![]() I do wonder if these LEDs from 2009 are somewhat obsolete! That limitation is unusual. Well, as was discussed in the earlier posts, to avoid reverse biasing, you would want to use a driver that only pulls cathodes low and anodes high, such as the MAX7221. Whether it actually matters in practice is another question, but I noticed that the initial responses seemed to deny that reverse biasing even occurs in this particular situation. He is correct here, yet nobody originally acknowledged that. The OP wrote "Well my question is the following: when I set the output pins to HIGH to turn off the led, wouldn't that cause a reverse voltage on the led?" But, when I do, I make sure to be under spec. Having said all of the above, I have used Charlieplexing before, and it does require reverse biasing if you max out the capability (ie. I should made a few more edits before posting so as not to mislead. My diagram was "lifted" from elsewhere, I did not mean to specify any actual pin numbers, but your warning is well advised. So I believe the best answer as to whether or not it is an issue is "it depends on what the data sheet indicates" and since the OP didn't specify the leds, we can't be certain if there is a problem or not. I have included a link below to a data sheet which warns that "Reverse biasing the dot matrix is not recommended, will cause damage to the leds." (the reverse voltage here is 3v). I mainly wanted to set the record straight on that.įor some LEDs, reverse biasing may be a problem according to the data sheet. The OP wrote "Well my question is the following: when I set the output pins to HIGH to turn off the led, wouldn't that cause a reverse voltage on the led?".I do feel some clarifications are necessary though: Thank you for responding Paul, you make some very good points. Having said that, Charlieplexing uses tri-state logic and yet still reverse biases the leds, so it may not necessarily be a big concern as long as you pay heed to the led data sheet's reverse biasing limits. I will add that if one can drive the display directly from logic level I/O, then changing the output to high impedance by dynamically changing it to an input can solve the reverse bias issue here. So unless I've either missed or misconstrued something, I believe we are in violent agreement! Furthermore, and of course, using transistors may also solve the problem of over driving the display directly from logic level outputs. ![]() ![]() That's exactly where I was going and as you correctly pointed out, that using transistors on the common anodes avoids this problem altogether (side note: I believe PNP is a good choice here). What you really want is a high impedance state on the cathode rather than a high output to turn it off. limit to high/low values) and correctly pointed out reverse biasing concerns that result from that design. It struck me that the OP was trying to do that (ie. I've seen where people have driven multi digit 7 segment displays directly from shift registers or directly from controller output pins (and with proper current limiting resistors) which are set in code to be either high or low. But perhaps to your point, I suppose it's better to be precise than not (notwithstanding that too often I'm accused of providing too much detail). My diagram was conceptual and intended to focus on the direction of current only which is why I omitted the current limiting resistors. ![]()
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