Strange behaviour on RGB Led and Arduino Pro Mini 3.3V

[xefil]

Hello,

I've got to work a RGB LED via Openhab/MQTT using 3 MOSFET.
But I've a strange problem. If I power the RGB LED only via 12V power adapter which is powering the Arduino itself as well via RAW pin, then the LEDS are very dim low in light. As soon I attach the USB via FT232 and set the output power of the FT232 on 5V (I've set it for mistrake) I get the leds in full power and light. If I set the FT232 to 3.3V (which should be the right setting 'cause it's an Arduino 3.3V), the leds become very dark.
If I disconnect the FT232 and use only the external power adapter, then they remain really dark as well. I can see the colors, but very poor.
The power adapter is a 12V 6A and now the led strip is only 1,8mt long, also the power drain should be low.
I've attached on the same radio the following adapters as well:

• radio
• DHT11
• Dallas DS18B20
  The two last sensors are, btw, not configured. They are powered using the VC pin on the arduino.

Ideas?

Thanks!

Simon

[AWI]

@xefil possibly the 3.3v is not enough to fully steer your mosfets. Can you share a circuit diagram?

[xefil]

@AWI , hello!
Thanks for your suggestion! Passing to an Arduino Pro MINI 5V version has solved the problem.
I've one question concerning the mosfet. I'm using a BUZ91A (600V - 8A). I don't have much knowledges on electronic, so, is it appropriate or should I consider different models? Does the 8A tell me only the limit or is it indicating a sort of gain the mosfet provides it to me?
Thanks!
Simon

[SiLeX]

I don't know this specific model, but I can recommend you to use TTL (logic level) types, which are to be used with 5V logic. I use the IRLZ34N with my 5V Arduino UNO and 12V common-Anode RGB Strip of 10 Meters without any issues ever since.

Always go and try to study the data sheet. You don't need to understand the graphs, but you really should understand the voltages between gate and source and how they work together.

[AWI]

@xefil Good to be of help. The BUZ91A is a good choice (one of the many). If you look at the data sheet you can see the Vgs which shows why less than 3 volts can give problems. And the Rds which show the resistance when switched on. The 0.8 Ohm value is rather high which means the Fet will dissipate (get hot) at high currents

Another type, like the BUZ71 do have a lower (0.1 Ohm) Rds the IRLZ34N mentioned by @SiLeX even has 0.035 Ohm Rds. So if the thing heats up to much these are alternatives.

[xefil]

Thanks a lot @AWI for the suggestions and explaination, even if it's a little difficult for me to understand.
I have to take some other for other LED strips. So I'll take IRLZ34N or BUZ71 instead my BUZ91A so they didn't get so much hot.
:-)

Simon