I am working on an art installation with high power LEDs.
But to get the basics to work I have setup a simple MOSFET LED circuit.
I seem to miss something or get it wrong.
The plan is ultimately to use an ESP32 with WLED to control MOSFETs that switch a high power circuit with 3W RGB LEDs.
To understand the basics I have setup a preliminary circuit with regular LEDs but I get strange behaviour and would love to ask for help.
For this I am using a Arduino Uno R3, One IRF740 MOSFET, a rotary potentiometer a lab power supply set to 6V and my Siglent Oscilloscope.
In the picture you'll see the scopes. One is at the source pin of the MOSFET and the other one at the Gate pin.
Both show the PWM signal. ?
As far as I understood it the IRF740 MOSFET should switch if a minimum of 4 V is applied to the Gate.
I know that the ESP32 can not put out a signal strength of above 3.3V but let's not discuss that at this time
Did I do a mistake in the circuit?
Do I have to pull down the Signal at Gate?
Or did I do a fundamental error and did not see it?
Welcome to the forum.
What is the odd behaviour ? Is that the voltage drop ?
Do you have a good multimeter. Then use the GND pin of the Arduino board as a ground, then measure everything: The 5V pin, the source pin, and so on.
I assume that the 5V pin of the Arduino is lower ? That is what happens when the USB power is used or when 6V is applied to VIN.
The DC barrel jack or VIN need at least 7.5V.
In the datasheet I see a mosfet that needs 9V at its gate (but 8.5V will also do):
thanks for the quick answers. I'll get right on it.
Regarding the odd behaviour:
If I increase the power supply on the Gate it lowers the voltage across drain & source pin.
If I increase the power on the drain & source (normally you would probably don't do that, I was just curious) it raises the gate voltage.
Gate voltage:
It is at 4.52V, I thought as well maybe it is not enough from the Arduino via USB. I'll make sure to beef it up. That though is anyway an issue as the ESP32 can only put out 3.3V. I need to get the MOSFET to switch fast enough for PWM to mix the LEDs.
In the final project turnout i want to use multiplexing to adress the 21 Light cubes with 3 Channels each. ... But basics first ;D
I'll attach a picture of the cubes that are to be lit up. (not in the arrangement that they will up, this was just a test stack-up).
@Koepel, maybe i got not the exact right datasheet. Same type though not the exact manufacturer. I'll re-check again and copy it in here as well.
Regarding setup:
Hard to see probably. I will clean it up make it new and post it so that my mistake or so hopefully becomes more obvious.
I did simulate it in everycircuit and it did work
First, from the looks of the picture you sent of the breadboard, The MOSFET is connected incorrectly. The legs of the MOSFET as it is physically laid out (tab to the rear) is GATE-DRAIN-SOURCE. It appears that you have it connected as you see it in the schematic.
Secondly, I would recommend a different MOSFET, one with a much lower Vgs(th). MOSFETs are not true on/off switches, they act more like a "dimmer" switch. The Gate threshold is the point where conduction starts, and the Rds will be relatively high. To use it as a true switch, you need to saturate it so that the Rds will be low (< 1 Ohm). In your case, you want to rapidly turn on and off the MOSFET in sync with the PWM output from the UNO. If you don't, the voltage drop caused by the MOSFET could throw off your output.
So i actually recorded two videos but finally found the solution. Though it bugs me of course that I partially don't yet grasp why it was / is not working.
It is now though. I changed out the MOSFET to a IRF3205 that has a GS (th)V of 2V.
Thank you for the help.
To better understand, I am looking into these questions:
Why is there a slope on the purple scope (probe is at the last LED's cathode) --> how do I influence the switching frequency?
How did I manage to break three (four? five?) MOSFETS... If they are mostly rated for 500V it could not be that and i never shorted them. The current flowing is also tiny.
Why did it not work before with the exact same parts, same layout etc.? Is it maybe just bad parts? (I bought a bunch of "Bojack 250 this and Aukenien 60 pices of that packages"
If you have any good pointers / reading, etc. i gladly dive into it to brush up my knowledge there.
Driving a mosfet is like driving a capacitor. The gate of a mosfet has a capacitance.
The IRF3205 has 4nF as input capacitance ! I think that I have not seen such a high value before.
You need a protection resistors to limit the current peak for the output pin of the Arduino board. That creates a low-pass filter with the gate capacitance.
There are special chips that blast a large current into the gate to be able to use the mosfet at a high frequency. They are called "mosfet drivers" or "gate drivers".
