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Hi I am entirely new to the PWM concept. I have an AVR based ATMEGA16 microcontroller (https://robokits.co.in/control-boards/rhino-robot-control-board/rhino-robot-control-board-avr-based-with-quick-c-compiler) which I successfully tested by running DC motor having rating of 12 V, 7.5 A. I now want to use the same microcontroller for controlling brightness of 3 high power CREE LEDs whose ratings are 72 V and 2 A each. I tried connecting output of the microcontroller to a DC-DC converter (https://www.amazon.in/Matefield-Converter-Step-up-10V-60V-12V-80V/dp/B07KZSMHKJ) but it didn't work.

When I control brightness using DC power supply, the brightness varies properly (from 0 to 50K Lux) with voltage. I used a MOSFET and the brightness varied from only 18200 to 18900 Lux when duty cycle was varied from 0 to 100 %.

Kindly help me out. Thanks in advance.

What other components do I need so that the PWM signal can be amplified or boosted?

The 12 V supply is input to the microcontroller board I use. Didn't know how to add a microcontroller board in the schematic. So just drew a block to represent it.

schematic

simulate this circuit – Schematic created using CircuitLab

I was wondering if it is feasible to use a step-up transformer so that the 12 Volt PWM output of the microcontroller board is stepped up to 72 volt PWM which will be fed to the LEDs.

Santosh
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    Welcome to EE.SE! Please draw a schematic of what you have done so far. – winny Mar 28 '19 at 08:19
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    You should use optocoupler in your design as the voltage different between Atmega side and high brightness LED side is 14 times. It gives your fail-safe isolation and confidence in design. – Prasan Dutt Mar 28 '19 at 12:33
  • Added the circuit diagram. – Santosh Mar 28 '19 at 14:41
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    What kind of MOSFET are you using, and what is the speed of your PWM? – ConcernedHobbit Mar 28 '19 at 15:12
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    That 10k resistor is going to limit your maximum brightness quite a bit. If you are still seeing lots of light at 0% PWM, you must not be shutting off that MOSFET properly. – evildemonic Mar 28 '19 at 15:21
  • MOSFET used is IRF540PBF. http://in.element14.com/vishay/irf540pbf/mosfet-n-100v-28a-to-220/dp/1653661 – Santosh Mar 28 '19 at 15:23
  • So should I reduce to 1K resistor? – Santosh Mar 28 '19 at 15:25
  • 16MHz crystal for maximum speed is what is mentioned – Santosh Mar 28 '19 at 15:32
  • 16MHz is much too fast for this. Is that the speed you are running the PWM? What type of PWM board is this? – evildemonic Mar 28 '19 at 15:38
  • Is there any microcontroller that gives 72 V PWM pulses and over 6 A? I would then use that instead if it is available. I searched a lot on Google but did't get any. – Santosh Mar 28 '19 at 15:38
  • No there is no microcontroller that can handle that voltage or current.. What voltage is your PWM putting out? – evildemonic Mar 28 '19 at 15:40
  • https://robokits.co.in/control-boards/rhino-robot-control-board/rhino-robot-control-board-avr-based-with-quick-c-compiler is an AVR based one – Santosh Mar 28 '19 at 15:40
  • I give 12 VDC input and so I guess output is therefore 12 V PWM. I do not know if I give 24 VDC input the output will be 24 V PWM. – Santosh Mar 28 '19 at 15:41
  • There is not enough documentation on that site for me to understand this PWM hardware. – evildemonic Mar 28 '19 at 15:43
  • Oh!! I just used this as one student suggested it and I found the coding for duty cycle very easy. However I do not know anything else about microcontrollers and PWM – Santosh Mar 28 '19 at 15:45
  • Please see this pdf. Will it be of use? https://robokits.co.in/documentation/Rhino.pdf – Santosh Mar 28 '19 at 15:47
  • That is a horrible, sideways diagram. Make it horizontal and take a few minutes to draw and write it more neatly, or use the built in schematic editor to provide your schematic. When you do these diagrams, sources are left, loads are right, higher voltage up and lower voltage down, you got that right, but also try to position things so the purpose is clear, and choose which lines cross based on that. Put the LEDs, switch and resistor in series vertically so their purpose is obvious, which brings me to my point: It appears you have 10k\$\Omega\$ in series to the LEDs, so at 12V, 1.2mA max. – K H Mar 29 '19 at 00:02
  • I have added the schematic. I hope it makes more sense – Santosh Apr 06 '19 at 14:28
  • The empty box represents the PWM microcontroller board. Don't know how to show it in the schematic. – Santosh May 01 '19 at 17:00

1 Answers1

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To start, check out

  1. this question, with answers explaining using a MOSFET as a switch.
  2. This question, with answers about using a MOSFET to drive LEDs.
  3. And this question, with answers discussing PWM for servos (basically the same concept).

For a better answer, you need to specify:

  1. What MCU pin or driver board output you are trying to use.
  2. What you are doing in code to configure the hardware & drive the LED
  3. What level electrical knowledge you actually possess. From the comments above, it doesn't seem like you have any idea what you are doing.

Hardware

In your schematic, you are driving a 72V, 2A LED from a 72V power source. When the FET is ON, the current is going through the LED, through the FET, and through R2 to ground. So, how much voltage is dropping across R2 during this time? I honestly don't know, since I don't have the data sheet for these LEDs, but I imagine it's not very much since the LED wants such a high voltage to turn on. If it were 1V, that's only 0.0001 A going through the LED.

Also, since FETs are voltage controlled, not current controlled like BJTs, you don't need a current limiting resistor going to the gate (R1). You can put one there for basic isolation, but it should be much higher (1k ~ 10k). You DO need a pull-down resistor pulled to ground at the gate to keep the FET OFF when the signal at the gate is low. This should also be (1k ~ 10k). If you do use a resistor between your driver and the gate, this pull-down should be on the driver side so you don't create a voltage divider at the gate.

I sincerely hope you are NOT doing this on the output of the motor driver. Those are pins designed to drive current to a motor, not what you are trying to do. If this is the case, you've likely destroyed the MOSTFET gate with the excessively high current output. All you need is a basic MCU output pin and a "logic-level" MOSFET, of which the FET linked in one of the comments is (meaning it can "turn on" at a lower logic level - look for the "gate-source threshold voltage" or Vgs parameter in the datasheet).

Kurt E. Clothier
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