Any way to control an N channel Mosfet by varying its gate connection to ground?

I've started playing around with the PWM LED driver ICs and wanted to control big 12v light groups using an N channel mosfet. The issue I ran into was that the led driver ICs don't supply the power to the leds but instead control how much varies the LEDs connection to ground on the exit end. This poses a problem as the N channel Mosfet i'm using needs to have 5v supplied to it's gate pin to open and close. If I were using shift registers, I could do it just fine but I wanted to have variable light intensity control and the LED driver ICs provided just that.

Is there a way to somehow wire the mosfet gate between 5v Ground and the LED IC pin to somehow make the current divert to the led IC when the IC opens up? Like maybe use the rule of the current flowing the path of lowers resistance? As in if the IC is fulyl closed, the path to the mosfet gate is the path of less resistance and thus the power flows there but as the LED IC pin opens up the 5v diverts there as it's now a less resistant path. This thoughts

The LED IC can control LEDs ranging from 5 to 20v max and the Mosfet opens fully when 5v is supplied to it's gate.

You could use another transistor as an inverter or you could use a P-type MOSFET on the high side and it will switch on when the gate is at ground.

TobiasRipper:
I've started playing around with the PWM LED driver ICs...

Which ones.

Most of them have a PWM/control pin.
No mosfet needed.

e.g. the common PT4115 has a "dim" input, that can directly connect to an Arduino PWM output.
Leo..

The driver IC's are low side (drain) power control. You feed power direct at the high side (source) and control how much current flows on the low side (drain) end. You do NOT need to control both source and drain.

The N channel MOSFETs are made to control low side.

I use 2N7777 MOSFETs (continuous power rating 200mA) to switch 12V led disks.

12V goes to one leg of the led disk, the other leg is wired to the DRAIN pin of the FET and the SOURCE pin connects to common ground, Arduino ground connects to common ground and Arduino pin connects directly to the GATE pin.

I believe that your driver ICs do the same job and likely have protection circuitry built in as well.

I feel that I haven't explained myself clear enough. I'm using the ws2811 IC. It can variably open up the ground side (Cathode side) the negative side of the LEDs by using built in PWM. So instead of controlling one LED I'm using the mosfet to control an array of big lights with the same precision that LEDs do. It takes 3 bytes of data to set the three LED values (usually for Red Green and Blue but i'm using it otherwise so one chip can control 3 light groups).

Anyways. I seem to have figured it out myself by connecting the N mosfets gate pin to 5v (which effectively opens it up completely) by using high rated resistor as well as connecting the gate pin to the IC directly. This way, as I suggested earlier, when the IC is closed, the current has no other way other than flow to the mosfet gate via the resistors but as soon as I send the data to the IC to open up the pin, the current flows there instead as it flows the path of lowers resistance. Works out in the end.

Post a diagram.
I think you are trying to use the three outputs of a WS2811 chip to drive three n-channel mosfets.
Should be possible, but 3Meg pull up is waaay too high.
That, and the unknown PWM frequency of the chip, will turn the mosfet on very slowly (= getting hot).
Drop the 3Meg to 1k.
The WS2811 datasheet states an absolute max Vout of 12volt (not 20volt).
Leo..

MOSFET gates work by voltage, not current. If voltage is present between the gate and source pins, the N FET conducts but not through the gate.

ws2811 leds are programmable. You feed them data on one line and power on another.

Are the big lights ws2811 clusters or ws2811 something?

If they're just lights then MOSFETs can switch them on and off, that's what I'm doing with my 36-led-junction disks.
Just wire the Arduino PWM pins to the gates and vary the PWM to change brightness.

The ws2811 chip is rated up to 20V but how much current? You do know that current rating and current use is important?

Wawa:
So you are trying to use the three outputs of a WS2811 chip to drive three n-channel mosfets.
Should be possible, but 3Meg pull up is waaay too high.
That, and the unknown PWM frequency of the chip, will turn the mosfet on very slowly (= getting hot).
Drop the 3Meg to 1k.
The WS2811 datasheet states an absolute max Vout of 12volt (not 20volt).
Leo..

Yeah I just realized it, I was trying to test while referencing the fill brightness of the lights so I was only testing fully on and fully off states. When I switched to testing gradual dimming, the 3m resistor, interestingly enough, made an actual ON OFF effect. So I toned it down to 240k and so far it's looking great. I tried a 1k resistor before and there was still voltage bleeding into the gate, thus partially opening it.

