so far I did not notice any difference between IRFZ44N and IRF520N. Will do more test.
Won't make a lot of difference unless your pwm frequency is high, say >10kHz.
Then you might get a slow turn-on of the IRFZ44 which would lose its advantage in Rds(on)
 
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