please sanity check LED multiplex current

Thread Starter

skk

Joined Mar 15, 2011
31
I changed my mind yet again, and now I am going with 2 sets of multiplexed leds going at the same time. Please validate my design and assumptions.

Circuit:
I have 2 different multiplexed sets of LEDS. The first set is 3 7-segment leds blocks, each with 7 20mA segments plus 1 10mA dot. The other set is 8 10mA leds x 4. These leds are common cathode red/green dual 3 lead leds.

Multiplexing:
I am multiplexing using the common cathode of both sets. Common cathode is switched by a 74HC595 ( source 35mA max ) with a 2N2907 for 350mA max. The rows are sunk using 2 TPIC6C595 shift registers. Each one will sinc 100mA per row and 500mA total.

My thinking is that since my 7-segments are on 1/3rd of the time, max that would be (20mA * 7 + 10mA ) * 3 or a pulse of 450mA with a 1/3 duty cycle to get 100% brightness. First, is this correct?

Assuming that, then I am not going to get enough current out of my source register because it gives only 35mA, with 2n2907 transistor that is 350mA. But since I only need 450mA for 100% brightness then that is like 77% brightness at 350mA, so I can live with that.

The second mutiplex set is 8 x 4 10mA LEDs, or 320mA < 350mA, so I am fine. On the sinc side, each line can drain 100mA each and 500mA total, so I am also fine.

So now I have the current limiting resistors for each of the 20mA segments ( 1v drop). The 7 segment ones are on 1/3rd of the time, so I need 3 times the normal current or

1v/ 20mA * 3 = 16.6 ohms
and the 'dot'
1v/ 10mA * 3 = 33.3 ohms
and the 8x4 10mA LEDS
1/ 10mA *4 = 25 ohms

Am I doing this right?

As for the base resistors on the transistors, that would be:

4.3v / ( 20mA * 7 + 10mA ) = 28.6 ohms

and the base resistor for the single LEDs

4.3v / (10mA * 8 ) = 53.7 ohms

Does this check out?

Thanks









http://www.ti.com/lit/ds/symlink/tpic6c595.pdf
 

praondevou

Joined Jul 9, 2011
2,942
Ok, I should have been more specific. Not everyone has always a schematic designer software at hand. Right now I'm accessing the site on a mobile device. No way I can open a .sch extension.
Diagrams should be posted as jpeg or png pictures unless someone asks specifically for something else.
 

Thread Starter

skk

Joined Mar 15, 2011
31
Here is the part of the schematic I was asking about. The 3 shift registers are at left. I am using STPIC65595s ( -100mA ) for the sinc and MM74HC595 ( +35mA) to drive the transistors. The emitter of the pnp is connected to vcc ( +5v ).
 

Attachments

thatoneguy

Joined Feb 19, 2009
6,359
Here is the part of the schematic I was asking about. The 3 shift registers are at left. I am using STPIC65595s ( -100mA ) for the sinc and MM74HC595 ( +35mA) to drive the transistors. The emitter of the pnp is connected to vcc ( +5v ).
Image is too small to read any text or numbers.
 

Thread Starter

skk

Joined Mar 15, 2011
31
OK, lets try again with text.

As I said before the shift registers for the sincing are STPIC6C595s and the sourcing shift register is MM74HC595 with help of 2N2907 transistors. In the circuit below I am running 4 columns of LEDs, each with their own transistors, each from different outputs of the MMHC74595. Only 1 of the 4 columns will be on at one time. The circuit below only shows 1 column:

Rich (BB code):
+----------+
| STPIC595 |
|          |--------/\/\/\/\----|<|---+
|          |--------/\/\/\/\----|<|---|
|          |--------/\/\/\/\----|<|---|
|          |--------/\/\/\/\----|<|---|
|          |--------/\/\/\/\----|<|---|
|          |--------/\/\/\/\----|<|---|
|          |--------/\/\/\/\----|<|---|
|          |--------/\/\/\/\----|<|---|     vcc +5v     
+----------+                          |      | 
                                      \     /
                                       \   v
                                    ---------- 2n2907
                                         |
+------------+                           |
| MMHC74595  |                           |
|            |--------------/\/\/\-------+
+------------+


The other set of multiplexed LEDs is similar, but just has 3 columns, but each LED is a double ( 20mA ), except for the 'decimal point' which is a single ( 10mA). Only one column shown below:


Rich (BB code):
+----------+
| STPIC595 |
|          |-----/\/\/\/\----|<|-|<|--+
|          |-----/\/\/\/\----|<|-|<|--|
|          |-----/\/\/\/\----|<|-|<|--|
|          |-----/\/\/\/\----|<|-|<|--|
|          |-----/\/\/\/\----|<|-|<|--|
|          |-----/\/\/\/\----|<|-|<|--|
|          |-----/\/\/\/\----|<|-|<|--|
|          |-----/\/\/\/\----|<|------|     vcc +5v     
+----------+                          |      | 
                                      \     /
                                       \   v
                                    ---------- 2n2907
                                         |
+------------+                           |
| MMHC74595  |                           |
|            |--------------/\/\/\-------+
+------------+


So top diagram on 1/4th of the time, bottom on 1/3rd of the time. Assume forward voltage drop 1v per led and max current is 10mA. The double LEDs are then 20mA and Vf=2v. The STPIC595 will sync 100mA per pin 500mA for the package, and the MMHC74595 can source 35mA, with the 2n2907 good up to ~500mA. I just wanted my original numbers for the base resisters and the current limiting resistors verified.

Thanks again.
 

Thread Starter

skk

Joined Mar 15, 2011
31
I hope I was not too simplistic in my text drawing, but I think I captured all the pertinent info. There are lots of components I was not asking about, and schematic is hard to read because I used a lot of buses. Anyway, the 3 shift registers are on the right. SINCEREG1 and SINCREG0 are the STPIC595s and the SRCREG is the MMHC74595. The two columns of common cathode LEDs form 4 columns of single LEDs 1 of 4 on at any time. The 7 segment displays at the right form the 1 of 3 multiplexed group. The anodes of both sets are driven by the MMHC74595 plus 2n2907 transistors. None of the resistors show values, because I still am not sure what values to use...
 
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