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

#### 012anonymousxyz

##### 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;

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

#### Paul__B

#1
##### May 21, 2016, 12:06 pm
Schematic does not seem to be visible.

#### 012anonymousxyz

#2
##### May 21, 2016, 08:28 pm
Does this work? Schematic Drawings > 8x8 Tester

#### Paul__B

#3
##### May 21, 2016, 10:31 pm
Not for me - don't know about anyone else.

#### chummer1010

#4
##### May 22, 2016, 02:44 amLast 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>

#### 012anonymousxyz

#5
##### May 22, 2016, 09:33 pmLast 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.

#### Paul__B

#6
##### 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.

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