Arduino Forum

Using Arduino => LEDs and Multiplexing => Topic started by: 012anonymousxyz on May 21, 2016, 08:37 am

Title: 8x8 LED Matrix Control
Post by: 012anonymousxyz on May 21, 2016, 08:37 am
Hi there! I'm building an 8x8x8 LED Cube and I'm following a reference design found here for an 8x8 grid;

https://www.dropbox.com/sh/fyvszu2xnow17fg/AADd5HBRnJvYwjZZD2eV0hJza/Schematic%20Drawings/8x8%20Panel%20Tester%20-%20Full%20Schematic.pdf?dl=0

I would appreciate insight into this schematic if you can figure out what is being done and why.

I have two questions related to this.

1) Here is the PMOS being used: http://www.irf.com/product-info/datasheets/data/irf9z34n.pdf
I've studied mosfets from a textbook and nowhere is mobility, width/length, or Cox given so how can I calculate the current through the PMOS given a gate voltage?

2) Why is there a 100 ohm resistor connected to ground on the anodes? Yet there is no resistor in series with the LEDS
Title: Re: 8x8 LED Matrix Control
Post by: Paul__B on May 21, 2016, 12:06 pm
Schematic does not seem to be visible.
Title: Re: 8x8 LED Matrix Control
Post by: 012anonymousxyz on May 21, 2016, 08:28 pm
Does this work? Schematic Drawings > 8x8 Tester

Dropbox link (https://www.dropbox.com/sh/fyvszu2xnow17fg/AACotfHOmN3eo0gS3QTL1lH1a?dl=0)
Title: Re: 8x8 LED Matrix Control
Post by: Paul__B on May 21, 2016, 10:31 pm
Not for me - don't know about anyone else.
Title: Re: 8x8 LED Matrix Control
Post by: chummer1010 on May 22, 2016, 02:44 am
Not for me - don't know about anyone else.

I think I got it - http://imgur.com/oehsj9L (http://imgur.com/oehsj9L)

Is this it OP?
Title: Re: 8x8 LED Matrix Control
Post by: 012anonymousxyz on May 22, 2016, 09:33 pm
Yes, that's it! I literally just put it on imgur as well. Thanks for that.

So I realized that the resistors are "pulldown" resistors. What's the problem with leaving them floating or unconnected?

As for the PMOS, here are the equations: PMOS Equation Sheet (https://inst.eecs.berkeley.edu/~ee105/fa05/handouts/discussions/Discussion5.pdf)
I read about the LED driver and I guess it uses a current mirror to draw a constant current. And the mistake I made is that I ignored Vds (Vgs is set 5V). But nevertheless, why are none of the parameters used to calculate the currents/voltages through/across the MOSFET given in any of the data sheets for any MOSFET (I've looked at a couple other transistors and they didn't have ratios or anything either). ATM my goal is to solve for Vds. That is, I have current (by the LED driver), and Vgs, and I want to calculate Vds to check that my LEDs will turn on.
Title: Re: 8x8 LED Matrix Control
Post by: Paul__B on May 23, 2016, 06:28 am
As pointed out elsewhere, you don't "solve" for VDS, you merely ensure you use a logic level FET which saturates at 4V VGS or less.