# please sanity check LED multiplex current

#### 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

#### praondevou

Joined Jul 9, 2011
2,942
It would really make things easier if you attach a complete circuit diagram.

#### skk

Joined Mar 15, 2011
31
OK, I uploaded the complete schematic ( attached ).

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• 118.3 KB Views: 16

#### 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.

#### 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 ).

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#### 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.

#### 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.

#### SgtWookie

Joined Jul 17, 2007
22,221
Here, I exported the Eagle schematic in b&w scaled at 150DPI, saved it as a .png and attached it.
[eta]
Gosh, the schematic doesn't resemble what you posted at all.

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