tdg8934

I have never really used FETs before but finding out slowly how to use them. At least P-Ch FETs to allow more current for some LED Matrix modules.

I currently understand that P Channel FETs allow a positive voltage (e.g. 5vdc or 0VDC) into it's Gate (G) from a microcontroller output. The Source is connected to 5VDC and the Drain is connected to a current limiting resistor and then to an LED Anode or series of LED Anodes. If I send 0VDC (GND) from the microcontroller output to the Gate of the P-CH FET then the LED turns on. Likewise if I send 5VDC from the microcontroller output to the Gate of the P-CH FET, then the LED or LEDs turn off. This works fine.

However, when using an N-CH FET, I would assume just the opposite is true but it is not because I am missing something due to never using them before.

Is it not possible to control the LED Cathode using an N-CH FET in the same or similar fashion?

The reason I ask is that I currently have a Parallax SX-28 microcontroller (similar to a PIC). On the 8 RB outputs I have it going to 8 P-CH FET Gates and then it's drains going to 8 LED Anodes of an 8x8 LED Matrix module. The 8 sources are all tied to 5VDC. The Cathodes I currently have running back to the 8 outputs on RC.

I wrote a program that the P-FET Anode rows are scanning and the data is being pushed out into RC (Cathode Columns). All is working well with this setup.

If I swap RB with RC in the code, the P-FETs anodes are no longer scanning the rows so that data is moving across the columns AND the message scrolling the other way is not as bright because it is not using the P-FETs effectively. I would like to scroll data from top to bottom as well as from side to side. This tells me that I will need some FET's of some sort between RC and the Columns.

I thought I could simply use N-FETs like I did P-FETs but I get nothing. However, after reading a little bit more I think they are only used in combination with P-FETs for using higher voltages like 12V (TTL or something greater than 5V).

Can anyone confirm this. ?

I suppose I can use UDN2803's between RC and the cathode columns but I would like the option of having full current draw like I have with the P-FETs.

Is this possible or how can it be done?

Thanks for your help.

Tim

PS: See attached SX-28 working code (SX Basic language - but I changed the .sxb extention to .txt so you can view it and also to use it as an attachment).

Attached Files

8x8_LEDTest.txt (6.3 KB, 19 views)

tdg8934

I beleive I have this figured out. I needed to connect the Sources to GND and then the Drains to the current limiting resistors on the Cathode Columns. Also needed to change some 0's to 1's in the code.

Thanks for listening.

audioguru

Ordinary Mosfets need 10V from gate to source to completely turn on.
"Logic level" Mosfets need only 4.5V or more.

__________________
Uncle $crooge

Hero999

It also depends on what the load draws, a MOSFET will pass lower currents if it's below the fully on voltage - look at the transfer curve on the datasheet.

__________________
What's so bad about Microsoft?

Get Opera it's simply a superb browser.