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# Simple Linear MOSFET Dimming Circuit + PWM

#### 123splat

##### 0
It could be important to anybody researching and trying to learn, so it is a relative post, not a necro-post. quit playing moderator.
Good effort Illuminum. Keep it up.

#### Bionic-Badger

##### 0
The math works out in both power calculation equations -- which are the same thing, and must be -- as ~435mW, give or take some rounding error.

I'm curious how you arrived at 326mW though.

#### ped

##### 0
Necro 'ing is fine if it adds to the original discussion.

It's when posters put "Thats sooo c00l" and things like that, that it gets deleted.

#### hozone

##### 0
Sorry for this dumb question:
what if a power a 100mW diode led laser, using the 750mW proposed circuit?

thank you

#### upaa27

##### 0
Sorry for this dumb question:
what if a power a 100mW diode led laser, using the 750mW proposed circuit?

thank you

Your diode will pop. That's way too much current

#### hozone

##### 0
Ok, thank you!
So my diode will "suck" all the current i give him

#### Grumbl

##### 0
Hi,
my english is not so good and i am an electronic nooby so i want to ask if someone help me to change the circuit so that i can use it with the following parts:

9V power supply

Diode:
-Max Current 1400mA (so drive with 1300mA)
-Voltage drop at nominal I 4,45V
-Nominal power Output 1000mQ

I hope u can help me

#### lasersbee

##### 0
Hi,
my english is not so good and i am an electronic nooby so i want to ask if someone help me to change the circuit so that i can use it with the following parts:

9V power supply

Diode:
-Max Current 1400mA (so drive with 1300mA)
-Voltage drop at nominal I 4,45V
-Nominal power Output 1000mQ

I hope u can help me
Are you aware that this post is over 2 years old?
Perhaps if you introduce yourself in the Welcome
section you would get more responses.

Jerry

#### Sputnicker

##### 0
How much voltage should I supply to this circuit to feed an 5v 1000 chinese mW (also known as 100mW) 405nm laser? Just 5v + 1.2v overhead, or it can handle higher voltages?

Last edited:

#### Cyparagon

##### 0
6.2V would be the absolute minimum, assuming those are the actual values. 7V or 9V are common voltages and would also be ideal. The limiting component would be the FET. In the case of the suggested IRF520, 100V or 60W, whichever is less.

As with all linear drivers, the higher the voltage, the more waste heat is generated. 12V would also work, but your efficiency percentage drops to the 40s.

#### emilts

##### New member
Hi.

I am testing this circuit and it works well with PWM and with a digital signal with High and Low levels (digitalWrite of arduino), but I have problems to increase the frequency to which I want the led to work (infrared of 905nm). When the pulse width is 1msg, it almost no longer emits and at that point it goes off.

I have tested with these three FETs (IRF530, IRF520N and FQP50N06L) and the same thing happens and the voltage is 7.6v. In the following image I show the voltage wave on pin 13 output of ESP32 at 3.3v and the Gate pin of the FET (this voltage decreases with increasing frequency and the waveform is not square).

Thanks

#### rajhlinux

##### Member
When the pulse width is 1msg, it almost no longer emits and at that point it goes off.

1msg equates to what frequency?

I am contemplating if I should use OP's circuit discussion or use the tried and true LM317 + Mosfet.

Designing a low powered (50 mW) UV laser engraver for a CNC to make PCBs, needs around 5 to 30 KHz TTL/PWM for the laser diode.

#### WizardG

##### 0
LPF Site Supporter
Hi.

I am testing this circuit and it works well with PWM and with a digital signal with High and Low levels (digitalWrite of arduino), but I have problems to increase the frequency to which I want the led to work (infrared of 905nm). When the pulse width is 1msg, it almost no longer emits and at that point it goes off.

I have tested with these three FETs (IRF530, IRF520N and FQP50N06L) and the same thing happens and the voltage is 7.6v. In the following image I show the voltage wave on pin 13 output of ESP32 at 3.3v and the Gate pin of the FET (this voltage decreases with increasing frequency and the waveform is not square).

ThanksView attachment 63837
Your microcontroller can't deliver the current needed to drive a MOSFET gate at high frequency. There are MOSFET driver chips available. Some of these are rated for several amps of drive current and might eliminate the need for the discrete MOSFET altogether.

#### rajhlinux

##### Member
Your microcontroller can't deliver the current needed to drive a MOSFET gate at high frequency. There are MOSFET driver chips available. Some of these are rated for several amps of drive current and might eliminate the need for the discrete MOSFET altogether.

You think the IRLZ44N Mofset can be used by microcontroller to modulate the laser diode?

It is a logic level Mosfet, wondering if I could use it, so does this means OP’s circuit discussion of using two transistors is still required?

Thanks.

#### WizardG

##### 0
LPF Site Supporter
You think the IRLZ44N Mofset can be used by microcontroller to modulate the laser diode?

It is a logic level Mosfet, wondering if I could use it, so does this means OP’s circuit discussion of using two transistors is still required?

Thanks.
If you wanna do PWM with arduino then I'm suggesting scrapping the discrete MOSFET altogether and using something like this: https://www.digikey.com/en/products...tALjEpSkJOi2AgqyKtNCJCSLQVIWYXGIaAho9EALw_wcB

If you need lots of current you can keep the discrete MOSFET, but use a gate driver IC as a buffer between it and the microcontroller.

#### rajhlinux

##### Member

Thanks for the reply.

It is possible to use a gate driver IC to modulate the laser diode alone without the Mosfet?
If so, I did not know this could be done. Have any schematic on how it is implemented?

The laser diode I have is a 50mW UV 405nm, requires a high frequency range so that I can use it to sensitize PCB boards for high resolution.