I was all set to build a 8x8 grid of LED's that would use the arduino and 2 x 74HC595 shift registers to control them and I wanted to determine what my current limits were and to help me choose my resistors so I modelled it up in ltspice.
As soon as I started to run the simulations I realised that my design was really dependant on not exceeding the reverse breakdown voltage of the LED's in order for it to work. More critically I didn't want it to fail over time as bulbs burn out.
I am left thinking that multiplexing LED's is critically dependant on staying below the reverse breakdown voltage of the LED. But I cannot seem to find that written anywhere.
Is it just one of those things that is so simple and obvious that no one ever bothers to mention it?
I've tried looking up in my LED datasheet what the reverse breakdown voltage but the only reverse info it has is a reverse current at 5v of 10uA. For my 5v circuit I'm reading that as being acceptable as it would indicate that it's able to limit reverse current flow at 5v and that I don't need to be dropping in signal diodes to protect it all.
Is my thinking correct? (probably not :)) or do I have to put in some means of actively protecting against reverse voltage?
There is nothing to worry about, all leds I know have reverse voltage > 5 V. And in second, how come leds could get reverse polarity?, your drawings probably not correct.
There is nothing to worry about, all leds I know have reverse voltage > 5 V.
Yes i’m thinking that’s the case.
And in second, how come leds could get reverse polarity?, your drawings probably not correct.
If you want to turn on a row at a time you wouldn’t get reverse polarity but if you want to turn just a single diode in a row you need to drop the other inputs low.
If you want an entire column on you set all the outputs low but you need to set all but one high if you just want one LED on.
The end result is a lot of reverse potential in the circuit which the LED’s would need to be able to handle.
Diagram attached (ignore the resistor configuration as they’re what i’m playing with)
LED_8x8_example.pdf (58.5 KB)
There should be no problem with 5V and multiplexing. Also most of the LED Matrix drivers (which are more or less two shift registers and multiplexing logic) are working up to 5V.
For the current limit you need to take care of the average & maximum allowed current specified in the datasheet.
I prefer using the "ready to go" Drivers since they are easier.
BTW: there are two very basic info slides here on multiplxing & crossplexing:
http://www.digikey.cn/PTM/PTMPartList.page?site=cn&lang=en&ptm=25206 (slide 5)
Great links. Thanks.
Those cross-plexing examples certainly remove any doubts I had about the processing relying on reverse breakdown voltages in order to function.