Go Down

Topic: 8x8 LED Matrix Control (Read 1 time) previous topic - next topic


Hi there! I'm building an 8x8x8 LED Cube and I'm following a reference design found here for an 8x8 grid;


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


Schematic does not seem to be visible.


Does this work? Schematic Drawings > 8x8 Tester

Dropbox link


Not for me - don't know about anyone else.


May 22, 2016, 02:44 am Last Edit: May 22, 2016, 02:45 am by chummer1010
Not for me - don't know about anyone else.

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

Is this it OP?
#include <signature.h>


May 22, 2016, 09:33 pm Last Edit: May 22, 2016, 10:05 pm by 012anonymousxyz
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
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.


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.

Go Up