Result:

So to answer all your other questions. Yes I'm using standalone WS2811 chips (NOT ones packaged in with LEDs) to control the mosfets. Mainly because I'm using an Uno for this project and need about 80 different light clusters all with PWM ability. So obviously Uno does not provide enough pins and not all digital pins are PWM pins and I don't want to be going into software pwm. Also I wanted to figure outhe ws2811 chips so this was an ample opportunity and lastly by using the ws2811 this lighting system is very vastly expandable with very little additional effort.

TobiasRipper:
I tried a 1k resistor before and there was still current bleeding into the gate, thus partially opening it.

GoForSmoke:
MOSFET gates work by voltage, not current. If voltage is present between the gate and source pins, the N FET conducts but not through the gate.

Delta_G:

Sorry, this was posted before I read your answer.

TobiasRipper:
Yeah I just realized it, I was trying to test while referencing the fill brightness of the lights so I was only testing fully on and fully off states. When I switched to testing gradual dimming, the 3m resistor, interestingly enough, made an actual ON OFF effect. So I toned it down to 240k and so far it's looking great. I tried a 1k resistor before and there was still voltage bleeding into the gate, thus partially opening it.

You're supposed to switch FETs fully ON or OFF because otherwise you get a lot of heat and lost power. Might as well use BJTs.

GoForSmoke:
You're supposed to switch FETs fully ON or OFF because otherwise you get a lot of heat and lost power. Might as well use BJTs.

What are BJTs? Just the terms, I'll look into what it is.

The LEDs prevents the output of the chip going fully to ground.
Remove the LEDs, and replace with a 1k pull up resistor (or as low as 470ohm if powered from 5volt).

TobiasRipper:
I'm using an Uno for this project and need about 80 different light clusters all with PWM ability.

Check out the PCA9685 datasheet.
16 channels of 12-bit PWM.
Can drive mosfets directly (page 34).
Daisy-chainable to 60*16 channels.
Boards on Adafruit, and cheap clones on ebay.
leo..

Hi,

Mainly because I'm using an Uno for this project and need about 80 different light clusters all with PWM ability. So obviously Uno does not provide enough pins and not all digital pins are PWM pins and I don't want to be going into software pwm.

80 different light clusters, is that 80 different WS2811 or 80 different groups of LEDs.

How are you controlling the IC now, I don't think you need a UNO PWM pin, you can use a library to control them.

I think adafruit Neopixel library will help.

I could be wrong... but thats how I read it.

Tom... :slight_smile:

TobiasRipper:
What are BJTs? Just the terms, I'll look into what it is.

Try this link, great service for answering questions like that.

TomGeorge:
Hi,80 different light clusters, is that 80 different WS2811 or 80 different groups of LEDs.

How are you controlling the IC now, I don't think you need a UNO PWM pin, you can use a library to control them.

http://www.edn.com/design/led/4438797/Tips-and-tricks-for-driving-WS2811-LED-strips---Part-1

I think adafruit Neopixel library will help.

I could be wrong... but thats how I read it.

Tom... :slight_smile:

I never said I'm using Uno's pwm for anything, I'm using the IC's built in PWM to set the brightness of the LED (or in this case the Mosfet which in turn controls a light set). I am however using one of many neopixel (WS2811 based) libraries to control the IC. So it really wan't the question of communication with the ICs, I've already figured that out from the datasheet, it was the question of how do I control the Mosfet which needs voltage fed into it's gate to open when the LED ICs don't send voltage but receive it (since you connect the cathodes of the LEDs to the IC - the ground pins).
Anyways, I've already gotten a solution figured out with resistance paths, really interesting.

Delta_G:
Try this link, great service for answering questions like that.

Took you longer to make that link than writing "Bipolar Junction Transistor" so I could google that. Good thing I look at urls before I click on them.

Hi,
This how they are usually connected together, how do you want to do it?

OR;


Tom.... :slight_smile:

WS2811.pdf (316 KB)

Nickel tour. Don't touch the exhibits, you might zap the FETs.

BJT is bipolar junction transistor -- the 'original' production transistors in products of the 60's and since.
-------- uses current through the gate to open a bigger current through the collector to emitter.
-------- more efficient than tubes but still makes heat in proportion to current.
-------- can take a bit of abuse without failing
===== good for loads up to 1A

MOSFET is metal oxide semiconductor field effect transistor -- I first saw in the 80's.
-------- uses voltage on the gate to open a current flow through the drain to source.
-------- highly efficient when fully open or fully closed, efficiency drops otherwise.
-------- can't take abuse, easily destroyed by even small static, some are 20V max.
===== good for rated loads small to large.

At this point if you're a Trump type you can think you know all you need.
Or if you're sane you can take this as a brief shallow look on the subjects, less than intro level.