# Improve Brightness in LED Multiplex

I’m doing a 16 rows & 48 column LED matrix design. The attachment shows only one row & some columns.

I’m scanning rows @ 1/16th rate. Designed for 4mA average current to the LEDs. I’m doing a 5V design (PIC & shift registers working on 5V), but For LEDs I can increase supply more than 5V.

My LEDs are bit dim.When I increase the LED supply voltage to 6.5V then the brightness improves.But I'm not sure is that voltage is too much for LEDs.

I use 3V LEDs.I need to make imrovements in my deisign.What modifications needs to do to make a brighter display?

• if the voltage was too high then the LEDs would be super bright and some of them would fail Feb 26, 2019 at 17:55
• What does the LED datasheet say for maximum current? Feb 26, 2019 at 18:00
• HI, LEDs are no brand cheap chinese LEDs (Oval Type 4.8mm).In packet label it shows 3.0-3.4V only. Feb 26, 2019 at 18:04
• I want to confirm this.Disable multiplex & turn ON a single row permanently.Enable the columns.Now check the R3 Current.Ex Now it shows 80mA & 3.2V across the LED.So that means when multiplexing the LED will get 80mA/16 = 5mA.Is this how you check?Am I correct? Feb 27, 2019 at 2:57

With those LEDs you can make a no brand cheap Chinese LED device that may work for long enough that you can sell it. This is why we pay extra for quality and documentation.

Figure 1. A typical datasheet will give (1) the forward (continuous) current, (2) the peak current and (3) the peak current maximum pulse width. Source: Cree C503B datasheet.

In the Cree C503B, above, the peak current maximum pulse is 100 μs per 1 ms.

There's a little more in (my) LED rated current.

• I want to confirm this.Disable multiplex & turn ON a single row permanently.Enable the columns.Now check the R3 Current.Ex Now it shows 80mA & 3.2V across the LED.So that means when multiplexing the LED will get 80mA/16 = 5mA.Is this how you check?Am I correct? Feb 27, 2019 at 2:58
• That would work but it sounds risky to run 80 mA through the LEDs. Feb 27, 2019 at 7:10
• I loaded my matrix as in my first schematic.When The load current reaching 2A rating the B772 transistors are getting really hot Also the ULN also getting heat.I'm planing to redesign the circuit to make it a proper 5V design by adding IRF9530 mosfets to row side & replace all ULN2803 with low saturation BJTs.TO drive the IRF9530 can I directly drive from PIC pin via a 100R & the Gate will pull up by a 10K? Feb 28, 2019 at 6:51

Reducing the resistors R3 would also increase the current.

4mA is quite low for an LED. I would assume most generic LEDS can stand around 20mA, especially if they are pulsed in a multiplexed display.

• You are correct.When I short circuit the R3 (27 Ohms) The brightness increases. Feb 27, 2019 at 2:49
• So that means 20mA X 16 = 320mA I should supply to LED? Feb 27, 2019 at 3:00
• @Joseph143 I'd be wary of eliminating the resistors entirely, as they are what limits the current. Everything else is just voltage drops in semiconductors. You may get away with short pulses of 320mA, but at the cost of shorter LED life. that's why you really need LEDs with datasheets. Those normally give ratings for constant and pulsed current. Feb 27, 2019 at 15:47
• I loaded my matrix as in my first schematic.When The load current reaching 2A rating the B772 transistors are getting really hot Also the ULN also getting heat.I'm planing to redesign the circuit to make it a proper 5V design by adding IRF9530 mosfets to row side & replace all ULN2803 with low saturation BJTs.TO drive the IRF9530 can I directly drive from PIC pin via a 100R & the Gate will pull up by a 10K? Feb 28, 2019 at 6:51