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I'M trying to use MOSFET as a driver for my 10W LED with 12V supply. This LED will be used as a transmitter with signal coming from Arduino nano. I'm using IRFZ44N MOSFET with a Vgs(min)=2V and Vgs(max)=4V. The problem is whenever I give the transmitting signal to gate of the MOSFET it doesnt turn ON to the fullest. The voltage drop across LED would be 6V and across Vds its 6V. When I checked the drop across Vgs the DMM reads only 2V. As of what I read the DMM averages the value for such signals but that should mean the drop across Vgs is aprox. 5v and hence turning on the MOSFET but it doesnt turn ON. Therefore, I tried giving the blink program's 1 second ON and 1 second OFF signal to the gate. What I noticed was the MOSFET turned ON completely with very less Vds and max. voltage drop across the LED but the same isn't true for higher frequency. I would like to know how to drive the mosfet with very high frequency like PWM.

LED DRIVER

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  • \$\begingroup\$ We need to know a bit more about that "LED" before you get a good answer. If it's really a "12V LED bulb" with built in current controller, Andy's answer is fine. Otherwise you're actually looking for a PWM-able current source, which adds at least one resistor to Andy's cct. \$\endgroup\$ Jun 3 '20 at 12:15
  • \$\begingroup\$ It's a regular 10W 12V COB LED. It's working just fine with current limiting resistor but when I use the mosfet to drive the LED only a part of it glows signifying that it is not being turned ON or maybe I'm wrong somewhere. I want to use it with its full intensity and also be able to turn ON and OFF at the same time to transmit data over a few feets. \$\endgroup\$
    – SurajB
    Jun 3 '20 at 12:52
  • \$\begingroup\$ The fact it has current limiting for 12V and it's not just a LED was unclear in the question. Andy's answer is fine. \$\endgroup\$ Jun 3 '20 at 12:58
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Summary of solution

  1. The 10 k series resistor will restrict the ability to drive sufficient gate voltage to most MOSFETs - this should be replaced by (say) 10 ohm or 100 ohm)
  2. This will also improve the high frequency activation of the MOSFET due to the low drive impedance charging gate capacitance up a lot quicker.
  3. The LEDs will look dimmer at a high frequency 50:50 drive compared to being switched on at a slow rate due to the persistence of the retina
  4. Using a DVM to measure 6 volts across the LEDs indicates that at a high frequency, the actual wave form is a fast on-off 12 volt signal that of course, averages to 6 volts. Ditto the voltage across the MOSFET.

Picture originally posted by OP: -

enter image description here

For a start the series 10 k resistor that connects to the Nano's output can be reduced to circa 100 ohms. At the moment it halves the drive voltage from the Nano and this will mean poor performance from the MOSFET in terms of switch on resistance.

If the Nano can produce 5 volts logic drive then you should be OK with the IRFZ44N: -

enter image description here

At higher frequencies, gate-source capacitance will degrade the signal seen at the gate if the driving current isn't sufficient. The gate source input capacitance is nearly 1.5 nF and with an effective 5 kohm source, this forms an RC low pass filter of cut-off 21.2 kHz. Significantly higher drive frequencies will turn into a mushy DC level at the gate. Try removing the series 10 kohm resistor and replacing it with 100 ohms for a start.

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  • \$\begingroup\$ Since I didn't have 100 ohm resistor, I connected 330 ohm in parallel with 10k. The voltage still read by DMM is 1.9~2V i.e. Vgs and the LED voltage drop remained same as before. Yes Nano does produce ~5V logic which also remained same during a slow one second ON and one second OFF. But as soon as I increase frequency the voltage drops, as read by DMM, to ~1.9-2V. \$\endgroup\$
    – SurajB
    Jun 3 '20 at 9:18
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    \$\begingroup\$ Measure the voltage output from the Nano with the circuit still connected - do the same measurement but on the Nano side of the 10 k ohm resistor please. \$\endgroup\$
    – Andy aka
    Jun 3 '20 at 9:20
  • \$\begingroup\$ ~5V for a slow one second ON and one second OFF. ~1.9-2V for a higher frequency signal as read by DMM. \$\endgroup\$
    – SurajB
    Jun 3 '20 at 9:26
  • \$\begingroup\$ OK, then the Nano cannot produce enough drive to the MOSFET's gate capacitance - what was the higher frequency? \$\endgroup\$
    – Andy aka
    Jun 3 '20 at 9:28
  • \$\begingroup\$ 1kHz pwm signal. \$\endgroup\$
    – SurajB
    Jun 3 '20 at 9:31
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List of things that I got wrong and how I overcame.

  1. So, I was wrong with circuit where the series 10k ohm resistor would act as voltage divider. So, removed the 10k ohm resistor connected to arduino but the pull down resistor is necessay. Thanks to @Andy aka.

  2. High frequency signals wont affect the dimming of the signal but its duty cycle will.

  3. The mosfet which I used was not a logical mosfet and hence it wouldn't turn ON properly. A logical mosfet like IRLZ44N did the trick.

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  • \$\begingroup\$ Listen me @surajB - you originally marked my answer as correct and now your summary of my answer you have decided to appoint as the correct answer. So, I'm going to get on your case about this and point out that if you want future help on this site you should show some respect to the people helping you. In no way, shape or form will anyone coming across your "correct" answer see it that way. For instance, point 1 I explained to you in my answer. Point 2 was delivered in comments/discussion under my answer.... \$\endgroup\$
    – Andy aka
    Jun 5 '20 at 13:03
  • \$\begingroup\$ ... Point 3 (the use of an IRFZ44) was mentioned in your question right from the start so, your point 3 is invalid. On this basis I have to downvote your poor answer and remind you that help you receive should be properly rewarded. If there was something about my answer that didn't deliver the goods please discuss it. Right now I'm disappointed in you. \$\endgroup\$
    – Andy aka
    Jun 5 '20 at 13:04
  • \$\begingroup\$ Well, Andy aka I'm extremely thankful to you for the amount of knowledge you have imparted on me. Let me clear things as:- 1st. I marked my answer as correct so that anyone who's looking for a similar question would find the solution for the exact same problem as mine. 2nd. I have thanked you in my first point in the answers. 3rd. Coming to point 2 we only talked about frequency therefore I had to mention it. \$\endgroup\$
    – SurajB
    Jun 5 '20 at 13:40
  • \$\begingroup\$ 4th As I mentioned earlier, to help anyone with similar question. I had to mention it as a solution . I do agree that it was my mistake and that's why I have mentioned in my answers as "List of things that I got wrong and how I overcame" stressing wrong again. \$\endgroup\$
    – SurajB
    Jun 5 '20 at 13:41
  • \$\begingroup\$ The solution was covered in my answer (and comments) and you are still disappointing me. If you wished my answer to cover a summary at the top, you should say so and I would have done this. "Thanks" on this site is applied by up-votes and formal question acceptance. That is how "thanks" is given on this site. I see what you have done as ingratitude. \$\endgroup\$
    – Andy aka
    Jun 5 '20 at 13:47

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