MAX7219 and many Leds

Thread Starter

FroceMaster

Joined Jan 28, 2012
662
Hi,
Will gave this setup, but i dont know if the MAX can handle it like this.
I need up to 6 LED in Series and there for i need 12v.
If i put a Transistor in, can i then handle it that way ?
If i can, do i still need the resistor(R1+R2) to Base of transistor ?

MAX with Led.png
Know i need a lot of components more, but for easy i only do this.
 

ElectricSpidey

Joined Dec 2, 2017
1,529
Only if the LEDs require less than 2 volts to light. (for 6)

Yes, you will still need the resistors...the ones on the base, and the series resistor for the LEDs.

And you should connect the emitters to ground, not an output pin.
 

ElectricSpidey

Joined Dec 2, 2017
1,529
You only need one transistor for each LED string, assuming the pin can provide the proper base current.

But driving the emitters from a pin defeats the purpose.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
662
I need all the Segment A-G and then 4 digits, so i will have 28 strings of LEDS . +one or 2 DP.
If i connect E to ground i cant select another digit.
 

ElectricSpidey

Joined Dec 2, 2017
1,529
I see, perhaps I am making the wrong assumption here, I was assuming you needed the transistor to provide more current.

Sorry.

As long as the digit outputs can handle the current, I see no need for another transistor,
 
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Thread Starter

FroceMaster

Joined Jan 28, 2012
662
Ok,, as i can read the datasheet, 320 mA but not sure, but if the MAX can handle ordinary LED Display, and i only use 14-16 mA pr string then i think it will be ok
 

Tonyr1084

Joined Sep 24, 2015
5,804
I think I missed something. "Control each digit"? Are you asking to be able to light only one LED at a time? Help me - I'm lost (again).
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
662
Or will it be better to make it this way, ?
MAX Parralel.png

Giving me 3*16mA = 48mA per segment, 48*8 = 400 mA ( if i write "8" and DOT ) , should have 500 max ?
As i can read from datasheet this should be possible ??

Max Spec.JPG
 
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Thread Starter

FroceMaster

Joined Jan 28, 2012
662
As i understand the data, i can have 100mA on each segment, and 500,mA on DIGITS
When connected like this, i use 48mA on each Segment, and yes(8*48) 400 mA on the DIG.
And the resistors can i also get rid off. only use 1 central on input of MAX ?

But again, maybee iam wrong,
 

ElectricSpidey

Joined Dec 2, 2017
1,529
Current isn't the only consideration, you also have to consider wattage.

And no, I don't think you can replace the resistors with a single one on the MAX input.
 

Thread Starter

FroceMaster

Joined Jan 28, 2012
662
Ahh calc the wattage, 24 pr LED and 17 pr R, = 195 mW pr Segment.
7- 8 segment , little to much, ok then i will have to go with 12v and Transistors.
But still i dont know if i can control the E from transistor by the DIGIT output from MAX or du i need a different approch ?
 

ElectricSpidey

Joined Dec 2, 2017
1,529
Ok so looking at the datasheet, if you drive the LEDs directly you control the current with a single resistor, but the problem is you will have parallel and series configuration, not just series and as many on this board will tell you...LEDs in parallel is a no no.

I personally disagree, but what the hell do I know.
 
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