EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: elcrni on November 08, 2023, 08:46:18 am
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hi all,
I have a small project and trying to drive different source LEDs with MAX7219 chip and external MOSFETs. I am not new to MAX7219 but although i have used MOSFETs in the past, i still have some questions.
So, the schematic below shows what i want to achieve:
1. External MCU has LED outputs and i want to be able to turn ON/OFF those LEDs with MAX7129 connected to main/different MCU. So, the signal (SEG_A (common cathode btw)) activates the MOSFET and then LED- is connected to MAX7219 GND thus turning the LED ON. I can then dimm it with MAX library brightness option, turn it off etc...
2. This option uses led strip or strip section, powered with external 12V, so the same routine, 12V power supply GND connects to main system GND, +12V connects to LED, MAX7219 SEG_B activates the MOSFET which then connects LED- side to DIG_2 (common GND in MAX chip). Again, i can dimm the led, turn it off/on using the MAX LED address.
My question here is, do i need any resistors on the signal side of the MOSFET coming from MAX chip? on my previous projects (not involving MAX7219) i always had a pulldown resistor connected between PIN signal and the MOSFET gate, do i need one now as well or the MAX chip already has me covered?
This schematic works! but what i've learned over the years in electronics hobby is just the fact that something works doesnt mean it's been set up correctly, so the last question is, is my setup correct?
Many thanks for all your help in advance,
Alek
P.S. MOSFET Used is irlml6246trpbf link to datasheet is https://eu.mouser.com/datasheet/2/196/Infineon_IRLML6246_DataSheet_v01_01_EN-3363535.pdf (https://eu.mouser.com/datasheet/2/196/Infineon_IRLML6246_DataSheet_v01_01_EN-3363535.pdf)
(https://i.ibb.co/N3LtByh/MAX7219.png) (https://ibb.co/Qc6pwKB)
de duplicate (https://dedupelist.com/)
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Please dont link to dodgy image hosting websites.
The LEDs appear to upside down.
For low speed switching you don't need a gate resistor. You do need a "logic level compatible" MOSFET, the one you have chosen will work well. An NPN will work fine too.
eg MBT3904WT1G The available MCU IO current varies from one MCU type to another. It's not smart to allow the load(s) to take it to the limit so a series resistor is a wise addition.
Read your MCU datasheet.
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Please dont link to dodgy image hosting websites.
The LEDs appear to upside down.
For low speed switching you don't need a gate resistor. You do need a "logic level compatible" MOSFET, the one you have chosen will work well. An NPN will work fine too.
eg MBT3904WT1G The available MCU IO current varies from one MCU type to another. It's not smart to allow the load(s) to take it to the limit so a series resistor is a wise addition.
Read your MCU datasheet.
Many thanks for your reply, somehow i missed it.
I am using ImgBB for image hosting, should be well known and reliable... not sure how you got it as dodgy
Yes, my diodes are upside down, did a quick mockup so made a mistake.
now, for a series resistor, i am not sure i follow where should i add it and what value?
Many thanks,
Alek
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I do not like the MAX7219 very much. First, it is far to expensive for what it does (which is common for MAXIM IC's), unless you use the Chinese clones. Further, I do not like it's very limited dimming capability (In just a few lineair steps).
It is a usable IC if you use it as intended, but If you start adding all the surrounding parts such as in your circuit, it's usefulness is reduced a lot. There are plenty of other LED driver IC's around, and you can even replace it with a dedicated microcontroller for driving the LED's. Maybe even add some 20ct shift registers.