Go Down

Topic: Help with increasing 8x8 LED Matrix brightness (Read 2 times) previous topic - next topic

Nick Gammon


The wire (ribbon cable) in the picture is 30 Ga wire and the current is being sources/sunk through several feet of what is essentially data grade wire


Are you referring to my picture? I had 1K resistors, so if my calculations are correct the most that each wire would have to take is 3 mA, and 3 * 8 = 24 mA for the Gnd return.

anarchy2465


Quote
The most current management I'm doing is switching it with the MOSFET which can take up to 9A

This means you will end up frying your shift registers.

I am having a bit of trouble down loading the data sheet for that FET, but almost certainly it is not a logic level FET and requires 10V to fully turn it on. You are supplying it with 5V and so it has a high, for a FET resistance. This is fine for small currents, but when a large current flows, with all your LEDs on the resistance drops a larger voltage and restrict what is flowing.

EDIT - just checked the data sheet and the FET is your problem, the Gate Threshold is only when the FET starts to turn on, not when it is fully on, you need 10V on the gate to get an on resistance of 0.4R which tells me it is a very old device newer devices have a much lower Ron ( resistance when on ) than this. So either boost the gate voltage or get another FET.


Can you elaborate on frying the shift registers?

Also you're right about the FET.  I disconnected it from the Arduino and connected the gate to a 9V battery.  HUGE difference.  However, I measured the current at the output of one shift register pin (i.e. one LED) and it was like 1.5mA while lit.  Not really sure what to make of that..  But with 9V driving the MOSFET I can could turn on 32 LEDs with comparable brightness to 8 LEDs.  I'm thinking of buying some TTL mosfets, do you have any ideas what series to go with?  Also how will changing the transistor affect the current?  Right now I've been able to get away with no resistors between the shift register outputs and the LEDs

Grumpy_Mike

Quote
Can you elaborate on frying the shift registers?

well you can:-
Quote
Right now I've been able to get away with no resistors between the shift register outputs and the LEDs

This means that the only thing limiting the current is the output impedance of the drivers. That means they are supplying too much current. You don't need resistors because you are killing your shift register outputs.
Read this about LEDs and the voltage / current relationship. http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

Quote
Also how will changing the transistor affect the current?

Because the on resistance will be much lower and that allows more current to flow.

Quote
However, I measured the current at the output of one shift register pin (i.e. one LED) and it was like 1.5mA while lit.

I find that very hard to believe. Meters don't tell the truth when subject to any sort of oscillation or chopping.

Quote
any ideas what series to go with

Try the IRLB8478


Nick Gammon


Go Up