When you look at raw multiplexing without a latch, the effect is the same as using PWM for brightness control The more digits, the lower the light output since each digit will be waiting for all the other digits to be scanned before it gets its turn to light up again.

Thanks, so latching may be the way to go. I'll need to delve into this topic.

 

[...]Except for blue....strange...

That very well may be a typo...

 

So, if I'm multiplexing 4 digits, that translates to a duty cycle of 25% per digit. Using the datasheet I linked on the previous page for the red LED display, an average of 20 mA per segment means (theoretically) I could drive each segment at 80 mA. This exceeds the maximum recommended current during pulsed operation, so I should instead use that value, which is 70 mA per segment.

Did I calculate that correctly? Also, it seems I'd need a pretty high-wattage resistor at this point...

 

Did I calculate that correctly? Also, it seems I'd need a pretty high-wattage resistor at this point..

I would go for 60mA, keep the LED heating down and prolong its life.
Why would the resistor wattage have to be high.? Its still around 20mA averaged.

 

A current-limiting resistor? You should have one per segment - only one means the LEDs will not share current equally and will result in, you guessed it, dimmer segments.

Put a current-limiting resistor on each segment.

See here for more information about driving LEDs.

Actually, no. See here for a data sheet for the 7219:

http://datasheets.maximintegrated.com/en/ds/MAX7219-MAX7221.pdf

A single resistor sets the current for a group of matched current sources, so a lot of the discussion doesn't apply.

LEDs have on and off times in the tens of nanoseconds. Unless you're multiplexing at MHz speeds, speed has almost zero effect on brightness. Duty cycle is everything.

Yes, you can reduce the value of the current-setting resistor. Yes, multiplexing permits "overdriving". This is a standard practice (despite your instinct). Yes, you must pay attention to the current capability details of your display and the driver. The 7219 can handle up to 45 mA per segment and 320 mA per digit. The standard solution is to go to the datasheet for the display and find its peak current capability. All "normal" LEDs can be pulsed at currents much higher than their continuous rating (blues and whites can be weird).

Note that the 7219 has a brightness PWM built in; you might have its value set incorrectly. Also, you have to set the scan limit register or it will scan 8 digits even though you have only 4, meaning unnecessary off time.

And finally, the 7219 was designed for a 5V world. There is a schematic on page 12 for driving 2.3" displays.

ak

 

Actually, no. See here for a data sheet for the 7219:

http://datasheets.maximintegrated.com/en/ds/MAX7219-MAX7221.pdf

A single resistor sets the current for a group of matched current sources, so a lot of the discussion doesn't apply.

LEDs have on and off times in the tens of nanoseconds. Unless you're multiplexing at MHz speeds, speed has almost zero effect on brightness. Duty cycle is everything.

Yes, you can reduce the value of the current-setting resistor. Yes, multiplexing permits "overdriving". This is a standard practice (despite your instinct). Yes, you must pay attention to the current capability details of your display and the driver. The 7219 can handle up to 45 mA per segment and 320 mA per digit. The standard solution is to go to the datasheet for the display and find its peak current capability. All "normal" LEDs can be pulsed at currents much higher than their continuous rating (blues and whites can be weird).

Note that the 7219 has a brightness PWM built in; you might have its value set incorrectly. Also, you have to set the scan limit register or it will scan 8 digits even though you have only 4, meaning unnecessary off time.

And finally, the 7219 was designed for a 5V world. There is a schematic on page 12 for driving 2.3" displays.

ak

I should have made this clear from the outset. What you have said is true if you are not using high-voltage displays. Maxim describes how to use the MAX7219 for such displays and then you can in fact use individual current-limiting resistors for each segment. I chose to do it this way because I am mixing differently colored LEDs which have different brightness.

I do have the brightness PWM cranked up all the way on the MAX7219 and am scanning only 4 digits


In any case, I think I have a strategy. Many thanks to all for your help!

 

I would go for 60mA, keep the LED heating down and prolong its life.
Why would the resistor wattage have to be high.? Its still around 20mA averaged.

Of course it is, I was thinking 70 mA continuous.